CIBSE How to Design a Heating System

How to design a heatingsystem

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CIBSE Knowledge Series — How to design a heating system

How to design aheating systemCIBSE Knowledge Series: KS8

Principal author Gay Lawrence Race

EditorHelen Carwardine

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11.1 Use of this guidance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

2 The heating design process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42.1 The design process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42.2 Heating system design process . . . . . . . . . . . . . . . . . . . . . . . . . . . .52.3 Key heating design calculation sequence . . . . . . . . . . . . . . . . . . . .82.4 Thermal comfort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

3 Key design steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .103.1 Step 1: pre-design and design brief . . . . . . . . . . . . . . . . . . . . . . .103.2 Step 2: gather design information . . . . . . . . . . . . . . . . . . . . . . . . .113.3 Step 3: design data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .123.4 Step 4: building thermal performance analysis . . . . . . . . . . . . . . .133.5 Step 5: heating system option analysis and selection . . . . . . . . . .153.6 Step 6: space heat losses and heat load . . . . . . . . . . . . . . . . . . . .203.7 Step 7: equipment sizing and selection . . . . . . . . . . . . . . . . . . . . .233.8 Step 8: heating load analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . .253.9 Step 9: plant sizing and selection . . . . . . . . . . . . . . . . . . . . . . . . .273.10 Step 10: system analysis and control performance . . . . . . . . . .273.11 Step 11: Final value engineering and energy targets assessment 293.12 Step 12: design review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29

4 Developing the design — key issues . . . . . . . . . . . . . . . . . . . . . . .314.1 Design data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .314.2 Design margins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .314.3 Energy efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .324.4 Quality control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34



1 Introduction

In cooler climates the provision of heating is an essential part of creatingcomfortable internal environments, and therefore heating system design is afundamental part of building services design.

Heating is a major sector within mechanical building services. There are some21 million domestic properties in the UK with gas-fired central heating, and afurther 200,000 commercial properties with heating. The UK market forheating systems is substantial, with around 1.65 million new domestic boilersinstalled per year and around 23,500 commercial boilers. There are around 9million radiators installed per year with a further 22 million metres ofunderfloor heating pipe (2005 figures)(1).

Heating is also a major consumer of energy within the UK, with space heatingaccounting for over 40% of all non-transport energy use and over 60% ofdomestic energy use(2), rising to over 80% if hot water is included (see Figure1). As major energy users, heating and hot water also generate a substantialproportion of CO2 emissions, delivering almost half the CO2 emissions fromnon-domestic buildings.

Given the current requirements to limit energy consumption and CO2

production, good design of heating systems is essential to ensure that systemsoperate efficiently and safely and make effective use of energy. Historicallythere have been problems with oversizing of heating systems which can leadto inefficient operation, particularly at part load operation, to controlproblems and to a reduction in plant operating life(3). The energyconsumption for oversized plant can be 50% more than necessary.

Although heating is often considered to be a simple, basic system, there aremany options and permutations to be considered. The majority of UKbuildings will require heating but different building types and locations willhave very different requirements and constraints — consider for example thechoices possible for a small ground floor flat in a city centre developmentagainst those for a holiday cottage in one of the National Parks, or thechoices for an urban industrial unit against those for a rural agricultural unitand farm shop.

The fundamental components of any heating system are:

— a means of generating heat, i.e. the heat source— a means of distributing the heat around the building or buildings, i.e.

the distribution medium and network— a means of delivering the heat into the space to be heated, i.e. the

heat emitter.

1

Heating

In 2005:

� 1.65 million new domestic boilers

� 23,500 commercial boilers

� 9 million radiators

� 22 million metres of underfloorheating pipe

were installed in the UK alone.

Sources: BSRIA domestic boilermarketing report March 2006, BSRIAcommercial boiler marketing reportMarch 2006.

Figure 1: UK non-transport energyuse (2002 figures) milliontonnes of oil equivalent

Space heatingWaterCooking/cateringLighting appliancesProcess useMotors/driversDrying/separationOther non-transport

41·4

11·8

4·42·4

15·0

9·7

3·3

12·9

/

Source: DTI Energy consumption tables:overall energy consumption. URN No:05/2008 Table 1.2 Non-transport energyconsumption by end use, 1990, 2000, 2001and 2002

There are many possible options to be considered, some of which are listedin Table 1 below. These can give many permutations, from the simple use ofelectric panel heating, using electricity both as the heat source anddistribution medium, to a conventional gas boiler system distributing lowtemperature water to a convector system. A more complex system would beone serving various buildings by using oil as the heat source to generate hightemperature water for the main distribution, which is then reduced intemperature and pressure to low temperature water, via heat exchangers, toserve a radiator system.

Whilst heating systems may seem relatively simple, in practice there are manyfactors to be considered during the design process, in order to achieve awell-designed system that delivers both the required comfort conditions andlevel of control whilst still minimising energy consumption. This publication,together with other CIBSE guidance, aims to assist the designer in achievingthat aim.

1.1 Use of this guidance

This publication provides a clear, step-by-step overview of the whole heatingdesign sequence:

— section 2 maps the heating design process, with flowcharts illustratingthe design steps sequence, and sets this in the context of the overallbuilding process

— section 3 outlines the key design procedures for each design step, and

CIBSE Knowledge Series — How to design a heating system2

Good design

Good design of heating systems isessential to ensure that systems operateefficiently and safely and make effectiveuse of energy.

Factors to consider

Building type:

� domestic

� school

� apartment building

� retail

� hospital

� factory

� office

Location:

� city centre

� urban

� suburban

� rural

Heat source gas CHP

LPG solar

oil biomass

coal off-peak electricity

electricity wind

air or water via heat pump

ground via ground source heat pump

Distribution medium water: low, medium or high temperature

air

steam

electricity

Emitter radiators ceiling panels

fan convectors natural convectors

panel heaters underfloor heating coils

unit heaters storage heaters

high temperature radiant panels

Table 1: Heating systems


provides guidance on data requirements and sources, design outputs,key design issues and potential problem points

— section 4 addresses additional design issues that affect the designprocess.

The publication links to the CIBSE Guides and also cross-references otherkey industry sources of design procedure guidance. Other relevant titles inthe Knowledge Series are:

— KS04: Understanding controls— KS06: Comfort— KS: Energy efficient heating (forthcoming title).

This guidance is intended to enable and assist building services engineersinvolved in design, installation and commissioning to appreciate the keydecisions and design steps involved in heating system design. It is likely to beof particular benefit to junior engineers and those whose main experience lieswithin other sectors of building services design. It can also be used by buildingservices engineers to facilitate discussion on design requirements and designdecisions with their clients.

The publication answers the following questions, which can be used to helpyou find the most relevant sections to you:

— What are the key stages in the heating design process? (Section 2.2)— What are the design criteria for thermal comfort? (Sections 2.4 and

3.3)— What should I consider when selecting a heating system? (Section 3.5)— How do I determine preheat requirements? (Section 3.6)— What should I consider to determine the required heating load?

(Section 3.8)— When should I consider load diversity? (Section 3.8)— What else should I consider during design? (Section 4).

Finally, a selected bibliography is provided for those who want further readingon the subject, subdivided to cover the main design steps and key topics suchas design data, design calculations, design checks, heating plant and controls.Detailed technical information on heating system design and design data canbe found in CIBSE Guide A (2006) and CIBSE Guide B (2001-2), chapter 1.

3

2 The heating design process

2.1 The design process

Design involves translating ideas, proposals and statements of needs andrequirements into precise descriptions of a specific product(4), which can thenbe delivered. (See Figure 2.) Two major features characterise the designprocess in general. Firstly, design tends to evolve through a series of stagesduring which the solution is increasingly designed at greater levels of detail,moving from broad outline through to fine detail. Secondly, design tends tocontain iterative cycles of activities during which designs, or designcomponents, are continually trialled, tested, evaluated and refined. Feedbackis therefore an essential component of the design process, as shown in Figure 2.

Within construction, design is a part of the larger construction process, asshown in Figure 3. Both the RIBA Plan of Work Stages(5) and the ACEConditions of Engagement Agreements A(2) and B(2)(6), which are commonlyused for mechanical and electrical building services design, divide design intothe separate stages of outline design, scheme design and further/detail design.In practice, therefore, the construction design process is invariably iterative,with many design steps being revisited and revised as the design evolves anddevelops, and this necessitates constant communication and clarificationbetween team members.


Figure 2: The design process

1. Clientneed

The design process

3. Design

Designperformance

Feedback/review

Feedback/review Inform

DevelopSelect

Implement

2. Designrequirements

4. Designdelivery


2.2 Heating design process

The problem with the standard design process is that it is both complex andlacking in design task details. Although design is a clear part of the process,detail of the design tasks involved is not given beyond global statements suchas ‘develop the design and prepare sufficient drawings…’.

Therefore, a simple straightforward design sequence for heating design hasbeen developed (see Figure 4 over the page) to both clarify the process andallow detail of specific design tasks to be added. This gives a simplified lineardesign sequence, from the pre-design stage through the various analysis,decision and calculation steps through to the final solution, enabling designtasks to be clearly linked to both preceding and succeeding actions. Althoughsome feedback loops are shown, in practice there are often feedback loopsbetween all tasks and even within specific tasks, reflecting the more iterativenature of real-life design. Further detail on all of these steps is available insection 3.

It is important to still set this in the context of the full design process. In practicethere are several design repetitions within the various stages, and overlaps fromone stage to another. For example, information on overall space requirementsand plant structural loadings is often required by other team members at theoutline design stage. This degree of detail is unknown at this early stagetherefore often assumptions and approximations have to be made in order toprovide information. It is vital that these are checked as the design progresses.

5

RIBA plan of work (1999) ACE Agreements A(2) &B(2) (2002)

A Inception/Identification of client requirements

B Strategic brief

C1 Appraisal stage

C2 Strategic briefing

Pre-

desig

n

C Outline proposals

D Detailed proposals

E Final proposals

C3 Outline proposals stage

C4 Detailed proposals stage

C5 Final proposals stage

Des

ign

F Production information C6 Production information stage

G Tender documentation

H Tender action

C7 Tender documentation and tender action stage

Con

stru

ctio

nJ Mobilisation/Project

planning

K Construction to practical completion

L After practical completion

C8 Mobilisation, construction and completion stage

Figure 3: Construction processstages


Step no. Key design steps Design tasks

1 Pre-design Obtain design brief. Identify client and building user needs and requirements. Refer to feedback and lessons learned from previous projects 2 Gather design information Gather information about site, including utilities provision and fuel options. Obtain information on use of building, occupancy hours and on possible building form, fabric, etc Establish and confirm key design requirements including Regulations and Codes of Practice. Establish planning conditions for use of on-site renewables 3 Design data Establish the key design data and parameters that relate to the design of the heating system, including building air tightness data, and potential use of renewables. Develop room design data sheets Check that design parameters comply with legislation, energy targets, etc 4 Building thermal performance Analyse building – establish fabric thermal performance and infiltration analysis Determine whether intermittent operation is likely and consider potential pre-heat requirements Estimate approximate building total heat loss to inform system selection process 5 Heating system option Consider zoning requirements. Consider alternative heat source (fuel) and heating analysis and selection system options. Establish contribution from renewable sources Consider operating and control strategies, and building usage and layout data. Assess options against client requirements, performance, risk, energy use, etc Select proposed system 6 Design calculations Calculate space heat losses. Assess ventilation requirements and provision. Assess Space heat losses and heat load HWS provision Check system selection choice still appropriate. Determine pre-heat requirements 7 Equipment selection and sizing Consider suitable emitter positions and connections. Check distribution layout considering balancing and regulating requirements. Consider circuit layouts and connections and pumping choices – variable or constant volume. Develop control requirements Size and select emitters and distribution network and determine any distribution losses 8 Design calculations Determine other loads such as HWS and process. Heating load analysis Calculate main heating loads. Analyse load diversity and pre-heat requirement and determine the total heating load

9 Plant sizing and selection Consider any standby requirement. Determine number of boilers /modules required and size and select main plant. Finalise control requirements Check layouts and services co-ordination for clashes and ease of commissioning and maintenance 10 Design calculations Review system design and check predicted system performance. System analysis Check part load performance Control performance Check that the selected controls are capable of achieving the required level of control, response and energy efficiency, particularly at part load 11 Final value engineering and Check that final system and components meet client requirements for energy targets assessment performance, quality, reliability, etc at acceptable cost; and also meet required energy targets and comply with Regulations, such as meeting the seasonal efficiency requirements 12 Review Design review

Figure 4: Heating system designprocess


As the design develops, these design steps are revisited and further detailadded with more accurate analysis as additional information becomes available.The steps and amount of repetition involved will differ from design to designbut an example is illustrated in Figure 5. This uses the same design stepnumbers as Figure 4 to show how the different steps are repeated andrevisited as the design develops. The detailed design tasks at each step havebeen omitted to keep the diagram to a manageable size.

7

Step no.

1

2

3

4

5

7 9

2

3

4

5

6

7

8

9

11

12

4

7

8

9

10

11

12

Design stage

Pre-design

Outline design

Scheme / Detail design

Design development/Final proposals/Production information

Key outputs

Design briefOutline drawingsand schematics.Provisional cost plan

Design drawings and schematics.Cost plan

Design drawings and specification for tender purposes.Possibly co-ordination drawings.Final cost appraisal

Key design steps

Pre design: obtain client brief. Refer to feedback and lessons learned from previous projects

Gather design information and establish key design requirements. Establish planning requirements Establish key design data Initial building thermal performance analysis.Approximate heat loss

Heating system – consider options and fuel choices

Consider system requirements, potential layout, etc Approximate total loads and plant size to arrive at cost plans, provide space requirements and structural load information, etc.

Gather further necessary design information and establish key design requirements

Establish key design data

Detailed building thermal performance analysis

Heating system choice and selection Design calculations: space heat losses

Equipment selection and sizing – emitters and distribution network. Control requirements

Design calculations: heating load analysis, possibly including thermal modelling

Initial plant and control selection

Value engineering workshops

Interim design review

Further building thermal performance analysis, to assist in modelling dynamic building and system performance (if required)

Final equipment selection and sizing Final heating load calculation and analysis Plant selection. Control requirements. Preparation of detailed design drawings and specifications for plant and equipment

Design calculations.System performance analysis, including part load performance and predicted energy use. Possible final dynamic modelling of building and system performance.Control performance Final value engineering exercise

Final design review

Post-occupancy review

Figure 5: Heating design processmapped against the maindesign work stages

2.3 Key heating design calculation sequence

Within the overall heating design sequence there are some specificcalculations that will need to be carried out, and the sequence of these canalso be illustrated as shown in Figure 6. These mainly take place during steps4, 6 and 8 — building performance analysis, heat losses and load analysis;continuing into system and equipment sizing in steps 7 and 9, and systemanalysis in step 10.


Infiltrationheat loss

Fabric heatloss

Site weatherdata

Internal and externaldesign conditions

Space heatloss

Space heatingload Pre-heat

margin

Emittersizing

Infiltration loaddiversity

Distribution systemsizing

Maximum simultaneousspace heating load

Standby capacity(if required)

Fuel supplysystem sizing

Distribution systemlosses

Part loadperformance

Final system andcontrol performance

analysis

Load diversityanalysis

Total heatingload

Boiler/heatingplant sizing

Fluesizing

Natural ventilationload (if any)

Internal gains (onlyif both heating and

gains are continuous)

Building thermalresponse analysis

Intermittentoperation

assessment

Building air-tightness details

Fabricdetails

Condensationrisk analysis

U-values

Central fresh airventilation heating load

HWSload

Processload

Figure 6: Key steps for heatingdesign calculationsequence


2.4 Thermal comfort

For heating design, thermal comfort could be regarded as the main output ofthe design process, as shown in Figure 7. Certainly most clients do not askfor a heating system as part of their design brief — their focus is on whatsystems deliver and not how they do it. What clients really require is thebuilding services design to deliver comfortable working or living conditions toenable their business to function efficiently. An understanding of thermalcomfort is therefore central to good heating system design.

Although there are many factors to take into account, thermal comfort isfundamentally about how people interact with their thermal environment.Generally, a reasonable level of comfort is achieved where there is broadsatisfaction with the thermal environment, i.e. most people are neither toohot nor too cold.

The four main environmental factors that affect thermal comfort are:

— air temperature (ta)— relative humidity — mean radiant temperature (tr)— air velocity (v).

All of these are affected by the choice of heating system and the way itdelivers heat to the space.

Building designers should aim to provide comfortable conditions for thegreatest possible number of occupants and to minimise discomfort. This isachieved by considering comfort requirements and setting appropriate designcriteria.

For the thermal environment, these would usually be the operativetemperature and humidity, together with a fresh air supply rate. A typicalinitial winter design condition might therefore be written as 21 °C and 50%RH for operative temperature and relative humidity respectively, with 10 l/sper person of fresh air required. More often some variation is allowed, i.e. 21 °C ±1 °C and 50% RH ±10%. Example design criteria for a range ofbuilding types are given in section 3.3.

For a further discussion of comfort, see CIBSE Knowledge Series KS06:Comfort, and CIBSE Guide A, chapter 1.

9

Designprocess

Clientneed

Thermalcomfort

Input Output

Figure 7: Design output

Thermal comfort

‘That condition of mind which expressessatisfaction with the thermal environmentand is assessed by subjective evaluation.’

ASHRAE Standard 55-2004

Key factors in thermal comfort

� temperature

� humidity

� air movement

� air quality.

3 Key design steps

This section covers the key steps in the heating design process given insections 2.2 and 2.3 in more detail to give some further guidance. Key designoutputs from each stage are summarised and additional reference sourcesprovided.

3.1 Step 1: pre-design/design brief

Depending on the type of project, the design brief may evolve during thecourse of the initial project stages. However, design briefs do not usually askfor specific heating systems, they tend to concentrate on the outcomes thatmust be achieved, i.e. the internal conditions that must be delivered. Thebrief may simply ask for a heated building, with specific comfortable workingconditions. Design of any system must therefore relate to the functional brief,and be seen in the context of the full design requirements.

During the initial design process the building services engineer can potentiallyprovide input on ways to optimise building performance and reduce energyloads, including advice on:

— building form and orientation to optimise the impact of solar gain— building air tightness, to reduce infiltration— fabric insulation— optimisation of glazing, balancing daylighting needs against thermal

performance— building thermal mass.

Much design data and information can be gained from the client brief andoccasionally additional input will be needed from the client to clarify points orto provide missing data in order to develop the design brief. Some clientbriefs will include the necessary initial design data such as internal designconditions, in some cases this will need to be advised. In both cases it issensible to check any data provided against good design practice.

Input to the design brief can include advice on:

— future need design requirements— comfort requirements— ventilation strategy— spatial requirements— standards and regulations— energy strategy, including the use of renewable energy sources— operating strategy including facilities maintenance requirements— plant life expectancy and replacement strategies


CIBSE Knowledge Series — How to design a heating system 11

— control strategy.

Information required from the design brief can include:

— required functional performance— occupancy— usage details and potential internal loads— internal design conditions— cost plan.

(Further detail of this is given in step 2.)

3.2 Step 2: gather design information

A large amount of information is necessary to inform the various designstages, and as such this task is ongoing throughout the design stages. Much ofthe information is available from the original client brief or statement ofrequirements, and additional information can be sought by additionalquestions. Other data must be gathered from other sources such as sitevisits, etc. Some key initial information is given in Figure 8.

Key design outputs for step 1: pre-design

� functional design brief.

Key design outputs for step 2:information gathering

� key design requirements

� necessary information to establishinternal and external design data

� site assessment and utilityprovision

� statutory and regulatory designrequirements and targets.

Siteinformation

Clientbrief

Standards andregulations

Location: Geographical location andheight above sea levelLocal microclimate, wind Information on local conditions – pollution, noise

Specific information required Outputs

Operating strategy: Client approach to building design and operation includingsustainability, energy strategy, control,maintenance, etc

Orientation: Details of surroundingbuildings, shading, etc

Services: Utilities provision and positions

Functional performance: Specific deliverables

Costs: Cost plans and budgets

Future needs: Future proofing and flexibility requirements

Building use: Tasks, office equipment, etc

Occupancy: Information on occupancyactivity and densityHours of occupation, etc

Access: Access to site

External designconditions

Availableservices

Cost budgetsand constraints

Internal designconditionsAssessment ofintermittent system operationInternal loads –small power,lighting, etc

Possible comfortor energyrequirements

Additional systemrequirements

Possible systemconstraints orrequirements

Statutory and regulatory requirements

Design requirementsEnergy targets,including % energyto be provided from renewable sources

Figure 8: Information gathering

The building services engineer will also need to provide information to otherdesign team members throughout the project. As outlined in section 3.1, atthe initial design stages this can include advice on optimising buildingperformance, and can also include information on potential spatialrequirements, which can be refined as the design develops.

The new Building Regulations Part L (2006) requires that both fabric andservices heat losses are limited and that energy efficient services witheffective controls are provided. Details are provided in the second tierdocuments such as the Non-domestic heating, cooling and ventilationcompliance guide and the Domestic heating compliance guide.

3.3 Step 3: design data

The fundamental initial design data needed for design of a heating system todeliver comfortable conditions are the:

— internal design conditions— external design conditions.

The design conditions selected can have a substantial impact on both systemloads and subsequent system performance and therefore care must be takento select appropriate values. See section 4.1 for further discussion.

Internal design criteria may be specified in the brief, or a required functionalperformance may be asked for and the designer will have to specify therequired conditions. In either case these will need to be checked against goodpractice design standards.

Table 2 gives example winter internal design conditions for thermal comfortfor a range of common building types. More detailed guidance for a widerrange of building and room types is given in CIBSE Guide A, Table 1.5, whichalso relates the design guidance to the expected clothing and metabolic ratesof occupants to achieve a predicted percentage persons dissatisfied (PPD) ofaround 5%. For design purposes reference should be made to the full tabletogether with the associated footnotes.


Building Regulations Part L 2006

Heating systems should be designed tominimise carbon emissions and make iteasier for the whole building to achieve abuilding CO2 emission rate (BER) lowerthan the set target (TER) and thuscomply with Part L requirements, whichimplement the EPBD directive.

Key design outputs for step 3: design data

� internal thermal comfort designconditions

� schedule of internal designcriteria for each space (e.g. onroom data sheets)

� external design conditions.


Selection of appropriate external design criteria requires information on thesite location, development details and local microclimate, as outlined insection 3.2, as well as meteorological data. The type of building and thethermal inertia will also help to determine what may be an acceptable risk ofexceedence of conditions, and this will need to be discussed and agreed withthe client. Further guidance is provided in CIBSE Guide A, chapter 2 and inCIBSE Guide J (2002).

3.4 Step 4: building thermal performance analysis

The thermal performance of the building will need to be established toenable the calculation of heat losses, assess preheat requirements andcalculate the heating loads. Some key information is given in Figure 9.

Building/room type Winter operative temprange °C

Suggested air supply rate

l/s per person

(unless stated otherwise)

Dwellings

bathrooms 20–22 15 l/s

bedrooms 17–19 0.4–1 ACH

halls, stairs 19–24 –

kitchen 17–19 60 l/s

living rooms 22–23 0.4-1 ACH

Offices

conference/board rooms 22–23 10

computer rooms 19–21 10

corridors 19–21 10

drawing office 19–21 10

entrance halls/lobbies 19–21 10

general office space 21–23 10

open plan 21–23 10

toilets 19–21 >5 ACH

Retail

department stores 19–21 10

small shops 19–21 10

supermarkets 19–21 10

shopping malls 12–19 10

Schools

teaching spaces 19–21 10

Notes:

1. ACH stands for air changes per hour.

2. For design purposes, please refer to the full Table 1.5 in CIBSE Guide A.

Table 2: Recommended winter thermalcomfort criteria for someselected building types

External design conditions

Appropriate design criteria should beagreed with the client, taking intoconsideration the acceptable risk ofexceedence of design conditions.

(Source: CIBSE Guide A, Table 1.5)


Calculation procedures and data required to establish the fabric thermalproperties, including the transmittance details, i.e. the fabric and glazing U-values, are given in CIBSE Guide A, chapter 3, together with U-values forstandard constructions. This information, together with the design conditionsfrom step 3 (section 3.3), and site data from step 2 (section 3.2), will alsoenable the analysis of condensation risk, if this is part of the agreed designduties. Key steps in the calculation sequence related to this and the buildingthermal response are shown in Figure 10 in dark blue.

14


Site weatherdata



Condensationrisk analysis

Fabric heatloss

Space heatloss

Fabricdetails

U-values

Building thermalresponse analysis

Figure 10: Key steps to analyse buildingthermal performance

Buildinginformation

Fabricinformation

Building plan and form: Details ofbuilding plan and formBuilding orientation and shadingBuilding layoutGlazing locations, etc

Specific information required Outputs

Internal layout: Layout drawingsPotential space use and fit outPartitioning

Fabric: Detail of building materials and constructionFabric thermal performance

Plant and distribution space:Potential location and space required/available (should be discussed and agreedwith rest of design team as early aspossible in the design)

Glazing: Glazing information – type, dimensions, including glazing height,and thermal performance

Air tightness: Construction qualityBuilding air tightness prediction

Room dimensionsConstraints onemitterpositioningConstraints ondistribution space

Possible systemconstraints orrequirements

Thermal massassessment(heavy orlight weight)Fabric and glazingU-valuesFabric admittanceY-valuesWindow leakageratesInfiltration data

Layout information toinform serviceslocation, zoningstrategy, etc

Figure 9: Building form and fabric

Glazing height

Glazing height influences comfort withinthe occupied space both due todowndraughts and to cold radiationwhich affects the mean radianttemperature.


As the thermal insulation performance of the building fabric has improved,the infiltration component of heat loss can now comprise a substantialpercentage and therefore needs to be estimated as accurately as possible.Although building air leakage testing will be required for most buildings, andwill form part of the design requirements, this sets an expected standard,generally specified for a specific applied pressure difference such as 50 Pa,and therefore does not provide data for infiltration calculations. Methods forestimating infiltration rates are given in CIBSE Guide A, chapter 4, withadditional guidance in CIBSE AM10.

An initial assessment of building use and hours of occupancy will determine ifintermittent, rather than continuous, operation is likely. Details of the overallbuilding thermal response will be needed to determine the likely preheatrequirements and the impact on heating system performance (see alsosection 3.6). More detailed modelling of the building and system dynamicperformance can then be carried out at a later design stage if required.

An initial estimate of total building heat loss can be useful at this stage to helpinform system choices, just to give an approximate global figure. The systemchoices that are reasonable for a 50 kW loss can be very different from thosefor a heat loss of 1500 kW, for example.

3.5 Step 5: heating system option analysis and selection

Heating system choice depends on many factors. These can be looselygrouped into two areas relating to practical system installation and toperformance and use factors.

Installation factors include:

— space required/available: both for plant and for distribution— potential plant room locations related to the spaces to be served— cost plan: capital cost of installation— zoning requirements— flexibility: any requirements for future change of use or changes in

fitout— ease of installation: access, materials, etc— ease of commissioning.

Performance and use factors include:

— cost— comfort— control— convenience.

15

Key design outputs for step 4:building thermal performance

� fabric thermal transmittancedetails, i.e. the fabric and glazingU-values

� building thermal response (anddynamic thermal performancecharacteristics includingadmittance values, if required)

� infiltration assessment forindividual spaces and for thewhole building

� assessment of intermittentoperation to inform preheatrequirements

� estimation of approximate totalbuilding heat loss.

Infiltration estimation

A useful cross check for infiltrationestimation is to convert the estimatedinfiltration total to a room or wholebuilding air change rate, as appropriate.

Zoning

Zoning strategy needs to be agreed withthe client. Some variation in internalconditions may be acceptable, which canhelp to minimise the number of zonesand improve operating efficiency.

To determine the most appropriate system to meet the client’s requirements,an assessment of options against some of these factors can be helpful. Systemchoices can be compared using, for example, a ranking and weighting matrixto assess suitability using some of the key usage factors related to systemchoice. Information on the client’s operating and control strategy will alsoinform the decision process.

Key design decisions will include the choice of:

— heat media and distribution system— system: centralised or de-centralised — heat emitter— heat source.

Tables 5, 6 and 7 provide further information on some system options, givingsome characteristics and relative advantages and disadvantages together with


Cost operating and maintenance costs

energy efficiency

carbon emissions and energy usage.

Comfort balance of radiant and convective heat output to providecomfort conditions

time taken to achieve comfort conditions from start up

evenness of heat distribution throughout space

noise level.

Control ability to provide accurate control of space temperature

ability to provide localised control

speed of response to changing conditions.

Convenience ease of use

location

potential lettable/usable space taken up by emitters/outletsand distribution

ease of maintenance.

Table 3: System performance and usefactors

Table 4: Heating system design choices

Heat media the balance between radiant and convective outputrequired from the system

space required for distribution

speed of response to changing conditions, and on start up.

System centralised or de-centralised – potential plant locations.

Heat emitter characteristics including the balance between radiant andconvective output

location to provide uniform temperatures

noise level

space required.

Heat source conventional boilers or other heat sources such as heatpumps, CHP, etc

boiler and fuel type, any storage requirements

central plant location.

Key design outputs for step 5:heating system selection

� zoning strategy for building — togive details of building zones andrequired operating conditions —hours of use and internal designconditions

� selection of heating system(s) inprinciple — fuel/heat source,system, distribution medium andemitter types.

Low and zero carbon technologies

Part L (2006) of the Building Regulationsencourages the use of low and zerocarbon (LZC) technologies, such asrenewables, CHP and heat pumps, as away of meeting the required carbonemission reductions, and implementingthe requirements of the EPBD directive.Many local planning authorities alsoencourage the use of these technologies,in some cases making it a specificplanning requirement.

some selection flow charts for heating systems and fuels. Although notincluded on the selection charts in Figures 11 and 12, note that, in addition toCHP, other low and zero carbon technologies such as renewables should alsobe considered as heat source options. Further information on heat emittersand heating systems is given in CIBSE Guide B, chapter 1, with guidance onrenewable energy sources covered in CIBSE TM38: Renewable energy sourcesfor buildings (2006).


Constraints on combustion appliances in workplace?

Considering CHP, waste fuel or local communityheating system available as source of heat?

Most areas have similar heating requirementsin terms of times and temperatures?

Significant spot heating(>50% of heated space)?

Above average ventilation rates?N Y

N Y

N Y

N Y

N Y

N Y

N Y

N Y

N YN Y

N Y

N Y

Non-sedentary workforce?

Radiant heat acceptableto process?

Note: This selection chart is intended to give initial guidance only; it is not intended to replace more rigorous option appraisal

Low temperatureradiant system

Medium or high temperatureradiant system

Convectivesystem

Convectivesystem

Centralised system

Start here

Decentralised system

Waste fuel or local community heatingavailable as source of heat?

Strategic need for back-up fuel supply?

Natural gas required?

Radiant heat required?

Natural gas +oil back-up

Communityor waste heat

Communityor waste with

oil or LPGback-up

Communityor wastewith gasback-up

Oil + LPGelectricityback-up

Electricity forhigh temperature

systems, LPGfor medium

temperature systems

Naturalgas

N Y

N Y

N Y

N Y

Oil orLPG

N Y

N Y

N Y

Centralised systemDecentralised system

Figure 11: Selection chart: heating systems

Source: CIBSE Guide B, chapter 1, Figure 1.2,itself based on the Carbon Trust Good PracticeGuide 303(7)

Figure 12: Selection chart: fuel

Source: CIBSE Guide B, chapter 1, Figure 1.3,itself based on the Carbon Trust Good PracticeGuide 303(7)


Medium Principal characteristics Advantages Disadvantages

Air Low specific heat capacity, lowdensity and small temperaturedifference permissible betweensupply and return, compared towater, therefore larger volumeneeded to transfer given heatquantity

No heat emitters needed

No intermediate medium orheat exchanger is needed

Large volume of air required —large ducts require moredistribution space

Fans can require high energyconsumption

Water High specific heat capacity, highdensity and large temperaturedifference permissible betweensupply and return, compared toair, therefore smaller volumeneeded to transfer given heatquantity. Usually classifiedaccording to water temperature/pressure:

Small volume of water required— pipes require littledistribution space

Requires heat emitters totransfer heat to occupied space

— LTHW (LPHW) Low temperature/pressure hotwater systems operate attemperatures of less than 90 °C(approx.), and at low pressuresthat can be generated by anopen or sealed expansion vessel

Generally recognised as simpleto install and safe in operation.

Use with condensing boilers tomaximise energy efficiency

Output is limited by systemtemperatures

— MTHW (MPHW) Medium temperature/pressurehot water systems operate atbetween 90–120 °C (approx.),with a greater drop in watertemperature around the system.This category includespressurisation up to 5 barabsolute

Higher temperatures andtemperature drops give smallerpipework, which may be anadvantage on larger systems

Pressurisation necessitatesadditional plant and controls, andadditional safety requirements

— HTHW (HPHW) High temperature/pressure hotwater systems operate at over120 °C, often with highertemperatures — perhaps up to200 °C, with even greatertemperature drops in thesystem. These temperatures willrequire pressurisation up toaround 10 bar absolute

Higher temperatures andtemperature drops give evensmaller pipework

Safety requires that all pipeworkmust be welded, and to thestandards applicable to steampipework. This is unlikely to be acost-effective choice except forthe transportation of heat overlong distances

Steam Exploits the latent heat ofcondensation to provide veryhigh transfer capacity. Operatesat high pressures. Principallyused in hospitals and buildingswith large kitchens or processesrequiring steam

High maintenance and watertreatment requirements

Table 5: Heat distribution media

18


Table 6: Centralised versus non-centralised systems

Centralised Non-centralised

Capital cost Capital cost per unit output falls with increasedcapacity of central plant.

Capital cost of distribution systems is high

Low overall capital cost, savings made on minimisingthe use of air and water distribution systems

Space requirements Space requirements of central plant and distributionsystems are significant, particularly ductwork

Large, high flues needed

Smaller or balanced flues can often be used

Flueing arrangements can be more difficult in somelocations

System efficiency Central plant tends to be better engineered, operatingat higher system efficiencies (where load factors arehigh) and more durable

As the load factor falls, the total efficiency falls asdistribution losses become more significant

Energy performance in buildings with diverse patternsof use is usually better

System operation Convenient for some institutions to have centralisedplant

Distribution losses can be significant

May require more control systems

Zoning of the systems can be matched more easily tooccupancy patterns

System maintenanceand operational life

Central plant tends to be better engineered, moredurable

Less resilience if no standby plant provided

Can be readily altered and extended

Equipment tends to be less robust with shorteroperational life

Plant failure only affects the area served

Maintenance less specialised

Fuel choice Flexibility in the choice of fuel, boilers can be dual fuel

Better utilisation of CHP, etc

Some systems will naturally require central plant, e.g.heavy oil and coal burning plant

Fuel needs to be supplied throughout the site

Boilers are single fuel

Based on data from CIBSE Guide F (2004), chapter 10.

19

3.6 Step 6: design calculations: space heat losses and heat load

The next step in the design sequence is to take the information on thebuilding fabric and infiltration performance from step 4 and use this toestablish both infiltration and fabric heat losses for each space to give anindividual heat loss for each building space that will require heating.Information on the type of heating system and emitter selected is alsorequired, as both manual calculations and the majority of software packageswill require information on the relative radiant and convective outputs as partof the input data.

CIBSE Guide A, chapter 5 provides details of the required calculationprocedures for heat losses, covering both a steady state heat loss approachand a dynamic approach which can provide more detailed analysis if required,including modelling of building and system thermal response. Section 5.6.2 ofCIBSE Guide A provides a worked example for the steady state heat losscalculation.

Key steps in the calculation sequence for space heat loss are shown in Figure13 in dark blue.


Table 7: Common emitter/system types

Design points Advantages Disadvantages

Radiators Output up to 70% convective

Check for limit on surfacetemperature in some applications,e.g. hospitals

Good temperature control

Balance of radiant and convectiveoutput gives good thermal comfort

Low maintenance

Cheap to install

Fairly slow response to control

Slow thermal response

Natural convectors Quicker response to control

Skirting or floor trench convectorscan be unobtrusive

Can occupy more floor space

Can get higher temperaturestratification in space

Underfloor heating Check required output can beachieved with acceptable floorsurface temperatures

Unobtrusive

Good space temperaturedistribution with little stratification

Heat output limited

Slow response to control

Fan convectors Can also be used to deliverventilation air

Quick thermal response Can be noisy

Higher maintenance

Occupies more floor space

Warm air heaters Can be direct fired units Quick thermal response Can be noisy

Can get considerable temperaturestratification in space

Low temperature radiantpanels

Ceiling panels need relatively lowtemperatures to avoid discomfort

Unobtrusive

Low maintenance

Slow response to control

High temperature radiantheaters

Can be direct gas or oil fired units

Check that irradiance levels areacceptable for comfort

Quicker thermal response

Can be used in spaces with highair change rates and high ceilings

Need to be mounted at high levelto avoid local high intensityradiation and discomfort

Heat losses

A useful cross check for heat losses is toconvert the calculated values to W/m2 orW/m3 figures to check against reasonablebenchmarks.


With better fabric insulation the infiltration heat loss can now account for upto 50% of the total heat loss in some smaller buildings and thereforeinfiltration rates need to be estimated as accurately as possible — see section3.4.

To move from the heat loss to the heat load for a space, additional factorsneed to be considered, including any additional loads within the space andany preheat requirements, as shown in Figure 14.

An assessment of ventilation provision is required at this stage, as althoughthis is likely to be met by a separate system in most buildings, it will in somecases be met by natural ventilation, in which case it will add an additional heatload directly to the space. Further information on naturally ventilatedbuildings is given in CIBSE AM10 and on mixed mode buildings in CIBSEAM13.

A preliminary assessment of other loads that may also need to be met by themain heating source, such as any HWS load, can also be made at this stage toprovide information for the next calculation step (see also section 3.8).

21

Heat losses — temperatures

Care needs to be taken when consideringthe temperatures to use for heat losscalculations. Design criteria are usuallygiven as operative temperatures (to).Fabric heat losses should use the internalenvironmental temperature (tei) andinfiltration loss the internal airtemperature (tai). These can differsubstantially for some buildings and someheating types. CIBSE provides a methodfor steady state heat losses that appliescorrection factors F1 and F2 to enable thedesign internal operative temperature tobe used — see CIBSE Guide A, section5.6.2. (Note: for very well insulatedbuildings, without large areas of glass, andwith low air change rates, there is oftenlittle difference between operative,environmental and air temperatures.)


Site weatherdata



U-valuesCondensationrisk analysis

Fabric heatloss

Space heatloss

Fabricdetails

Figure 13: Key steps to establish individualspace heat losses


Fabricheat loss

Natural ventilationload (if any)

Internal gains (onlyif both heating and

gains are continuous)

Spaceheat loss

Spaceheating load

Pre-heatmargin

Intermittentoperation

assessment

Building thermalresponseanalysis

Figure 14: Key steps to establish spaceheating loads

Radiant systems

For high temperature radiant systems thestandard heat loss calculation methodsare not appropriate for equipmentselection. Instead the distribution ofradiant energy in the space should bedetermined, utilising a radiant polardiagram for the emitter. Further guidanceis given in CIBSE Guide A, section5.10.3.7 and CIBSE Guide B, section1.4.6. Medium and low temperatureradiant systems can be sized using theusual heat loss calculation methods.

Internal gains

Normally no allowance would be made for internal gains in establishing spaceheating loads as a worst-case scenario is always considered, i.e. to bring theunoccupied building up to temperature. However, exceptionally, if the heatingwill be operating continuously and there are constant heat sources such aselectric lights and occupants in a continuously occupied building, then thesteady state heat requirement can be reduced by the amount of the constantgains. However the risks of this should always be made overt to the client asif any gains are removed or reduced or the building is operated intermittentlythen the system may not be able to achieve the design temperatures.

Preheat requirements

The building thermal capacity will affect the way the building responds toheat input, meaning the rate at which it warms up and cools down. For anybuilding that is heated intermittently this will need to be considered as thebuilding will cool down during the unoccupied periods and then need to bebrought back to temperature. For heavyweight buildings with a high thermalcapacity, and/or those intermittently occupied, some additional heatingcapacity will be required to ensure that the building can warm up and achievethe design temperature before the start of the occupied period: the preheattime (see Figure 15). This additional capacity is required by the space heatingsystem, i.e. the emitters, as well as by the main heating plant.

In order to assess the preheat requirements, information on bothintermittent operation and on the building thermal response is needed. Fornormal intermittent operation the plant and equipment will need to be largerthan the steady state requirements, with the required capacity calculated byapplying an ‘intermittency factor’, F3, based on the thermal response factorfor the building and the total hours of plant operation:


HWS

HWS requirements and options shouldbe assessed, e.g. consider whetherstorage or instantaneous water heating ismore appropriate. For hot water storageconsider the options of a dedicated boileror a standalone hot water generator(direct-fired storage system). Forinstantaneous hot water consider thechoice and availability of fuel and whetherpoint-of-use provision or multi-outlet ismore suitable.

Insi

de t

empe

ratu

re

Plant off

Preheattime

Optimisedstart time

Start ofoccupancy

Design insidetemperature

Time

Intermittent operation

Intermittent heating occurs when theheating plant is switched off at or nearthe end of a period of occupancy andthen turned back on at full capacity priorto the next period of occupancy in orderto bring the building back to the designtemperature. There are two main typesof intermittent operation:

� normal intermittent operation iswhere the output is reducedwhen the building is unoccupied— for example to a level of 10 °C to protect the buildingfabric and contents

� highly intermittent is where thebuilding is occupied for shortperiods only and therefore needsto be brought back totemperature quickly prior to use.

Figure 15: Preheat


Peak heating load = F3 x space heat load

Details are given in CIBSE Guide A, section 5.10.3.3 and Appendix 5.A8, andin CIBSE Guide B, section 1.4.7.3.

If the calculated value of F3 is less than 1.2, CIBSE suggests that the value betaken as 1.2 to ensure that a reasonable margin of 20% for preheat isapplied, although other values may be used, for example by using a dynamicsimulation model to more accurately assess the required excess capacity. Fullanalysis of building thermal response can require dynamic rather than steadystate modelling and this is discussed further in CIBSE Guide A, chapter 5.

CIBSE suggests in Guide B, chapter 1 that acceptable values for F3 lie in the range1.2–2.0, with research(8) indicating that values over 2.0 cannot be economicallyjustified for most buildings and could result in considerably oversized plant. Thesame research found that a value of 1.5 was a more typical economical value forthe cases investigated. For small buildings and small plants the optimum valueswill be even lower. The use of optimum start control, as illustrated in Figure 15,can help to ensure adequate preheat time in cold weather.

For highly intermittent systems, a steady state heat loss is inappropriate tosize the system and a dynamic simulation model that considers the way heatis absorbed by the building fabric is required. Details are given in CIBSEGuide A, section 5.10.3.3.

3.7 Step 7: equipment sizing and selection

Once the individual room losses and space heating loads have beendetermined and decisions have been made on the system, emitters, etc, thenthe system can be sized and emitters selected. Key steps for this are shownin Figure 16 below. It is possible that alternative solutions are still beinginvestigated at this stage, in which case further comparison in terms of cost,performance and energy efficiency may be required to reach a final decision.

Plant size ratio

The intermittency factor F3 can also beexpressed as a plant size ratio (PSR)defined as:

PSR = installed heat emission

design peak steady state heat load

Key design outputs for step 6: spaceheat losses and heat load

� schedule of individual space andzone heat losses, subdivided intofabric and infiltration losses,together with details of theinternal design conditions

� assessment of preheatrequirements for the building

� schedule of space heating loads.

Emittersizing

Space heatingload

Infiltrationload diversity

Preheatmargin

Distributionsystem sizing

Maximumsimultaneous

space heating load

Distributionsystem losses

Figure 16: Key steps for emitter anddistribution system sizing

Heat transfer correction factors

The type of heat emitter can have asignificant effect on the calculated designsteady state heating load, so it is essentialthat appropriate values for the heattransfer correction factors F1 and F2 wereused at step 6.

The heat output from the emitter, and therefore the size required, will beaffected by its position within the space and local effects such as furniturepositions, etc. For example if emitters are situated behind furnishings thenmost of the immediate radiant heat output will be lost, and in some caseseven the convective heat output can be obstructed and reduced. Althoughmuch of the heat will eventually enter the space it may not be availableduring preheat and therefore an allowance may be need to be made and therequired heat output increased to compensate. Details are given in CIBSEGuide A, section 5.10.3.2.

Some heating systems, such as warm air, can lead to considerabletemperature stratification in the space — see Figure 17. This means that theinside temperature at high level is much higher than that used in heat losscalculations and therefore the heat loss through the ceiling/roof will begreater than anticipated. A correction to the heat loss, to allow for the heightof space and system used, will need to be applied — for example a 5–15%increase in the fabric component of heat loss for a low level forced warm airsystem used in a space 5–10 m high. Further guidance is given in CIBSEGuide A, section 5.10.3.2 and in Table 5.15.

These corrections can now mean that, for certain heating systems, therequired emitter load is larger than the original space heating load. Once theemitters have been sized then the distribution layout can be determined andthe system sized. Guidance on pipe and duct sizing is given in CIBSE Guide C(2001), chapter 4. When determining the most appropriate layout for thedistribution system, balancing and regulating requirements should beconsidered, e.g. the use of reverse return pipework layouts to aid systembalancing during commissioning.

The system distribution losses will need to be assessed. Those from withinthe space can contribute to the required space heating load. However anynon-useful distribution losses will need to be allowed for within the overall


15 20 25 15 20 25 15 20

Radiator Underfloor heating Warm air heater at high level

25

3·0

2·0

1·0

0

Room

hei

ght

/ m

Air temperatures / °C

Heat emitters

Check that the manufacturer’s publisheddata is applicable to the conditions atwhich the emitter will be operating andapply any relevant corrections for spacetemperature, water temperatures, etc.Note that manufacturers’ outputs arebased on particular space and watertemperatures which may differ from thedesign operating conditions.

Key design outputs for step 7:emitter and distribution system

sizing

� schedule of emitters withrequired output, and with surfaceand water temperature forhydronic systems

� initial control requirements

� layout drawings with emitterpositions

� schematic of pipework layoutswith required flowrates forhydronic systems.

Figure 17: Vertical air temperaturegradients for different heatingtypesSource: CIBSE Guide A, Figure 5.6.


heating load for the building. Whilst for energy efficiency distribution lossesshould be minimised, for example by insulating pipes that run through non-occupied areas, an allowance will still need to be made. Guidance is given inCIBSE Guide C, chapter 3.

3.8 Step 8: design calculations — heating load analysis

Once individual space heating loads have been determined, and the emittersand distribution system sized, an overall heating load can be determined. Thiswill require establishing all the various heat loads that may need to be met,such as:

— space heating loads— any system distribution losses— HWS load— central fresh air ventilation heating load (if ventilation air is provided

centrally by mechanical ventilation systems)— any potential process load.

The first step is to establish the maximum simultaneous space heating load —see Figure 18. Having already considered the preheat requirements for thespace(s), and sized the emitters, an allowance needs to be made for any non-useful distribution losses, as discussed in step 7.

Infiltration load diversity

For individual spaces the maximum heat loss is always required to size anyemitters for that space. However when considering the total space heatingload for sizing central plant, some diversity can be applied to infiltration, toallow for the fact that infiltration of outdoor air will only take place on thewindward side of the building at any one time, with the flow on the leewardside being outwards. This suggests that the total net infiltration load is usuallyabout half of the summation total for the individual spaces, although theinfiltration patterns for individual building configurations should always beconsidered carefully. This exercise is important as, given current high levels of

25

Emittersizing

Space heatingload

Infiltrationload diversity

Preheatmargin

Distributionsystem sizing

Maximumsimultaneous

space heating load

Distributionsystem losses

Figure 18: Key steps to establish themaximum simultaneous spaceheating load

fabric insulation, the infiltration component of heat loss is now substantial,often accounting for up to 50% of the total in small buildings. CIBSE Guide A,chapter 4 provides further guidance on infiltration.

The next step is to consider the other loads that may need to be met by theheating plant and carry out an assessment and analysis of load diversity — seeFigure 19.

Load diversity analysis

An analysis of load diversity is needed as the maximum demands for eachseparate part of the overall load are unlikely to coincide. In addition tothe infiltration diversity within the total space heating load, there can bezone diversities, perhaps due to differing hours of occupancy. Processloads could be intermittent and the HWS load could perhaps peak at themiddle or towards the end of the occupied period, rather than thebeginning.

The individual and zone space heating loads should be reviewed to checkwhen the peak demand occurs. While it is most likely that the worst casescenario will be for all spaces to require heating at the same time it ispossible in certain buildings that there could be spaces or zones which onlyhave very occasional use and do not coincide with the main demand timesfrom other areas.

For intermittent heating, the period of maximum demand for the heatingsystems will be during the preheat period. In practice the preheat periodsfor all spaces and zones will generally be co-incident and therefore themaximum space heating load will be the sum of these, after consideringinfiltration diversity as discussed above.

For continuous heating some diversity can be expected between the variouszone heating loads. This is discussed in CIBSE Guide A, section 5.10.3.5, with



Central fresh airventilation

heating load

Processload

HWSload

Preheatmargin


Maximumsimultaneous

space heating load

Total heatingload

Figure 19: Key steps to establish the totalheating load

Key design outputs for step 8:heating load analysis

� assessment and analysis of loaddiversity

� total heating load to enable boileror other heating plant to besized.


Table 5.18 suggesting that central plant diversity factors ranging from 0.7–1.0may be appropriate depending on building type and system control.

3.9 Step 9: plant sizing and selection

Once the overall heating load has been determined, then the heating plantcan be sized and selected, see Figure 20, together with other plant itemssuch as the flue and fuel supply system if required.

Standby capacity

Occasionally standby capacity may be required so that, in the event ofpartial system failure or plant maintenance, the main loads can still be metand the building continues to function. The decision on this can require riskassessment. However this can add still more additional capacity to thesystem increasing the overall risk of oversizing and poor performance,therefore this should be considered together with the load diversity analysisas there may already be sufficient capacity within the system. Wherefurther capacity is required careful consideration is needed of the loadbreakdown to ensure that the various expected load combinations can bemet efficiently, for example considering the optimum module size formodular boiler installations. If the heating plant consists of modular boilersthen adding one extra module may be sufficient to both meet therequirement and still ensure system operating efficiency.

Control requirements should be finalised, considering the required systemoperation. With the main system design layouts completed, the final layoutsand services co-ordination should be checked again for any clashes and forease of commissioning and maintenance.

3.10 Step 10: system analysis and control performance

With the system selected and plant and equipment sized and plant selected, itis now possible to more accurately predict system performance and checkenergy performance targets are still met.

27

Figure 20: Key steps for boiler/heating plantsizing and selection


Total heatingload


Standby capacity(if required)

Fuel supplysystem sizing

Final system and control performance

analysis

Fluesizing

Key design outputs for step 9: plant sizing and selection

� schedule of plant, giving requiredoutput, flowrates, etc

� control requirements

� schematic of plant layout,connections, etc.

Control system

Both the heating system and its controlsystem should be appropriate for therequirements of the building and theoperation it supports. Ideally theapproach should always be to use thesimplest control system that meetsbuilding owner, operator and user needsand capabilities, and efficiently deliversthe required quality of system operation.

Predicted system performance, including part load performance, should beinvestigated to check that the selected systems can operate efficiently underall predicted load conditions, see Figure 21. This is particularly important ifadditional capacity has been added, for example for preheat or standby, asthis effectively adds a margin. It is important to check that this does notunduly oversize the system, leading to poor performance at normal operatingconditions. It is also essential to check whether other margins have beenadded at any stage in the design process, including those that will occur byselecting standard plant sizes.

System control performance

In order to achieve an energy efficient building that delivers the required levelof functionality and occupant comfort it is essential to form a clear andintegrated control strategy at a very early design stage. In all cases the controlstrategy should be set out first so that the control options can be evaluatedagainst the required level of functionality. As such, the controls should beconsidered at an early stage as an integral part of the system design.

At this design stage the task is to carry out a final evaluation of the controls,now that the final system design is complete and part-load performanceevaluated, to ensure that they can deliver the required level of control,response and energy efficiency.

Controls are discussed further in CIBSE KS04: Understanding controls, whichalso explains terms such as weather compensation, optimum start controls,etc; with further information on heating system controls given in CIBSEGuide B, chapter 1, CIBSE Guide F, CIBSE Guide H (2000), and in other textssuch as Heating systems — plant and control (2003).


Normal system operation

The initial system design is often basedon design conditions that occur for lessthan 1% of the occupied time. For themajority of the heating season occupiedperiod the system will be operating on afraction of the installed load andtherefore it is essential to ensure that thesystem can operate efficiently at theselow load conditions.


Total heatingload


Fluesizing

Maximumsimultaneous

space heating load

Final system andcontrol performance

analysis


Figure 21: System analysis

Key design outputs for step 10:system analysis

� analysis of system part-loadperformance

� system control strategy statementand flowcharts

� schematics of plant and systems

� required control systemfunctionality

� control system specification.


3.11 Step 11: final value engineering and energy targets assessment

Final value engineering assessment

Value engineering should be carried out at several stages within the project toensure that the design is on track to meet the client requirements forperformance, quality, reliability, etc at least cost. For example, valueengineering workshops can be held during both the scheme and detail designstages to ensure that the design decisions made are the ones that achievebest value.

Energy targets

The final system performance will need to be checked again to ensure itcomplies with regulations and meets required energy targets, for examplemeeting the seasonal efficiency requirements and achieving a buildingemission rate (BER) less than the target emission rate (TER).

3.12 Step 12: design review

There are a number of different interim reviews that can be done throughoutthe design stages of a project, from the feasibility and innovation review tostraightforward progress reviews, culminating in a post-project review afterproject completion which can provide valuable feedback lessons to informfuture work.

During the design stages there should be review meetings of the design teamat regular intervals to review design progress, agree changes, checkcompliance with the brief, etc. The intent of these is to monitor the progressof the design against the programme and cost targets, anticipate potentialproblems, and ensure that required information will be available whenneeded. Review meetings can involve one or several design disciplines.

Some design practices hold a formal peer group in-house design review nearthe end of the design stages, presenting to other design teams, perhaps fromother regional offices. This can be a useful part of the project quality checks,and provide additional valuable cross-checks on the proposed designsolutions, as well as sharing experience and expertise within the organisation.

Post-project review is usually held by the in-house design team at the end ofthe project, after completion and handover, to review the inputs andoutcomes and provide the opportunity to summarise key points learnt. Thiscan provide the opportunity to review both the technical content of thedesign and the management of the design process to provide feedback toinform future work, including the provision of design benchmark data for

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Value engineering

A systematic approach to achieving therequired project functions at least costwithout detriment to quality,performance and reliability.

Key design outputs for step 11: value engineering and energy targets

� value engineering review

� energy target and emission valuecalculations.

Safety in design

Reviews should include consideration ofsafety in design to ensure that theprovision of the design can beconstructed, operated, maintained andde-commissioned safely, to comply withthe Construction (Design andMangement) Regulations (CDM)requirements. Helpful guidance ondesigners’ responsibilities under CDM isgiven on the HSE website:www.hse.gov.uk/construction/designers/index.htm.

future projects. A post-project review meeting can also be held with theentire project team.

Sometimes there is the opportunity to obtain further feedback after handoverand occupation, e.g. via post-occupancy surveys. The client may also requireadditional duties to include monitoring system operation. For example, theenergy performance of the system can be monitored using the CIBSElogbook approach, and the actual operation of the system and comfortperformance monitored for compliance with the intended design operation.This can provide valuable feedback to inform briefing and design guidance forfuture projects. Further guidance on feedback can be found in BSRIA AG21/98: Feedback for better building services design.


Key design outputs for step 12:design review

� quality checks on the designtechnical content

� feedback lessons and designbenchmark data to inform futurework.


4 Developing the design — key issues

This section covers some key areas relevant to the overall design of heatingsystems.

4.1 Design conditions

The choice of both internal and external design conditions can have asubstantial impact on initial system loads and subsequent systemperformance. These are a fundamental part of heating load calculations andthe choice should be very carefully considered. For example the differencebetween using a temperature difference of 21 K (–1 °C to 20 °C) and one of25 K (–4 °C to 21 °C) for a particular building is nearly a 20% increase in theheat loss. By the time allowance has been made for reduction in emitteroutput and preheat requirements the difference could be as much as 40%.When considering energy efficiency the fundamentals need to be consideredfirst.

It is also important to consider what system performance criteria areacceptable and agree this with the client. Establishing the required systemperformance criteria at the briefing stage is one of the most critical tasks inthe design and it is vital that clients and their designers have a thoroughunderstanding of what conditions are required and what can practically beachieved. For example the difference between specifying an internalcondition of 21 °C ± 1 °C or a condition of 21 °C ± 2 °C can have aconsiderable impact on energy consumption, control choice and systemperformance. The closer the control the more expensive the system. Ifconditions can be relaxed a little and allowed to vary (within reasonablelimits) the system can be simpler and cheaper to install and to operate.

Further guidance can be found in CIBSE KS06: Comfort on practical issues ontemperature and design criteria, etc, with guidance on design conditions inCIBSE Guide A, chapters 1 and 2, and on the margins that can occur atdifferent design stages in CIBSE RR04: Engineering design calculations and theuse of margins (1998).

4.2 Design margins

Margins should never be added during a calculation process without anadequate reason for doing so and only with the approval of a senior engineer.Excessive margins can result in system oversizing and poor operationalperformance and control. If any margins are used they should be clearlyidentified and a justification given for their use, which should be recorded inthe design file. It is also important to check for any inbuilt assumptions andmargins in software calculation packages. The use of margins should be

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reviewed at several stages during the design process to check theirappropriateness and avoid any duplication or excess, e.g. at the end of acalculation procedure, at design review stage, etc. Figure 22 illustrates theconsequences of oversizing for heating system performance.

(For more information on the use of margins in engineering design refer toDesign Checks for HVAC — a quality control framework for building servicesengineers, topic sheet number 1 Design margins and CIBSE Research ReportRR04: Engineering design calculations and the use of margins.)

4.3 Energy efficiency

Energy efficiency should be considered throughout the design process. Ingeneral, energy efficient heating should:

— incorporate the most efficient primary plant to generate heat/hotwater

— optimise the use of renewable energy sources— ensure that heat/hot water is distributed effectively and efficiently— include effective controls on primary plant and distribution systems to

ensure that heat is only provided when and where it is needed and atthe correct temperature

— be responsive to changes in climate, solar gains, occupancy, activity andinternal gains.


Variable temperature

If terminal units are oversized, space temperatures drift higher than required and energy is wasted. If coils are oversized, too much water is pumped through the system and performance and control is compromised if laminar flow results when flow rate is reduced

Oversized pumps consume excess energy as too much cold water is pumped and/or they are inefficient because they are not operating at their most efficient operating point. They can often cause balancing problems during morning start-up and constant temperature pumps may turn off and on at maximum demand

Return water temperatures are lower than expected if oversized constant temperature (constant flow) radiators are installed. Boilers can corrode if they are not protected

Variable flow constanttemperature

Constant flowconstant temperature

Boiler plant

Oversized valves reduce effective control and fail prematurely. They can often cause balancing problems during morning start up

Boilers that are oversized will cycle at maximum demand. Under medium and low loads burner fraction on-time is small (especially if cycling rates are high) and reduction in plant dynamic efficiency occurs. Operating costs increase because of the reduced plant load operating efficiency. Oversized plant permanently operating at low loads can reduce plant life. Accelerated wear can also arise from unstable control caused by plant oversizing. For example: many oversized steam traps fail prematurely because they operate too close to their closed position

Variable temperature circuit and variable flow circuit return water temperatures are higher than expected

Figure 22: The impact of oversizing onheating system performance

(Source: BSRIA AG 1/2000 Enhancing theperformance of oversized plant by Barry Crozier,BSRIA 2000)


Designers should:

— select fuels and tariffs that promote efficiency and minimise runningcosts

— segregate hot water services generation wherever possible— consider de-centralised heating and hot water services generation plant

on large sites to reduce standing losses and improve load matching — locate plant to minimise distribution system and losses— ensure distribution systems are sized correctly to minimise pump and

fan energy consumption— insulate pipework, valves, etc effectively — ensure the base load is provided by the most efficient plant— utilise condensing boilers where feasible and appropriate— consider energy recovery where feasible, e.g. from air exhaust streams.

Further guidance is given in the CIBSE Knowledge Series on Energy efficientheating and CIBSE Guide F, chapter 10.

4.4 Quality control

The design information, including the design calculations, is part of the designprocess and therefore will form part of the project design file and recordsand be subject to standard in-company quality assurance (QA) and qualitycontrol (QC) procedures. As such all information and data should be properlyrecorded and checked. Good practice includes:

— clearly identify and record all data sources to enable input informationto be adequately verified

— clearly state all assumptions, and identify, and flag, where moreaccurate data will be required (e.g. from client, manufacturer, etc) asthe design progresses

— review any assumptions as the design progresses to check they are stillvalid, and replace with more accurate information as received

— clearly identify, record and review the required design inputs anddesign outputs

— record calculations clearly, with sufficient detail to ensure the work canbe followed by others (be aware that if a problem arises on a projectthis could mean revisiting calculations several years after they wereoriginally done)

— identify and record calculation checks and cross-checks clearly — verify the design to ensure it can meet the design requirements— review the overall design.

Further guidance on design quality control is given in BSRIA AG 1/2002:Design checks for HVAC.

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References

1 BSRIA domestic boiler marketing report (Bracknell: BSRIA Ltd) (March 2006) and BSRIA commercial boiler marketing report (Bracknell: BSRIA Ltd) (March 2006)

2 DTI UK Energy Consumption in the United Kingdom (www.dti.gov.uk/energy/statistics/publications/energy-consumption/page17658.html)

3 Crozier B Enhancing the performance of oversized plant BSRIA AG 1/2000 (Bracknell: BSRIA Ltd) (2000) and Brittain J Oversized heating plant BSRIA GN 12/97 (Bracknell: BSRIA Ltd) (1997)

4 Cross N Design: principles and practice — product planning and the design brief (Open University) (1995)

5 RIBA Plan of Work (London: Royal Institute of British Architects) (1999)

6 ACE Agreement A(2) 2002 and B(2) 2002 (revised 2004) Mechanical and Electrical Engineering Services (London: Association for Consultancy and Engineering) (2002/2004)

7 The designer’s guide to energy-efficient buildings for industry GPG 303 (Carbon Trust) (2000)

8 Day A, Ratcliffe M and Shephed K Sizing central boiler plant using an economic optimisation model (Proc CIBSE National Conference) (2001)

Selected bibliography

Overall heating system design process

Heating, ventilation, air conditioning and refrigeration CIBSE Guide B , chapter 1(London: Chartered

Institution of Building Services Engineers) (2001-2)

Lawrence Race, G, Design checks for HVAC BSRIA AG 1/02 (Bracknell: BSRIA Ltd) (2002)

Lawrence Race, G, Mitchell, S, A practical guide to HVAC building services calculations BSRIA/CIBSE

BG 30/03 (2003)

Building Regulations compliance guides

Domestic heating compliance guide (London: TSO) (2006)

Non-domestic heating, cooling and ventilation compliance guide (London: TSO) (2006)

Comfort

Comfort CIBSE KS06 (London: Chartered Institution of Building Services Engineers) (2006)

Environmental Design CIBSE Guide A (London: Chartered Institution of Building Services Engineers)

(2006), chapter 1

CDM guidance for designers

www.hse.gov.uk/construction/designers/index.htm

Design data

Environmental Design CIBSE Guide A , chapters 1 and 2 (London: Chartered Institution of Building

Services Engineers) (2006)

Reference data CIBSE Guide C (London: Chartered Institution of Building Services Engineers) (2001)


Weather, solar and illuminance data CIBSE Guide J (London: Chartered Institution of Building Services

Engineers) (2002)

Design management

Parsloe, C, Wild, L, Project Management Handbook BSRIA AG 11/98 (Bracknell: BSRIA Ltd) (1998)

Parsloe, C, Allocation of design responsibilities for building engineering services BSRIA TN21/97

(Bracknell: BSRIA Ltd) (1997) (New edition due in 2006)

Design margins

Engineering design calculations and the use of margins CIBSE Research Report RR04 (London:

Chartered Institution of Building Services Engineers) (1998)

Design quality control

Lawrence Race, G, Design checks for HVAC BSRIA AG 1/02 (Bracknell: BSRIA Ltd) (2002)

Design review and feedback

Lawrence Race, G, Pearson, C, de Saulles, T, Feedback for better building services design BSRIA

AG 21/98 (Bracknell: BSRIA Ltd) (1998)

Domestic heating

CIBSE Domestic building services panel, Domestic heating design guide (London: Chartered Institution

of Building Services Engineers) (2003)

Energy efficiency

Energy efficiency in buildings CIBSE Guide F (London: Chartered Institution of Building Services

Engineers) (2004)

Energy efficient heating CIBSE KS (London: Chartered Institution of Building Services Engineers) (to

be published)

Fabric thermal performance



Heating design calculations with worked examples

Lawrence Race, G, Mitchell, S, A practical guide to HVAC building services calculations BSRIA/CIBSE,

BG 30/03 (2003)



Heating, ventilation, air conditioning and refrigeration CIBSE Guide B , chapter 1 (London: Chartered


Sands, J, Parsloe, C, Churcher, D, Model demonstration projectBSRIA BG 1/2006 (Bracknell: BSRIA

Ltd) (2006)

Heating plant and controls

Heating, ventilation, air conditioning and refrigeration CIBSE Guide B , chapter 1 (London: Chartered



Building Control Systems CIBSE Guide H (London: Chartered Institution of Building Services

Engineers) (2000)

Understanding controls CIBSE KS04 (London: Chartered Institution of Building Services Engineers)

(2005)

Renewable energy issues for buildings CIBSE TM 38 (London: Chartered Institution of Building Services

Engineers) (2006)

Day, A, Ratcliffe, M, Shepherd , K, Heating systems — plant and control (Oxford: Butterworth-

Heinemann) (2003)

Heating systems

Heating, ventilation, air conditioning and refrigeration CIBSE Guide B (London: Chartered Institution of

Building Services Engineers) (2001-2), chapter 1

CIBSE Domestic building services panel, Underfloor heating design guide (London: Chartered

Institution of Building Services Engineers) (2004)

Sands, J, Underfloor heating — the designers guide BSRIA AG 12/01 (Bracknell: BSRIA Ltd) (2001)

Brown, R, Radiant Heating BSRIA AG3/96 (Bracknell: BSRIA Ltd) (1996)

Infiltration estimation

Environmental design CIBSE Guide A , chapter 4 (London: Chartered Institution of Building Services

Engineers) (2006)

Natural ventilation in non-domestic buildings CIBSE AM10 (London: Chartered Institution of Building


Natural ventilation

Natural ventilation in non-domestic buildings CIBSE AM10 (London: Chartered Institution of Building


Mixed mode ventilation CIBSE AM13 (London: Chartered Institution of Building Services Engineers)

(2000)

Renewable energy

Renewable energy sources for buildings CIBSE TM38 (London: Chartered Institution of Building


Value engineering

Hayden, G, Parsloe, C, Value engineering of building services BSRIA Application Guide 15/96

(Bracknell: BSRIA Ltd) (1996)

CIBSE Knowledge Series — Variable flow pipework systems36

General textbooks covering heatingsystems and design aspects

Oughton, D, Hodkinson, S, Faber andKell’s Heating and air-conditioning ofbuildings, 9th ed (Oxford: Elsevier) (2002)

Moss K, Heating and water services designin buildings (London: Taylor & Francis)(2003)

Day, A, Ratcliffe, M, Shepherd K, Heatingsystems — plant and control (Oxford:Butterworth-Heinemann) (2003)

Kavanaugh, S, HVAC simplified (AtlantaGA: American Society of Heating,Refrigerating and Air-ConditioningEngineers) (2006)

CIBSE How to Design a Heating System

Documents

design review294

design margins

design information

design data123

design brief

heating design process42

design key issues

key design steps103