Energy Efficiency E-module - Guidance 1... · Energy Efficiency E-module - Guidance Efficient Operation of Low Temperature Hot Water Boilers in ... This increases the efficiency of

Energy Efficiency E-module - Guidance

Efficient Operation of Low Temperature Hot Water Boilers in

the Public Sector

Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 2

Contents

1 Introduction 3

2 Learning Objectives and Outcomes 3

2.1 Learning Objectives 3

2.2 Learning Outcomes 3

3 Overview and Principles of LTHW Heating Systems 4

3.1 Conventional Boilers 4

3.2 Condensing Boilers 5

3.3 Wall Hung Boilers 6

3.4 Modular Boilers 7

3.5 Boiler Efficiencies 7

3.6 System Components 8

3.7 How to Survey a Boiler House 10

4 Energy Saving Opportunities 12

4.1 Boiler Replacement 12

4.2 Boiler Compliance - Ventilation for Gas Systems 13

4.3 Boiler Compliance - Gas Safety Regulations 14

4.4 Improving an Existing Boiler House 14

4.4.1 Insulation Improvements 14

4.4.2 Hydraulic Layout 14

4.4.3 Pump Upgrades 14

4.4.4 Combustion Efficiency Checks 15

4.4.5 Heating System Maintenance - Poor Practice 15

5 Building the Business Case 17

5.1 Before Considering a Boiler Upgrade Project 17

5.2 Information Gathering 17

5.3 Business Case - Case Study 17

6 Useful Links and References 19


1 Introduction

This guidance follows the format of the associated e-module, “Efficient Operation of Low

Temperature Hot Water (LTHW) Boilers in the Public Sector”. It provides further details on

the subjects covered in the module.

Please note that module users working in a healthcare environment should always refer to

the relevant Scottish Health Technical Memorandum (SHTM) prior to considering installation

of the measures suggested in the module. The advice given in the SHTM may conflict with

the advice given in this module, as it has been developed for the wider public sector. The

relevant SHTM can be found on the Health Facilities Scotland website.

2 Learning Objectives and Outcomes

2.1 Learning Objectives

The learning objectives from this module are to:

Understand the different LTHW boiler types and their applications;

Understand the main principles of how LTHW boilers and distribution systems operate;

and

Understand the different measures which can be implemented to improve boiler house

efficiency.

2.2 Learning Outcomes

The learning outcomes from this module are for the reader to be able to:

Understand the basic principles regarding how different boiler types work;

Understand where the opportunities for improving LTHW heating systems exist in

Scottish public sector sites and buildings;

Describe the main boiler technologies including their typical efficiency range, and the

advantages and disadvantages of each technology;

Carry out an audit of a boiler house and identify opportunities for making

improvements;

Identify when a boiler should be replaced and what technology could be applied;

Prioritise the opportunities for improving LTHW systems in public sector buildings; and

Understand the key aspects in relation to LTHW boiler projects when building a business

case.


3 Overview and Principles of LTHW Heating Systems

Table 3.1 provides a brief overview of some of the different LTHW boilers that are currently in operation.

Table 3.1 – An Overview of LTHW Boilers

Boiler Type Description Typical Seasonal

Efficiency

Conventional

Floor Standing, atmospheric or

pressure jet burners,

large in size. Boilers of this type are likely to be 15 years or older

45 -70% (depending

on condition)

Condensing (Floor Standing)

Smaller than conventional boilers. Have extra heat exchanger surfaces. Installed since 2005

85 - 90% (depending on heat

emitters)

Condensing (Wall Hung)

Smaller than floor standing units. Have

extra heat exchanger surfaces. Installed since 2005. Typically

used for domestic applications

85 - 90% (depending on heat

emitters)

Modular (condensing/

condensing and conventional)

Several boilers linked to give more flexible and

efficient output to service larger heat loads

Depends on boiler type

3.1 Conventional Boilers

This section introduces the basic principles of LTHW heating systems and the different boiler types available.

A LTHW boiler works by burning a fuel and then using the heat energy from this combustion

to heat water which is pumped around the heating distribution system, typically to

radiators, air handling units or fan convectors. A basic cross section of a conventional gas

boiler layout is shown in Error! Reference source not found..


Figure 3.1 - Conventional Boiler

When the boiler starts up, the burner fan first blows cold air through the combustion

chamber to purge any residual gases from the previous combustion cycle. Once this process

is complete, the burner then ignites and burns fuel in the combustion chamber. The hot

gases from this combustion process then pass over the boiler heat exchanger, heating the water before passing out of the boiler through the vent.

On the ‘wet’ side of the system, when the boiler fires, the heating circulating pump switches

on to ensure that there is a flow of water going through the boiler heat exchanger. The

heated water is then circulated round the heating distribution system. This process is

described in more detail in the following section. Low temperature hot water boilers

generally heat water up to a maximum flow temperature of 90°C, with systems typically designed based on flow and return temperatures of 82°C and 71°C respectively.

Most boilers have an integral temperature sensor which controls the burner to meet the

target flow temperature. In smaller, simpler boilers this can be achieved by switching the

burner on and off. Larger, more complex boilers can have two stage burners which allow

them to alternate between low fire, high fire and off. Others have fully modulating burners

which can alter the amount of fuel burnt to meet variations in the heat load. Fully

modulating burners tend to have the highest overall operating efficiency, whilst single stage

on/off burners are least efficient. A typical conventional boiler can achieve seasonal

efficiencies of around 70-75%. This capacity to modify boiler output to meet heat demand is

a feature of gas and oil fired systems. As shown in the biomass e-learning module, this

ability is not shared by boilers burning solid fuels. These require different control strategies as a result.

Since 1997, conventional, non-condensing boilers that meet the minimum efficiency

requirements of the boiler efficiency regulations are usually marketed as ‘high efficiency’ boilers. These boilers tend to have higher seasonal efficiencies of around 82%.

Conventional boilers can come in a variety of shapes and sizes. They tend to be floor

standing and draw their combustion air from the space around the boiler making the

provision of sufficient ventilation essential. Burners tend to be either: atmospheric, relying

on convection; or forced draft. The latter use a fan to draw combustion air into the burner.

Forced draft burners allow for closer control of the flow of combustion air into the burner. This reduces flue gas volumes and thus the size of the flue required.

3.2 Condensing Boilers

Condensing boilers differ from conventional boilers in that they have a secondary heat

exchanger on the boiler flue through which the heating return water passes prior to entering


the main boiler heat exchanger. This increases the efficiency of the boiler as more heat is

extracted from the combustion gases prior to them being exhausted to the surrounding atmosphere.

Where the system can operate with return temperatures below 50°C, higher levels of

efficiency can be achieved. This low water temperature causes the moisture in the flue

gases to condense thus releasing the latent heat contained in the vapour. This is why these

boilers are known as ‘condensing boilers’. Figure 3.2 shows a basic cross section of a condensing gas boiler layout

Figure 3.2 - Condensing Boiler

Condensing boilers are best applied to systems where weather compensation can be

applied, ensuring that return temperatures can be minimised and that the boiler can

operate in condensing mode for as much time as possible. For more information on weather

compensation control, see the accompanying e-module on controls. However, it is worth

noting here that this can usually only be applied to systems which exclusively serve heating

circuits, with domestic hot water provided separately. LTHW boiler systems which also serve

domestic hot water tanks usually require constant temperature supplies which negate the system’s ability to compensate at all.

Condensing boilers tend to have fully modulating burners allowing them to vary output to meet a fluctuating load.

Condensing boilers can come in many forms including floor standing, wall hung, and

modular. Modular and wall hung boilers tend to have much lower water content than conventional boilers.

3.3 Wall Hung Boilers

Wall hung boilers tend to be smaller than floor standing boilers. These boilers can also be

flued horizontally through walls, provided a balanced flue is used and the flue terminal is sufficiently far away from any nearby openings such as windows or ventilation louvres.

Balanced or ‘concentric’ flues have two orifices. The inner tube transports flue gases to

atmosphere and the external tube supplies air to the boiler. When this arrangement is used

the ventilation requirements for the boiler room or cupboard are reduced as the combustion

air is provided via the flue. The same effect can also be achieved by using separate flues,

although these cannot be installed horizontally and must terminate at least 1 metre above

the roof level of the building in question.


Wall hung boilers are generally available in sizes ranging from 30 kW up to 120 kW. Larger

boilers are floor standing. Additionally, wall hung boilers tend to have a low water content

and therefore a high hydraulic resistance. This is an important consideration when replacing

old high water content boilers with wall hung boilers as additional pumping capability may be required.

3.4 Modular Boilers

Modular boiler systems are arrangements where several boilers are linked together to meet

the heating demand of a building. Typically, these boiler systems are made up of between 2

to 12 identical modules, although sometimes a combination of condensing and non-

condensing boilers will be used.

Modular boilers have a number of benefits. Like wall hung boilers, they have lower water

content, taking up less physical space than a conventional boiler. In addition, modular

boilers can offer an attractive solution to effectively and efficiently meeting the varying heat

demand of a large commercial building by allowing modules to be sequenced to operate at

maximum efficiency for as much of the time as possible.

For example, condensing boilers operate most efficiently at part load, whereas non-

condensing boilers are generally most efficient at peak load. Consider a building with a peak

heating demand of 500 kW but a summer base load of only 100 kW and it is evident that a

modular boiler can help improve seasonal efficiency. If a single 500 kW non-condensing

boiler was to be installed, it would only operate at optimum efficiency during the coldest

months of the year while the rest of the time it would be operating at part load with reduced

efficiency. If on the other hand a boiler with five separate 100 kW modules was installed,

the boilers could be sequenced to ensure that modules operate at peak load (and thus peak

efficiency) for a much greater time period.

For condensing boilers, where optimum efficiency is at part load, similar benefits can be

achieved by ensuring that the boilers are sequenced to operate at part load for as much

time as possible. There can also be benefits to using a combination of condensing and non-

condensing boilers to meet varying loads as efficiently as possible, especially in applications with a high demand for hot water.

As with wall hung boilers, the low water content of modular systems leads to high hydraulic resistance and additional pumping capability may be required as a result.

3.5 Boiler Efficiencies

Table 3.1 shows typically quoted values for boiler efficiencies.

Table 3.1 – reported boiler efficiencies

Reported Efficiency Efficiency Type

Boiler 1 102% Net Efficiency

Boiler 2 94% Gross Efficiency

System 87% Seasonal Efficiency


This shows that there is an important point to be considered regarding boiler efficiency and how they can be compared

It is not uncommon to see boiler literature advertising boiler efficiency of 102%, whereas

other figures may suggest a peak efficiency of 94%. This is because the first figure refers to

net efficiency, which assumes that the energy contained in the water vapour in the

combustion gases is recovered. The second value is gross efficiency and assumes that the energy is not recovered.

Another efficiency term often used in reference to boilers or heating systems is seasonal

efficiency. This is a weighted average of the efficiencies of the boiler at 15%, 30% and

100% output. The overall system seasonal efficiency will also be influenced by the type of

heat emitters used. For example, a condensing boiler serving only an underfloor heating

system and operating at relatively low temperatures will have a higher seasonal efficiency

than the same boiler serving a system including fixed temperature heat emitters with a flow

temperature of 82°C.

It is important to be aware of the various ways of reporting boiler efficiency when evaluating which boiler to install and to undertake comparisons on a like-for-like basis.

3.6 System Components

How the boiler is connected to the rest of the heating system and the key components of that system must be considered.

Error! Reference source not found. is an energy efficient schematic boiler house with multiple condensing boilers, connected in parallel.

Figure 3.3 - Boiler House Schematic

From the diagram, it can be noted that the boilers are connected and can be isolated in

such a way as to allow any one of them to be taken offline or even removed without the

system having to be shut down. Within the boiler flow pipework there will typically be

temperature and pressure gauges or sensors as part of the control and safety systems.


The system shown has a single circuit, though in practice there can be any number of

circuits depending on the heating load being serviced. All flow circuits should be prior to any

return connections, ensuring that each circuit gets boiler temperature water. In systems

where there are a mixture of variable and constant temperature circuits the flow temperature will be varied on the variable temperature circuits, using mixing valves.

Another component of a system of this type will be a dirt separator and deaerator. This will

be located at the hottest part of the system. These devices remove circulating air, micro-

bubbles and any small particles of dirt in the system. This is important as it ensures that air

and dirt are removed from the system to prevent corrosion, improve heat transfer, and prevent unnecessary deterioration in other system components such as boilers and pumps.

All modern boiler installations include a common header which acts as a barrier between

primary and secondary circuits, allowing for optimum control and hydraulic performance. The benefits of this component are explored in more detail later in this document.

The pressurisation unit and expansion vessel are also important components of the system.

The pressurisation unit automatically maintains pressure in sealed systems by pumping in

mains cold water when the system pressure drops below a predetermined level. As the

name suggests, the expansion vessel accommodates expansion of the fluid in the system as

it heats up. It consists of a tank with a rubber diaphragm separating the fluid from a charge

of nitrogen gas. As the system heats up, the diaphragm expands to accommodate the

increased volume. Some older systems will have a feed and expansion tank at high level which performs both functions for systems operating at atmospheric pressure.

The primary pumpset, usually comprises a twin-head pump with automatic changeover, to

give redundancy to the system. Finally on the water side, a number of balancing or

commissioning valves are present, to allow the system to be set up with adequate flow through each boiler.

Other important items are the condensate drain (often via a tundish) for condensing boilers,

and the various gas isolation valves, allowing for individual isolation of each appliance and

isolation of the gas as it enters the boiler house. This is also a requirement under the gas

safety regulations. It is also good practice to include a strainer to capture any pipeline

debris such as scale or rust before it reaches the pumps and boilers. It is important that this component is located correctly to protect these expensive plant items.

Table 3.2 contains a list of LTHW boiler system components.


Table 3.2 - Boiler system components for LTHW systems

Component Purpose Impact on Efficiency

Isolation Valves To isolate boilers, components and for gas safety

Allows removal/isolation of boilers, components etc. for maintenance or replacement but the system can still operate in most cases

Balancing and

conditioning Valves

To maintain adequate flow through boiler

Ensures the water quantity is met whilst avoiding

the entry of large quantities of potentially damaging cold water

Mixing Valves Mixing temperatures Ensures each circuit and emitter gets the correct temperature

Temperature

Gauge/Sensor

To measure the temperature within the

boiler and heat circuit

If the temperature reads below/above the range set by the manufacturer this could indicate a fault

within the system or a component

Pressure Gauge/Sensor

To measure the pressure within the boiler and

heat circuit

If the pressure reads below/above the range set by the manufacturer this could indicate a fault within the system or a component. The pressure within a system affects the quantity and the temperature of the water. Both low and high pressure can be dangerous

Dirt Separator and deaerator

Removes dirt and air from the system

Improves heat transfer, prevents corrosion and damage to pipeline and components

Common Header

Acts as a barrier

between primary and secondary circuits

Allows for optimum control and hydraulic performance

Secondary flow and return circuits

Secondary circuits providing different areas

of the building

Ensures each circuit gets boiler temperature water

Pressurisation Unit and Expansion

Vessel

Maintains pressure and contains the expansion

of system as it heats up

The pressure within a system affects the quantity and the temperature of the water. Both low and

high pressure can be dangerous

Strainer Removes pipeline debris including scale, rust etc.

Prevents damage to components

Pump Set Twin set of pumps

The pumps allow the water to flow through the

entire hot water and heating system. Two pumps allows for redundancy

Condensate Drain Removes condensate from condensing boiler

Essential for boiler operation

3.7 How to Survey a Boiler House

There are key items to look out for when surveying a boiler house. A good exercise when

surveying a boiler house is to try and sketch out a schematic (as shown in Figure 3.3) to show how the system is laid out.

Step-by-step guide to surveying a boiler house:

Identify the boiler type, capacity and age;

Identify the flow and return pipes – note arrangement of pipework and pumps;

Identify condensate drain if appropriate;

Identify expansion vessel and pressurisation unit;

Identify header and number circuits;

Identify ancillary pipework e.g. dirt separators, deaerators and location of components;

Identify water provision - separate heater, calorifiers, pump sets etc.;

Identify the flue (or flues) and where they exit the building;

Identify ventilation;

Identify control systems and how they operate; and


Check insulation.

Things to look out for:

Insulation. Are all pipes and valves well insulated? Have valve jackets been replaced

after maintenance work?

Are temperatures on heating and hot water pipes as expected?

Are multiple boilers firing together? If this is occurring, particularly in summer, it could

suggest an issue with the boiler controls.

Are there any water leaks in the system? Is the pressurisation unit coming on and off

frequently?

Are there any unusual noises suggesting plant items may have failed?

Is there appropriate isolation on the boiler gas supply?

How old are the boilers? How far are they from replacement?

In most cases a gas safe register approved heating engineer should carry out any repair and maintenance work that is identified.


4 Energy Saving Opportunities

4.1 Boiler Replacement

This section covers boiler house layout and other issues to be considered when replacing

existing boilers. First, consider again the schematic of an energy efficient low temperature

hot water system, with multiple boilers as shown in Error! Reference source not found..

Figure 4.1 - Boiler House Schematic

When replacing older boilers with new, it is likely that the new units will be low water

content boilers. As a result it is recommended that a separate flow circuit is created for the

boilers to protect them from pressure fluctuations on the heating side for example when

occupants open and close thermostatic radiator valves. There are many ways to do this,

however the most common is to install a primary circuit, feeding an oversized header or

buffer vessel, which is then connected to the secondary circuit or circuits. The secondary

side would then circulate hot water to heat emitters such as radiators and fan convectors.

The return pipework for modular boiler systems should be plumbed in a “reverse return”

arrangement to prevent short circuiting of water through one boiler, i.e. ensuring the water

leaving the first boiler will return to it last. It is still common to find boilers installed in the

Scottish public sector estate feeding separate flow and return headers, even when boilers

are relatively new as they have simply been replaced in a one out/one in fashion. A

common associated issue is hot water flowing in circuits that are supposed to be isolated for example radiators being heated when only hot water for hand washing is required.

Having the boilers plumbed with a primary header has many additional benefits. These

include more stable control, better sequence control of multiple boilers, options for

connection of future heating circuits and simpler integration of renewable or alternative energy systems such as biomass or heat pumps.

If replacing conventional boilers with condensing boilers, there are key differences to be

aware of. The flue fitted to the existing boilers is unlikely to be suitable, as the combustion

products from condensing boilers would corrode a standard flue. Finding a flue route for

new boilers may also be problematic for an existing building, particularly if the boilers are located in a basement.


Another requirement is that a drainage connection is needed to remove the condensed

water from the system. There may be health and safety issues that need to be addressed,

for example finding a route for the new drainage pipework that doesn’t create a trip hazard

in the boiler room. In addition, a common issue that can be encountered in cold winters is

the potential for externally located condensate drains freezing, causing the boilers to trip.

Both issues require careful consideration of the location and design of the condensate drains.

A common problem with retrofitting modern condensing boilers to existing heating

installations is that the return water temperatures are too high to allow the boilers to

condense, and this reduces the savings that can be achieved from installing the new

equipment. Separating out the hot water systems that provide hot water to taps, commonly

referred to as domestic hot water (DHW), by installing gas fired water heaters or point of

use electric water heaters, would allow lower water temperatures to be maintained in the

system. Condensing operation can be maintained for most of the year, even if the system is

serving heat emitters that require constant temperature such as air handling units and fan

convectors, by varying the temperature of the primary circuit related with changes to

external temperature. Water temperatures should be maintained above the thermal cut out settings on fan convectors for example, typically around 50°C.

Getting the design right can mean the difference between making negligible or no energy

savings, to saving 25% or more on fuel. Strathclyde Fire & Rescue reduced the energy

consumption of their estate by over 40% since 1990 with the majority of the savings being

achieved through a rolling boiler replacement programme using the techniques described here.

4.2 Boiler Compliance - Ventilation for Gas Systems

When making improvements to a boiler house, it is important to consider compliance with

relevant standards and legislation. Two areas where older plant rooms are often non-

compliant are ventilation and gas safety.

The requirements for ventilation are set out in BS6644:20111 which is the specification for

the installation and maintenance of gas fired hot water boilers of rated inputs between 70

kW (net) and 1.8 MW (net). The quantity of ventilation required is dependent on the type

of boiler, its size, the flue type, and the method of ventilation whether natural or

mechanical. Usually, natural ventilation is preferable as it avoids the need for complicated and costly controls interlocks.

In almost all cases where boiler houses are naturally ventilated, both high and low level

ventilation will be required. This is particularly important where boilers with conventional

flues are used as the ventilation supplies air for combustion, whereas when balanced flues

are used ventilation is only required to prevent excessive temperatures. Without sufficient

ventilation, the boiler is starved of oxygen which can lead to incomplete combustion leading to production of Carbon Monoxide (CO).

A good first step when evaluating compliance is to check that the boiler house in question

has both high and low level ventilation. High level louvres must be as high as reasonably

practicable, whilst low level louvres must be as low as reasonable practicable and within 1 metre of the floor.

If there is any doubt about the compliance of a boiler house, refer to the British Standard document.

1 A quick reference guide to this standard can be found at www.mhgheating.co.uk/wp-content/uploads/2012/04/BS-5440-BS-6644-IGEUP10-Quick-Reference-Guide-010813.pdf

http://www.mhgheating.co.uk/wp-content/uploads/2012/04/BS-5440-BS-6644-IGEUP10-Quick-Reference-Guide-010813.pdf

http://www.mhgheating.co.uk/wp-content/uploads/2012/04/BS-5440-BS-6644-IGEUP10-Quick-Reference-Guide-010813.pdf


4.3 Boiler Compliance - Gas Safety Regulations

All gas fired boiler equipment is required to comply with the legislation as set out in The Gas

Safety (Installation and Use) Regulations 19982. Some key points include:

There must be an emergency control valve (ECV) on the gas supply pipe immediately

after it enters the boiler house, as close as is practicable to the point of entry in order to

allow the safe isolation of the gas supply should a fire break out. Often, modern boiler

houses will have both a manual valve (which is compulsory) and an automatic valve,

which is optional. This offers the peace of mind that if the system develops a fault the

gas valve will close automatically isolating the gas supply;

The gas pipe should never pass through enclosed, unventilated voids. This is to ensure

that there can never be an undetected build-up of gas that could lead to a risk of

explosion; and There should be valves located before each user to allow individual isolation.

4.4 Improving an Existing Boiler House

There are a number of areas to consider when thinking about improving an existing boiler

house. These include:

Insulation;

Amending hydraulic layout;

Upgrading old pumps, and Boiler combustion checks

4.4.1 Insulation Improvements

For more detailed information on the theory and practice of upgrading heating system insulation, see the accompanying insulation e-module.

A good first step when evaluating an existing boiler house for potential savings

opportunities is to identify all items of pipework and valves that are not insulated and

address these.

An infrared thermal imaging survey is a quick and relatively easy method of assessing the

performance of existing insulation and heating elements within the boiler house. The

brighter the image (yellows, orange) the hotter the component and the least efficient insulation. If the image is dark (blues, purples) the insulation is working well.

4.4.2 Hydraulic Layout

It is still common in the public sector to find heating systems with separate flow and return

headers. Often, these can be converted into a common header arrangement relatively

easily. This can help with addressing common issues as noted earlier, improving control and saving energy.

4.4.3 Pump Upgrades

Pumps account for the majority of electricity consumption on LTHW heating systems and

there can be opportunities for making savings in this area. The biggest savings in the public

sector are likely to be achieved by replacing old fixed speed pumps with new, variable speed ones.

2 Approved code of practice and guidance can be found at www.hseni.gov.uk/l56_safety_in_the_installation_and_use_of_gas_systems_and_appliances.pdf

http://www.hseni.gov.uk/l56_safety_in_the_installation_and_use_of_gas_systems_and_appliances.pdf


It used to be normal practice to fix the system flow rate during commissioning using

regulating valves. This works by increasing the system pressure by closing the valve until

the correct duty point is reached on the pumps system curve to provide the desired flow

rate. Any increase in system pressure means a proportional increase in power consumption.

Studies suggest that only 20% of the pump drive motors operating in this way are running

at their full rated input, mainly due to the individuals sizing them adding an additional 10% to 15% to err on the side of caution.

In addition to this, most pumps used in building services installations serve circuits with

varying pressure, for example heating circuits with thermostatic radiator valves (TRVs) that

open and close. The rate of flow through the radiator is reduced by closing the TRVs, always resulting in a duty point shift on the pump curve towards higher heads.

A pump set with an integral inverter can be used in place of regulating valves, adjusting the

pump speed instead of artificially adding system pressure to reach the desired duty point.

Variable speed pumps can therefore respond to changes in demand by varying the motor speed and saving energy when speed reduces.

4.4.4 Combustion Efficiency Checks

Having the combustion efficiency of boilers regularly checked is a good way of ensuring that

the boilers are operating efficiently, as well as giving notice of any potentially serious

issues, such as high carbon monoxide content in the flue gases. Boiler combustion efficiency

test results typically report the oxygen, carbon monoxide and carbon dioxide content in the

flue gas, along with the net and flue gas temperatures, the net efficiency and the

percentage of excess air. The net temperature relates to the temperature difference between the flue gas and the ambient air temperature in the boiler house.

It is recommended to have boiler combustion efficiency checked at least once a year.

4.4.5 Heating System Maintenance - Poor Practice

Some examples of poor practice are shown in Figures 4.2. The first image shows an older

type boiler in poor condition. There are various signs of rust and fouling across the unit, as

well as signs of sulphur around the boiler. The second image shows signs of poor

maintenance, there is corrosion on the pipe and the pipes and valves are also uninsulated.

Figure 4.2 – Examples of poor practice with heating systems


Providing effective maintenance of LTHW boilers is one way to obtain optimum efficiency.

Two types of maintenance exist, preventative and reactive with the former preventing

problems and the latter being carried out after a fault has been identified. Experience shows

that in the long term, preventative maintenance can be far more cost effective than waiting for problems to occur and then responding to them.


5 Building the Business Case

5.1 Before Considering a Boiler Upgrade Project

When thinking about replacing boilers, it is important to consider all other low cost

measures which could be implemented to save fuel consumption prior to proceeding, so as

not to distort the business case. For example, it may be prudent to ask the following

questions of the project before proceeding:

1. Have the heating and boiler controls been optimised? It is important to make sure that

energy is not being wasted through poor control as this can usually be rectified for

relatively low cost.

2. Is insulation up to standard? Insulating pipework and valves is a low cost way to save

energy and money, and this should be taken account of before considering a new boiler

plant.

3. Is the best fuel being used? If the site uses oil, is there a natural gas connection

available? Could gas burners be retrofitted to the existing boilers?

4. Is the building appropriately zoned? Could improved zoning help with better control? If

so, this could be done before replacing boiler plant, or it could be included as part of the

same project.

Once all of the above have been considered, an effective business case can be made.

5.2 Information Gathering

To build an effective business case, one of the most important things is to gather accurate

information. In particular, collect the following data:

The building or boiler total fuel consumption;

The boiler combustion efficiency;

The building peak heating demand; and

The age of the existing boilers.

In addition, consider any other site specific actions which may influence the viability of the

project, for example:

Is the boiler house in the basement?

Could flue routes be complicated?

Is redundancy required?

How will hot water be provided?

5.3 Business Case - Case Study

Consider the following system:

A system comprising two 200 kW boilers was installed in 1985;

Total annual gas consumption is 712,600 kWh;

Existing boiler efficiency is 75%;

Gas price is 3.2 p/kWh;

Current annual running cost is £22,800; and

Building peak heat demand is 180 kW.

For the current system:

Heating demand = gas consumption x efficiency, which is 712,600 x 0.75 giving a total

demand of 534,450 kWh.


If a boiler with a seasonal efficiency of 91% is installed:

o New gas consumption = 534,450/0.91, equating to 587,308 kWh.

o Annual saving = 712,600 – 587,308 = 125,292 kWh.

o Cost saving = 125,292 x 0.032 = £4,009.

Assume that three 90 kW boilers will be installed, giving a scenario where one boiler could

fail but the other two could still meet the building peak heating demand.

A full boiler house upgrade, with new boilers, pipework and all ancillaries installed would cost in the region of £90,000, giving a simple payback of 22 years.

This serves to demonstrate how difficult it is to make the economics of a new boiler stack

up on energy cost saving alone, unless the existing boiler has reached the end of its

economic life and needs replacing anyway. CIBSE estimate that the life of LTHW boilers is

20-25 years. Therefore, in our example the boiler has exceeded the normal economic working lifetime and it is likely that it should be replaced.


6 Useful Links and References

Title Source Field Link

Low temperature hot water boilers The Carbon Trust LTHW Boilers www.carbontrust.com/media/7411/ctv051

_low_temperature_hot_water_boilers.pdf

BS6644:2011 Specification for the

installation and maintenance of gas-fired hot

water boilers of rated inputs between 70 kW

(net) and 1.8 MW (net) (2nd and 3rd family

gases)

British Standards Gas fired hot Water

Boilers

Guide B1 – Heating CIBSE Heating

Options Appraisal Toolkit Resource

Efficient Scotland Options Appraisal

www.resourceefficientscotland.com/resourc

e/options-appraisal-toolkit

http://www.carbontrust.com/media/7411/ctv051_low_temperature_hot_water_boilers.pdf

http://www.carbontrust.com/media/7411/ctv051_low_temperature_hot_water_boilers.pdf

http://www.resourceefficientscotland.com/resource/options-appraisal-toolkit

http://www.resourceefficientscotland.com/resource/options-appraisal-toolkit


Energy Efficiency E-module - Guidance 1... · Energy Efficiency E-module - Guidance Efficient Operation of Low Temperature Hot Water Boilers in ... This increases the efficiency of

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