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SERIES

JULY/AUGUST 2004 SERIES 2 MODULE 04

ENERGY-EFFICIENTBOILERS & BURNERS

CURRENT THINKING ON...

The Continuing Professional Development programme

There have been a lot of changes in theboiler world over the last ten years, much

of which has been a drive towards greater

energy efficiency. Condensing boilers are

now becoming a common option; even the

entry level is what used to be called 'high

efficiency'. You may think that one boiler is

much the same as another; they all look

roughly the same on the outside but there

are a wide range of products and their

selection ultimately determines the

efficiency of the heating system.

Back to basics! Most heating boilers

don't boil, they generally produce Low

Temperature Hot Water (LTHW) at 80-900C. Only steam boilers actually boil the

water and they are mainly used in very

large industrial sites these days. Some

large multi building sites operate on

medium (MTHW) or even high (HTHW)

temperature hot water allowing the

designer to minimise the diameter of

distribution pipework and hence capital

costs. Some heating systems (e.g.

underfloor) operate as low as 400C and

these are ideal for condensing boilers (see

page 27).

Space heating boilers are mostcommonly fuelled (fired) by natural gas but

oil is still widely used. Different grades of

oil are used, rated by their viscosity

(thickness). Light kerosene and fuel oil are

used in smaller buildings while heavy fuel

oil that requires some heat to make it

viscous enough to pump to the boiler, is

generally used in very large multi-building

sites. Liquid Petroleum Gas (LPG) is also

used where natural gas is not available,

although this can be expensive. Bio-fuels

are becoming more popular, fired on crops

specifically raised as a fuel or where

organic waste is readily available (e.g. treepruning in local authorities).

25FUNDAMENTAL SERIES JULY/AUGUST 2004

By Phil Jones, Building Energy Solutions

Produced in association with

WELCOMEEnergy in Buildings and

Industryand the Energy

Institute are delighted to have teamed up to bring you this

Continuing Professional Development initiative aimed at

energy managers.

This is the fourth module in this series and focuses on

boilers and burners. It is accompanied by a set of multiple-

choice questions. To qualify for a CPD certificate readers

must submit at least eight of the ten sets of questions from

this series of modules to EiBIfor the Energy Institute to

mark. Anyone achieving at least eight out of ten correct

answers on eight separate articles qualifies for an Energy

Institute CPD certificate. This can be obtained, on

successful completion of the course, for a fee of 15 (for

members) or 25 (for non-members).

The articles, each written by a qualified member of the

Energy Institute, will appeal to those new to energy

management and those with more experience of the

subject. The following topics will be coming up: drives and

controls; air conditioning; monitoring and targeting;

training; utility purchasing; and building management

systems. Previous modules covered metering, lighting, and

space heating.

If you miss any of the modules during the year let us

know ([email protected]) and we will send the

missing modules to you by e-mail in pdf format.

We hope you enjoy the series and hope they provide

useful information. Let us know what you think of them and

what subjects you would like to see covered in the future.MARK THROWER, MANAGING EDITOR

Boilers generally consist of a burner thatproduces a flame that raises the

temperature of a heat exchanger which

then passes the heat to the water in the

heating system. Waste gases called

"products of combustion" are then emitted

to the atmosphere via a chimney (flue)

including carbon dioxide (CO2) which is a

contributor to global warming. There is a

wide range of burner and heat exchanger

designs, all with different configurations

and materials combined in arrangements

that are manufacturer specific.

BurnersThe objective of a burner is to achieve

combustion with the correct mix of fuel and

air so that all the fuel is burnt efficiently. A

poor air/fuel ratio giving poor combustion

of fuels like oil can lead to "carry-over" of

unburnt oil into the chimney, wasting fuel

and increasing emissions.

There are three main categories of

burner (see Fig. 1):

Atmospheric burners - gas is injected

through the burner which entrains the air

necessary for combustion. This is the most

basic and least-efficient approach, and one

that the market is moving away from as aresult of tighter building regulations.

BOILERS & BURNERS


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capacity, flexibility in maintenance and

allowing the most efficient boilers to take

the base load. Overall energy savings of 5-

10 per cent are typical.

BoilersBoilers generally fall into three main

categories (see Fig. 4):

Standard boilers - based on upgraded

traditional designs, often cast iron with

atmospheric burners. Due to

improvements in heat exchange and

insulation they provide the entry level

technology with a clear efficiency margin

over older existing plant.

While they meet the full load efficiency

requirements of the building regulationsmany fail at part load. This is due to the fact

that at part load, an atmospheric boiler

draws an excess volume of combustion air,

because the flue is sized for the maximum

full load, and unless the excess air is

controlled, then the boiler efficiency

performance is reduced.

Reducing the excess air using a flue

damper can result in gross efficiencies up

to 85 per cent at part load, well above the

minimum requirements of Part L.

Alternatively, standard boilers could be

used as part of a mixed installation withhigher efficiency boilers in order to meet

Part L.

High-efficiency boilers - These boilers

generally have low water content (and/or

low thermal mass) with even greater heat

exchange surface and insulation. They

achieve around 85 per cent at full load

falling slightly to around 83 per cent at 30

per cent part load and comply with Part L

requirements. The higher part load

efficiencies make them particularly

suitable for applications with a wide range

of loads. They sometimes come in a

packaged modular arrangement or assmaller individual wall hung boilers.

27

BOILERS & BURNERSFUNDAMENTAL SERIES JULY/AUGUST 2004

Condensing boilers - use an additional

heat exchanger to extract extra heat by

condensing water vapour from the

products of combustion. They operate at a

minimum efficiency of around 85 per cent,

even when not condensing and can achieve

efficiencies in the range 85-95 per cent

depending upon the system return watertemperature. Condensation begins to

occur at return water temperatures below

55C and the lower the return the more

efficient the boiler.

In underfloor heating systems that

operate at 30-400C they can achieve

seasonal efficiencies over 90 per cent

However, the more common approach for

standard radiator systems is direct

weather compensation to achieve around

88 per cent. Constant temperature 80C

flow systems for fan coil units or air

handling units are less appropriate forcondensing boilers as payback periods will

be less attractive.

Condensing boiler efficiency is often

higher at part load than at full load and they

easily comply with Part L requirements with

85 per cent at full load rising to around 90

per cent at 30 per cent load. Condensing

boilers provide typical energy savings of 10-

20 per cent when replacing existing older

plant resulting in paybacks of 2-5 years

depending on the installation.

Condensing boilers can be more

expensive than the standard boiler. To

keep capital cost to a minimum while stillretaining high efficiencies it is sensible to

mix and match condensing and non-

condensing boilers. Other than very low

temperature systems, combinations of

condensing and non-condensing boilers

are normally more cost effective than all

condensing boilers. Specifying the lead

boiler(s) as condensing, with high

efficiency or standard boiler(s) to top-up,

optimises capital cost while still keeping

overall plant efficiency high. It is common

to find that half to three quarters

condensing plant provides the most

economic approach. Condensing boilers

should always be the first choice for 'lead'

gas boilers in multiple installations.

ControlsIt is very difficult to separate boilers from

the heating system as the two interact to

form an overall system efficiency. Equally

important is the dynamic nature of heating

systems with heat demand changing

almost constantly. The most efficient

systems have efficient boilers, good heat

distribution systems and good controls.

The key requirement is to provide heat

only when and where it is needed and at

the right temperature while minimisingboiler cycling. Use optimum start/stop for

time control and weather compensation

for temperature control, trimmed by

motorised valves or TRVs for zone control.

Good sequence control is fundamental to

achieving an energy-efficient multiple

boiler installation. In particular, careful

location of the sensor in a representative

part of a constant flow primary circuit is

essential for stable control. All boilers have

a boiler control thermostat and a high limit

thermostat for safety purposes. In multiple

boiler installations these should be set

much higher then the sequence controls sothat they allow the sequence control to act

without interference.

Where boilers are more efficient in 'low'

fire than in 'high' fire then the sequence

should begin by bringing the more efficient

'low' stages on first. The sequence should

therefore go LOW-LOW-LOW then HIGH-

HIGH-HIGH.


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Name.......................................................................................................................................................................................(Mr. Mrs, Ms)

Business Address..........................................................................................................................................................................................

Town ................................................................................................................................................................................................................

...........................................................................................................................................................................................................................

Post Code................................................................................................... email address..........................................................................

Tel No...............................................................................................................................................................................................................

Completed answers should be mailed to:

The Education Department,

Energy in Buildings & Industry,

P. O. Box 825,

GUILDFORD, GU4 8WQ

MODULE 04 TEST QUESTIONSPlease mark your answers on the sheet below by placing a cross in the box next to the correct answer. Only mark one box for each question. You may find it helpful to mark the answers in pencilfirst before filling in the final answers in ink. Once you have completed the answer sheet in ink, return it to the address below. Photocopies are acceptable.

FURTHER READING

Heating CIBSE Guide B1 (London: Chartered Institution ofBuilding Services Engineers) (2002) Heating systems and their control GIR 40 (Action Energy0800 585794) www.actionenergy.org.uk (1996) Heating controls for wet central heating systems insmall, commercial and multi-residential buildings GPG 132(Action Energy 0800 585794) www.actionenergy.org.uk (2000) Domestic Heating Design Guide published by HVCA andothers, November 2000. (ISBN 0-903783-33-9) Condensing boilers CIBSE Applications Manual AM3(London: Chartered Institution of Building Services Engineers) (1989) Heating system option appraisal - an engineer's guide for

FUNDAMENTAL SERIES JULY/AUGUST 2004

Other observationsIt is essential to select the most efficient

plant and ensure that plant and equipment

are not oversized. Plant that is too large

will operate further down the part load

curve and hence at lower efficiencies

unless it is condensing.

Where possible, segregate domestic hot

water from space heating in order to avoid

poor summertime efficiencies. Plant sized

to meet space heating and hot water will

effectively be far too large for small

summer hot water demands and this could

reduce seasonal efficiency significantly.

Even a well-designed system can

perform badly with poor controls.

Conversely, you can't fix a poor heating

design by just adding controls. The boilers,

heating distribution and controls have to

be seen as one overall system.

Regularly carrying out good boilermaintenance is essential to ensure

continual high efficiencies. This includes

cleaning and setting up the burner,

cleaning the heat exchanger to ensure

good heat transfer and setting the boiler

controls correctly.

Above all, keep it simple! Over-complex

systems and controls can lead to

installation, commissioning and

maintenance problems which result in poor

operation and efficiency.

existing buildings GPG 187 (Action Energy 0800 585794)www.actionenergy.org.uk (1996) Building control systems CIBSE Guide H (London:Chartered Institution of Building Services Engineers) (2000) Energy Efficiency in Buildings - CIBSE Guide F (London:Chartered Institution of Building Services Engineers) (2003) Heating Systems Plant and Control by Day, Ratcliffe andShepherd (Blackwell Science, 2003)

Phil Jones is an independent energy consultant specialising inbuildings. He is the author of many guides on the subjectincluding CIBSE Guide F - Energy Efficiency In Buildings. Phil isa member of the CIBSE Carbon Task Group and chairman ofthe CIBSE CHP Group.

1. Approximately what return water temperaturedoes condensation begin to occur in acondensing boiler system?

A. below 1014C B. below 3014C C. below 5514C D. above 7514C

2. What is the most efficient mode of burneroperation?

A. Fully modulating B. High/low C. On/off D. Reverse return

3. What is an ideal application for condensingboilers?

A. High temperature hot water systems B. Fixed 82/7114C systems including fan

convectors C. Underfloor heating systems at 30-4014C D. Steam condensate systems

4. Typically, what is the optimum proportion of

condensing boilers in a multiple boilerinstallation with standard radiators?

A. 10-25 per cent condensing B. 25-50 per cent condensing C. 50-75 per cent condensing D. 75-100 per cent condensing

5. Which measure of boiler efficiency can reachover 100 per cent?

A. Forward B. Net C. Plus D. Gross

6. What can be used to improve the part loadefficiency of traditional and standard boilers?

A. Flue dampers B. Additional chimney height C. Introducing fan dilution D. Removing insulation

7. What is the most important form of control for

multiple boiler installations? A. Differential control B. Open loop control C. Linear control D. Sequence control

8. What renewable energy can be burnt in boilers? A. Wind energy B. Wave energy C. Biomass energy D. Solar energy

9. What is the efficiency requirement to meetPart L at full load in England & Wales?

A. 78 per cent B. 80 per cent C. 85 per cent D. 90 per cent

10. Which boilers should always be consideredfirst as lead boilers in an energy efficientmultiple boiler installation?

A. Atmospheric B. High efficiency C. Stratospheric D. Condensing

EDITORIALManaging Editor: Mark Thrower Tel: 01483 452854E-mail: [email protected]

PUBLISHER Pinede Publishing, 16-18 Hawkesyard Hall, Armitage Park, Nr. Rugeley, Staffordshire WS15 1PU.Tel: 01889 577222 Fax: 01889 579177

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