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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 (mark.thrower@btinternet.com) 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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28
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: mark.thrower@btinternet.com
PUBLISHER Pinede Publishing, 16-18 Hawkesyard Hall, Armitage Park, Nr. Rugeley, Staffordshire WS15 1PU.Tel: 01889 577222 Fax: 01889 579177
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