BUILDING STANDARDS DIVISION - Biomass Installations A guide to safe and sustainable construction
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BUILDING STANDARDS DIVISION Low carbon equipment and building regulations A guide to safe and sustainable construction Biomass Installations This guidance document is intended to be read in conjunction with the general introduction to low carbon equipment
Document Version Control
Title: Low Carbon Equipment & Building Regulations: Biomass Installations
Purpose: To provide a guide to safe and sustainable biomass installations
Version Date Notes
1.0 February 2012 Guidance for the installation of woody biomass equipment taking
account of the mandatory building standards set by the Scottish
Building Regulations.
We acknowledge with thanks the drawings and photographs kindly provided by others.
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Low carbon equipment and building regulations:
A guide to safe and sustainable installation
This chapter is one of a series that provides a basic introduction to different low carbon technologies and
describes their relationship to the building regulations in Scotland. It should be read alongside the
general introduction to this guide.
Biomass Installations
Section 1 INTRODUCTION 4
Section 2 WOOD BIOMASS 6
Section 3 LOG BURNING APPLIANCES 15
Section 4 WOODCHIP APPLIANCES 17
Section 5 PELLET BURNING APPLIANCES 20
Section 6 CHIMNEYS AND FLUES 22
Section 7 HEATING SYSTEM CONTROLS 24
Section 8 BUILDING REGULATIONS 27
Appendix A EXPLANATION OF TERMS 37
Appendix B GOOD PRACTICE ADVICE 39
Appendix C WORK NOT REQUIRING A 42
WARRANT
Appendix D RELEVANT STANDARDS 44
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Low carbon equipment and building regulations Biomass
1. Introduction
BIOMASS
This guidance is one of a series of chapters that provides a basic introduction to
different low carbon technologies and describes their relationship to the building
regulations in Scotland.
In the energy industry, the term biomass includes a wide variety of materials – not
just wood. The UK Biomass Task Force defines biomass as:
“literally, any biological mass derived from plant or animal matter. This includes
material from forests, crop-derived biomass including timber crops, short rotation
forestry, straw, chicken litter and waste material” The use of woody biomass, in the
form of wood chips, wood pellets, and logs may offer a viable alternative to fossil
fuels, particularly in areas not served by gas mains.
Biomass heating systems can be a simple stove used to heat a single room or with a
boiler to provide hot water and central heating. Wood logs and wood based products
can be included as part of a low carbon emissions strategy for buildings.
The majority of biomass boilers on sale in the UK, at the scale appropriate to this
document, are designed to run on dry and seasoned logs, woodchips and wood
pellets. Appliances should be designed in accordance with the British Standards
Institution (BS EN or BS) or the Design Guide produced by the Chartered Institution
of Building Service Engineers (CIBSE).
The types of wood biomass appliances covered are:
• Log boilers
• Pellet boilers
• Wood chip boilers
SCOPE OF GUIDANCE
This guidance has been prepared with domestic sized installations in mind, with
output ratings not more than 50kW. The guidance therefore, may not be relevant to a
biomass appliance with an output rating of more than 50kW. It is also important to
note that the wood fuel systems discussed within this guide are only space heating
and hot water systems.
CARBON MONOXIDE
Carbon Monoxide (CO) is an extremely poisonous gas that can be present in the
fumes from the combustion of fuel burnt under incorrect conditions. The gas cannot
be seen, smelt or tasted, making it difficult to detect.
Carbon monoxide can kill or cause permanent damage to a person’s health. Initial
symptoms include tiredness, drowsiness, headache, nausea and chest pains (similar
to flu). To reduce the risk of carbon monoxide poisoning the following should be
checked to ensure that:
• there is adequate ventilation
• there is correct installation of flue liner and chimney
• the draught in the flue meets manufacturers recommendations
• the correct fuel for the appliance is being used
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Low carbon equipment and building regulations Biomass
• there is regular maintenance, including sweeping chimney and
• there is a carbon monoxide detector to alert occupants.
HETAS recommends that a Carbon monoxide detector is installed with every solid
fuel appliance including existing ones. However the detector must never be
considered as a replacement for correct maintenance or chimney sweeping.
CLEAN AIR ACT
The Clean Air Act allows local authorities to create smoke control areas in which
smoke emission is prohibited unless arising from the burning of authorised fuel or
from the use of an exempt appliance. Wood fuel can be burnt in a smoke control
area, if you use an “exempt” appliance. An “exempt” appliance is one which is
permitted to burn unauthorised fuels in smoke control areas. These appliances have
passed tests to confirm that they are capable of burning an unauthorised or
inherently smoky solid fuel without emitting smoke.
EXEMPT APPLIANCES
The following website lists the appliances and fuels that are approved for use in
smoke control areas and contacts for local authorities - smokecontrol.defra.gov.uk
FUELS
To burn wood, wood chips and wood pellets in a smoke control area you must use
an exempt appliance, as listed in the above link. It is an offence to acquire any
unauthorised solid fuel for use in a building in a smoke control area other than in a
building or appliance that is exempt.
A definition of terms used in this guide can be found in Appendix A.
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Low carbon equipment and building regulations Biomass
2. Wood Biomass
WOOD BIOMASS
The most common biomass used for domestic sized appliances is wood. When
used as an energy source wood can be classed as a low carbon fuel because
although it emits CO2 during combustion, it absorbs CO2 during growth.
Wood is a very versatile fuel
which can be burned in many atmosphere
different forms and in a number CO2
of appliances. It can be used to
heat one or more rooms, a growing
whole house, or to produce hot trees
water and to cook. convert CO2
into plant
This guidance document looks wood material
burning
at biomass in the form of a
releases
wood derived fuel. There are CO2 back
three main forms: logs,
woodchips and reconstituted
fuels such as pellets.
IS WOOD BIOMASS RIGHT FOR ME?
Wood biomass may be suitable for you depending on:
• building location
• space available
• position of the fuel store in relation to the boiler
• the availability of fuel.
For many people in rural Scotland where a backup source of heating in the event of
a power-cut is useful, stoves which can burn logs may provide the ideal solution.
THE BENEFITS OF WOOD FUEL
The benefits of wood fuel as a renewable energy are:
Good for the local economy - wood fuel sourced locally can create business
opportunities and contribute to the local economy.
Security of supply – using wood to heat our buildings diversifies the fuel sources,
decreases dependence upon fossil fuel exporting countries and depending on the
amount of wood grown can increase the security of our energy supply.
Carbon neutral fuel – wood fuel is considered almost carbon neutral due to the
short time span of the carbon cycle if the wood is harvested and replanted. The
same quantity of atmospheric CO2 is absorbed when the tree is growing as released
when the wood is burned.
Low carbon process - the emission of carbon dioxide through harvesting,
processing and transportation of wood fuel can be minimised by using local
production and distribution networks.
Contributing to social amenity - parkland, woodland, forestry and agriculture are
generally perceived to be environmentally and socially attractive amenities by the UK
population, providing opportunities for recreation and leisure activities. The wood fuel
industry invests in these sectors.
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Low carbon equipment and building regulations Biomass
Avoids landfill - recycling and reuse is generally the best environmental option and
should ideally be the first choice for use of wood by-products and waste. Where this
is not done, the debris left from processing trees and the waste not used as fuel,
generating energy, or some other application are often consigned to landfill. This
imposes costs for disposal, additional burden on limited landfill resources and also
contributes to climate change by the creating landfill gases. These gases include a
high proportion of methane (CH4), which has a much higher global warming impact
than carbon dioxide.
FUEL TYPES
Wood logs - are easily available and are the
most common form of wood fuel. Logs have
been used for centuries in open fires and
more recently in stoves and boilers. The
amount of storage required for logs will
depend on whether the owner is splitting their
own or buying ready prepared logs from a
supplier. Log stores should allow plenty of air
flow and be designed to protect the logs from
rain. The moisture content of logs can have a
considerable effect on the useful heat yield. Between 35% and 60% of the weight of
freshly felled (green) wood can be water therefore, it is important that logs are
allowed to ‘dry’ or ‘season’ to reduce the moisture content.
For successful burning, the moisture content should be below 25% so it is advisable
to have covered storage facilities for at least one year's worth of fuel to allow for
seasoning. As the moisture content in the wood increases the amount of useful
energy available from the wood decreases; as energy is used up to drive off the
excess moisture. At 60% moisture, wood can have an energy content of typically
1.7kWh/kg, but at 25% moisture this can increase to 4kWh/kg.
Burning wet wood will result in a fire that smoulders and creates a lot of tar, steam
and smoke. This can result in corrosion and condensation problems in boilers and
flues and reduce the amount of heat produced. This applies to both log burning
boilers and stoves. Unburned fuel can result in tar like deposits on the lining of the
chimney thus contributing to the risk of chimney fires. It can also be detrimental to
the visual look of a stove as burning wet wood can blacken the glass at the front of
the stove.
Wood Chips - are made from whole trees,
branch wood or coppice products which
have been mechanically shredded by a
chipping machine. Wood chips are bulky
and sufficient storage and delivery access
needs to be considered when designing a
heating system with this type of fuel. They
are normally stored in a bunker or silo
designed so that the chips can be tipped in.
The chips are then fed by an auger to the
boiler. Transport costs can be high, but if wood chip is sourced within a 20 miles
radius it can be a very cost-effective fuel.
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Low carbon equipment and building regulations Biomass
For most users and suppliers of wood chips the two parameters that are critical to
efficient, trouble free operation of the appliance are moisture content and chip sizes.
Wet wood chips have a lower energy content (lower calorific value) than dry wood
chips. Chips with a moisture content greater than 30% should be stored in a
ventilated area to allow for seasoning. The wood species used also has a bearing on
the calorific value of the chips, with hardwood chips having a higher calorific value
than softwood. Heaps of wood chips can be very difficult to dry and should therefore
be ideally dried as logs and chipped once the wood is at the target moisture content.
Wood Pellets - are a type of wood fuel
generally made from highly compressed
waste sawdust. Because they are
compressed they need less storage space
and are easier to handle. They are usually
produced as a by-product of sawmilling and
other wood transformation activities. The
materials used, include ground woodchips,
sawdust and bark however, pellets can be
made from practically any biomass material
including straws, grasses, energy crops etc. No chemical additives are needed, the
properties within the wood itself serves as a binder, although sometimes small
quantities of maize starch are added to improve binding.
As with logs and chips, storage has to be considered; a whole house size wood
pellet boiler uses up to 10m3 or 6.5 tonnes per year. Depending on the heat loss
from a house, a stove which heats only one room is unlikely to use more than a
tonne a year.
Pellets can be delivered in pre-packed bags or by bulk tanker delivery. With bulk
storage, dust may accumulate creating health and safety and operational hazards.
The pellets are extremely dense and can be produced with a low humidity content
(about 6% to 10%) this allows them to be burned with a very high combustion
efficiency. There are only minor energy losses due to the need to burn off the
moisture content.
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Low carbon equipment and building regulations Biomass
CHOOSING THE RIGHT TYPE OF FUEL
Description Storage Advantages Disadvantages
Logs - usually During seasoning Familiar fuel, which can Not as suitable for automatic
seasoned for up to 2 store under cover be used on domestic feeds. Log burners require
years to decrease with good air fires, stoves and wood regular de-ashing. Large
moisture content. circulation. Keep burners storage space may be
Suitable for small to dry required
medium appliances
Wood Chips - Keep dry, store as More suited to automatic Moisture content > 30%
Depending on the loose piles handling results in a lower net calorific
moisture the chips value.
can be: Covered open air More efficient combustion When storing over long
wet: moisture storage is periods high moisture wood
content > 30% preferable Cheaper than pellets degrades quickly which can
dry: moisture content result in risk of self heating
< 30% and potential spontaneous
combustion. Turning over of
More suitable for stockpiles is necessary to
larger installations avoid build up of heat
e.g. over 25kW
output
Wood Pellets - Keep dry, stored Generally good storage Pellets must be kept dry as
usually under cover, characteristics exposure to water will
referred to by the preferably in breakdown pellets and cause
diameter of the enclosed areas Low moisture content them to swell considerably
pellets, typically
between 4-10mm. Can be shipped in bulk. Can degrade to saw dust as
a result of excessive, or
They should be Good combustion poorly designed, mechanical
clean, pleasant characteristics handling processes
smelling and
smooth to touch If breakage of pellets does
occur levels of dust can rise
Suitable for small to dramatically. In the most
medium appliances severe cases this has the
potential to create a dust
explosion hazard
FACTORS TO BE CONSIDERED
When considering what is the most suitable appliance for a particular building there
are many factors that will influence the choice, including the location and the need
for additional building work. For most houses, equipment and storage space needs
to be compact and easy to maintain. Logs or pellets may be best suited as they can
be more easily accommodated.
Other factors to take into account are:
• the source of fuel and the delivery method
• installations inside Smoke Control Areas must only be of ‘exempt appliances’
• owners of wood fuel boilers where the burn rate exceeds 45.4kg/hr must apply to
the local authority for chimney height approval
• storage facilities
• suitably sized log stores will be required to dry wood during the summer
• water heating stoves have higher installation costs than room heating stoves.
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Low carbon equipment and building regulations Biomass
COMBINED SYSTEMS (central heating)
The most economic and effective system to realise a fully low carbon heating system
for single family houses is at present, the combination of a wood biomass boiler, a
water tank and solar thermal collectors (2-3m³) (see also the low carbon equipment
chapter - Solar thermal systems or solar hot water (SHW)). In summer the solar
system provides hot water and stores it in the tank.
BOILER SIZING
Similar to other solid fuel systems, wood biomass systems do not respond well to
varying loads or long periods of low load. Therefore to get the best out of the system
it should be operated relatively continuously between 30% and 100% of its rated
output. This is because if used as part of a combined heating and hot water system,
more energy (heated water) will be produced during the combustion process than
may be required at that time. A heat store (highly insulated heat storage tank) is
useful for accommodating this excess energy (heated water) for use at different
times during the day.
The correct sizing of a wood biomass system is essential in helping reduce waste,
cost and providing a comfortable environment. The sizing of an appropriate boiler
should be carried out by a qualified heating engineer. However, there are a couple of
methods and online calculators for roughly establishing the boiler output required.
The following is an example of one of these methods.
To produce a room temperature of around 21º Celsius, when the outside
temperature is minus 1º Celsius, you will need about 1kW of heat for every 14 cubic
metres of an average insulated space. 1kW is the equivalent of approximately 1 bar
of an electric fire.
EXAMPLE
To establish the output of the wood burning stove required to heat a room,
ascertain the cubic capacity of the room in m3, the kilowatt requirement is then
calculated by dividing this figure by 14.
VARIABILITY DEPENDING ON INSULATION
These calculations are based on heating a room which has been averagely
insulated (1990s or earlier). To calculate the kW requirement for a poorly
insulated room (1950s or earlier) divide m3/10. To calculate the kW requirement
for a room constructed to comply with current energy efficiency standards divide
m3/24.
Worked example - a room 5 metres wide, 3 metres long and 2.4 metres high will
have a cubic capacity of 36m3. The required output in kW for the boiler would be
36/14 = 2.57.
A stove with 3 kW output would adequately heat this room.
A higher output stove may be required if the room has, stairs leading off it, is badly
insulated, or has lots of windows/external doors.
The actual size of the appliance will not only vary depending on the insulation but
also, the exposure of the room, the temperature requirements, the geographical
10Low carbon equipment and building regulations Biomass
location and the amount of time it will be used. Specialist advice should be sought
before a final decision is taken.
The rated output of each appliance is based on the performance of the appliance
when tested using a standard test fuel and with all doors closed. Appliances are
tested to BS EN 13240 if they are conventional room heaters/stoves burning wood
logs and/or smokeless fuels and to BS EN 14785 if designed to burn mechanically
fed wood pellets.
STORAGE
The storage facilities for the fuel can be a significant proportion of the overall capital
cost of a system. It is important that careful consideration is given to the design of
such facilities so that there is sufficient storage space for the fuel, appropriate access
to the boiler for loading and a local fuel supplier.
There is considerable variation between the amount of storage needed depending
on the different boiler systems and fuels such as wood pellets. Sufficient storage is
needed to avoid frequent transport deliveries which would reduce the carbon savings
generated from using biomass.
The main factors that influence the choice of storage are:
• type of fuel and appliance
• quantity of fuel required depending on heat demand
• frequency of deliveries
• space available and
• vehicle and handling equipment available.
VOLUME OF STORAGE REQUIRED
Courtesy of Forestry Commission
11Low carbon equipment and building regulations Biomass
To calculate the storage area necessary for the particular fuel type, the following
information is required:
• Energy demand
• Calorific Value of the fuel
• Density of the fuel
Fuel consumption (kg) = Energy demand (kWh)/Calorific value (kWh/kg)
Fuel volume (m3) = Fuel consumption (kg)/Fuel density (kg/m3)
EXAMPLE
Calculate the storage volume required for wood pellets if:
• The annual heat requirement is 24000kWh and the coldest monthly heat
requirement is 4000kWh
• Wood pellets density is 600kg/m3
• Wood pellets calorific value is 5kWh/kg
• Pellets are delivered on a monthly basis
Worked Example:
• Calculate the fuel consumption (FC) for the coldest month
FC = 4000kWh/5 kWh/kg = 800kg/ month
• Calculate the fuel volume (FV) required
FV = 800kg/600 kg/m3 = 1.33m3
The store volume required would be 1.33m3
OTHER FACTORS
Due to the cost of delivery it is usually prudent to order a minimum of 3 tonnes of
wood chips. Calculating the storage volume for logs is more problematic as the sizes
are not always consistent.
If a hopper is used for storing wood fuel, it is best if it can be attached to the outside
of the building in an accessible location, or in an underground lined pit.
The following table provides details on the size of storage for different fuel types
based on a 3 month delivery cycle to minimise the number of deliveries. Advice on
the sizing of storage for woody biomass fuel for larger buildings is given in the non-
domestic Technical Handbook.
Bulk woody biomass fuel storage: 100% primary heating and DHW
Dwelling size Wood pellets Wood chips Logs - stacked
< 80 m² 1.5 m³ 3.5 m³ 3 m³
80 -160 m² 2 m³ 5 m³ 4 m³
> 160 m² 3 m³ 6 m³ 5 m³
12Low carbon equipment and building regulations Biomass
In some dwellings the stove may only be used to provide secondary heating. The
following table provides details on size of storage based on a 6 month delivery cycle.
Bulk woody biomass fuel storage: secondary heating
Dwelling size Wood pellets Wood chips Logs - stacked
< 80 m² 0.3 m³ (9 bags) 1 m³ 0.5 m³
80 -160 m² 0.5 m³ (13 bags) 1.5 m³ 1 m³
> 160 m² 0.7 m³ (16 bags) 2 m³ 1 m³
SELECTING THE RIGHT FIREWOOD SUPPLIER
Environmentally, it is preferable to select a local supplier as this limits the
energy/emissions required to transport fuel. Using a local source where possible,
can result in local investment and employment, it also has the potential to create and
sustain jobs in rural areas.
There are various factors which could influence the choice of supplier:
• does the wood supply come from a sustainable source?
• what type of wood is being supplied – hardwood, softwood or a mixture of both?
• will the wood fuel suit the appliance?
• is it possible to buy the wood by volume rather than weight? (as this removes the
influence of moisture content).
UNAUTHORISED FUELS
Burning certain wood fuel types can affect the amount of tar and deposits which build
up on your chimney and release noxious chemicals into smoke. The materials
should be avoided for combustion as they can emit unpleasant odours, generate
emissions that damage the environment and are harmful to health:
• varnished or plastic-coated wood
• wood treated with preservatives
• household waste
• garden waste
AMOUNT OF ENERGY (calorific value)
Energy density - the term 'energy density' refers to the energy contained in a fuel per
unit weight. The energy density of fossil fuels such as coal, oil and gas are
considerably higher (up to three times) than that of wood biomass fuels such as
pellets or chips.
Indicative Calorific Values of Wood Biomass
Particle size Moisture content Gross Calorific
(%) value (MJ/kg)
Logs ≥100 mm 60 6
25 15
Chips from fresh 2 – 25 mm 50 8
wood (can be graded) 10 17
Wood Pellets 8-10 mm 10 17.5
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CARBON EMISSIONS
Research indicates that using wood biomass for Comparison of CO2 emissions
heating can give reductions in carbon emissions Fuel kg CO2/kWh
compared to using other fossil fuel heating fuels. Heating oil 0.274
This is dependent on good practice in fuel Electricity 0.517
production, processing and transport, the (standard tariff)
efficiency of the heating system and the eventual Mains gas 0.198
disposal. Woodchip 0.009
Wood pellets 0.028
PERFORMANCE Wood logs 0.008
The performance of a wood biomass boiler
depends upon the type of fuel used. The relationship between the boiler and the fuel
is crucial – boilers are usually designed to burn fuel of a specific type and quality.
Deviation from the specification without adjustments to the boiler design can lead to
poor efficiency and increased emissions of air pollutants.
14Low carbon equipment and building regulations Biomass
3. Log burning appliances
TYPE OF APPLIANCE
Appliances for burning logs are:
• open fires
• stoves (includes ranges)
• log boilers
OPEN FIRES
These appliances are the traditional way of burning
logs to provide space heating. Although not renowned
for their efficiency, they have a strong amenity value as
a feature in a room and a modern fireplace with a well
designed grate and flue can make a useful contribution
to space heating. For example, many modern designs
of fireplaces include chimney dampers to control air
flow to the chimney increasing the useful heat output.
Some also include a convection chamber which
pushes hot air into the room which greatly increases
their efficiency. A coal grate may not be suitable for an
open fire to burn wood, as a solid base to retain an ash
bed is required.
STOVES
A wood-burning stove is often the simplest and the
cheapest way of benefiting from efficient wood heating.
Stoves are available in a variety of forms, including
cast iron, steel and ceramic. This includes kitchen
ranges although relatively few are specifically designed
for wood-burning and tend to be designed as multi-fuel
systems.
Although many wood-burning stoves only heat the
room they are in, the higher output versions may have
an integral back boiler to provide domestic hot water
and if required, central heating.
Courtesy of Biomass Energy Centre
15Low carbon equipment and building regulations Biomass
An efficient wood burning stove or boiler produces flue gases that are cooler than an
open fire. For this reason it is beneficial to install an insulated liner within an existing
chimney when a wood burning stove is installed to ensure that there is a sufficient
draw of air. If there is no existing chimney, a wood burning stove or boiler installation
is normally fitted with a new system chimney, such as the insulated twin wall type.
LOG BOILERS
While wood logs are often used in stoves to heat the room they are situated in, they
can also heat water for central heating systems, either in a stove with a back boiler
or a log burning boiler designed for burning logs. Stoves burning logs often have an
efficiency of around 65% however, modern wood log boilers when burned at
maximum output can have efficiencies of up to 90%. Log boilers are the least
'automated' of all the wood heating options described here and require refuelling
every few hours as well as regular de-ashing.
Modern boilers have closely controlled combustion and a high level of efficiency.
They are often referred to as log wood gasification boilers where the term ‘gasifying’
means that the wood is superheated, resulting in high efficiency ratings.
Boilers can be designed with large combustion chambers to minimise how often they
need to be stoked. Log systems however, begin to become impractical for heating
requirements over 50kW because of the need to regularly re-fuel.
16Low carbon equipment and building regulations Biomass
4. Woodchip Appliances
WOODCHIP APPLIANCES
Woodchip burning technology has been widely developed and adopted in the Nordic
countries, North America, Austria, Germany and many other countries. Many
decades of development have resulted in robust and reliable systems ranging in size
from appliances suitable for a 4 bedroom domestic property up to a power-station.
Modern woodchip systems are typically used for larger buildings such as schools or
community centres but they are available from 20kW which is suitable for about
fifteen radiators. This means that the heating of larger homes is perfectly feasible
with wood chip. However, as these boilers require automation to feed the fuel to the
burner and need up to three times the storage space required for wood pellets, they
might not be cost-effective for a smaller home. The space requirements of wood chip
systems also means that their incorporation into existing buildings can be difficult,
especially where there is limited access for vehicles.
TYPES OF WOOD CHIP BOILERS
There are different types of woodchip boilers on the market, which are characterised
by the type of combustion and feed mechanisms. The main difference between wood
chip and wood pellet systems is the way the chips are fed into the boiler. As the chips
do not flow as easily as pellets an agitator is needed to stir the chips to keep them
moving through the boiler to the burner.
Modern small-scale woodchip heating systems are relatively simple in design and
generally contain a woodchip storage hopper and a supply pipe to the burner unit
attached to the boiler. Domestic sized appliances will normally use the underfeed
burner or the horizontal burner. The three types discussed here are:
• underfeed burner
• horizontal (stoker) burner
• moving grate burner
UNDERFEED BURNERS
In these systems the chips are fed from the bottom up using an auger system. The
primary air is supplied through the ring shaped steel grate with secondary air being
supplied from above. While fuel should ideally have a low moisture content this type
of burner can accept fuels with a moisture content of up to 50%. The removal of ash
from the bottom of the combustion chamber is sometimes by manual intervention,
but it is more common to have the ash augured from the bottom of the combustion
bed to an external ash bin.
Underfeed burner
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HORIZONTAL BURNERS
Commonly known as the stoker burner this appliance has a small self-contained fire
bed at the end of the fuel feed pipe which is fixed into the base of a boiler-water
jacket heat exchanger. A small fan blows air under and over the small heap of wood
fuel that is fed by the horizontal auger into the fire bed to produce a large flame
which heats the boiler surface and heat exchanger. It is cheaper than the moving
grate systems because it is less complicated. Ash is usually removed manually
especially on smaller units.
Horizontal Burner
MOVING GRATE BURNER
The moving grate burner, (also known as step-grate or inclined grate) is similar to
furnaces traditionally designed for coal firing. Woodchip fuel is fed via the hopper
and feed pipe onto an inclined fire grate. The grate gently oscillates to move the
woodchips down and along as it burns and forced combustion air is fed in
underneath the grate. Combustion happens at high temperatures within a ceramic
chamber, with a boiler heat exchanger above the firebox. Appliances are made in the
full range from 20kW upwards. While fuel should ideally have a low moisture content
this type of burner allows wet fuel to self-dry before combustion. Therefore it is less
sensitive to fuel moisture content and to a degree fuel quality; this allows it to burn a
wide range of fuels of varying moisture content. Once the wood has combusted, the
remaining ash falls from the lower end of the grate, and is removed mechanically into
the ash pan or bin.
Of the three general designs the moving grate burner type is the most complex in
construction and generally the most costly. Moving grate plants can use pellets or
woodchips, although woodchips are used more commonly because of the capacity of
the plants to burn wet fuel.
There are 3 main types of moving grate burners, these are:
• stepped
• tipping
• rotating
18Low carbon equipment and building regulations Biomass
Moving Grate Burner
Water
Ceramic
Combustion
Secondary
Feed
Primary 3-section reciprocating
WOODCHIP FUEL
Woodchip fuel should:
• be uniform in shape and size and suited to the heating appliance
• be free of oversize slivers or pieces of twig that could block the automatic feed
system
• have a low dust/fines content. Small airborne particles are injurious to health,
reduce the airflow through the chips, and tend to create more fly-ash
• be of low moisture content. Woodchip systems are not as sensitive to this as log
fuel systems. Woodchip moisture content below 30% are usually fine, but the
ability of each appliance to self-dry the fuel should be assessed.
19Low carbon equipment and building regulations Biomass
5. Pellet burning appliances
PELLET STOVES
Pellet stoves are often designed to be a focal point of a room, giving a direct view of
the fire. They are efficient and have a high level of control with automatic operation.
Rooms are heated radiantly through the glass door and by heat from the body of the
appliance and the boiler. They offer the most user-friendly form of wood heating.
These appliances generally have an auto-ignition system using a hot air blower,
allowing the stove to be used in conjunction with a heating system programmer and
boiler interlock. With thermostatic controls it can automatically stop burning when
demand is satisfied or when a time clock turns it off. The controls allow an accurate
supply of fuel and combustion air in response to the energy demand.
Most stoves have an integral hopper which can normally hold enough pellets for
several days burning. They are much easier to control than log burning stoves, and
can be left to burn all day with little attendance. Domestic sized stoves are filled by
hand using bags of pellets of 15kg upwards.
Wood pellet appliances should be tested to BS EN 14785 if designed to burn
mechanically fed wood pellets.
KEY CHARACTERISTICS
The stoves are designed to burn quality pellets to an approved standard. Pellet
stoves are generally available with heat outputs between 5kW and 18kW. Careful
consideration needs to be given to the flues to ensure they satisfy the building
regulations (see section 8).
OPERATION OF WOOD PELLET STOVE
Wood pellet fuel is loaded into the top of
the stove, with a pellet feed auger at the
bottom of the hopper. The auger turns
and pushes the pellets up into the top of
the chamber, where they drop down into
the combustion chamber. Once in the
combustion chamber ignition is usually by
a hot air blower which starts the fire
automatically. A combustion fan then
feeds the fire with the correct amount of
air to achieve maximum combustion
efficiency and minimum ash. Secondary
air is introduced above to create full
combustion of volatile gases resulting in
low levels of carbon monoxide and
unburned gas. The ash generated from
combustion falls through the grate into
the ash pan below although because of
the high efficiency of the appliance the
ash pan may only need to be emptied
every two months.
20Low carbon equipment and building regulations Biomass
Many units have pressure switches in the flue system to prevent or extinguish
combustion if the flue becomes blocked.
TOP FEED BURNER
This system allows for the accurate feeding of fuel to the combustion chamber and is
particularly suited to small appliances with on/off operation. After use the ash needs
to be cleaned from the grate. If low quality pellets are used clinkering or slagging of
the ash can occur.
Wood pellet burning stoves and boilers are designed and constructed to be very
efficient with low emissions and high levels of automation.
COMBINED HEATING
Wood pellet stoves with a back boiler can provide convenient hot water and
appliances are available that provide central heating and hot water for all building
types. These appliances range from simple 8kW boilers upwards to several hundred
kWs.
21Low carbon equipment and building regulations Biomass
6. Chimneys Hearths and Flues
CHIMNEYS
The purpose of a chimney is to take the combustion products (smoke and gasses)
from an appliance to the outside air and at the same time, to draw air for combustion
into the appliance. This movement of combustion air and exhaust is called draught.
For wood burning appliances to work successfully they must be connected to a
chimney with a correctly sized flue. For a flue to work properly the hot air must rise.
Factors such as running an appliance at a very slow rate or cold air leaking into a
flue may cool and slow down the combustion gases which will affect the performance
of the chimney.
For a chimney to operate satisfactorily it should be smooth internally, warm and as
straight as possible.
The use of the words 'chimney' and 'flue' can be confusing. BS EN 1443 defines a
chimney as 'a structure consisting of a wall or walls enclosing a flue or flues' and a
flue as 'the passage for conveying the products of combustion to the outside
atmosphere’. For the purposes of the building regulations a 'flue pipe' is defined as a
pipe that connects a combustion appliance to the flue in a chimney. See diagram
below:
Open Fire Example
Flue Liner – the material which forms the passage
through which combustion gases pass
Chimney – the structure that surrounds,
carries and supports the flue
Flue – the passage through which the
products of combustion are discharged to Gather, lintel and throat above the fireplace
the external air. recess formed by brick / blockwork or a pre-cast
chamber
Superimposed Hearth
Constructional Hearth
Fireplace Recess
22Low carbon equipment and building regulations Biomass
If the property does not have an
existing chimney then one can often
be constructed without too much
disturbance.
CONSTRUCTION MATERIALS
Chimney are generally made of
brick or stone but other materials
such as metal (system chimney)
can also be used. By using a double
skin insulated chimney system, a
stove can be installed practically
anywhere, and no existing chimney
is needed to fit the flue system.
Modern stove / boiler connection to chimney
CONNECTION OF THE APPLIANCE
The flue connection should be:
• vertical for at least 600mm from a top outlet appliance
• no more than 150mm long horizontally from a rear outlet appliance
• airtight
• no more than 450 from the vertical and
• easy to sweep both the connecting flue pipe and chimney.
HEARTHS
A solid fuel appliance (including wood biomass) should be provided with a solid, non-
combustible hearth that will prevent the heat of the appliance from igniting
combustible materials. Also any part of a dwelling that abuts or is adjacent to a
hearth should be constructed in such a way as to minimise the risk of ignition by
direct radiation or conduction from a solid fuel appliance located upon the hearth.
23Low carbon equipment and building regulations Biomass
7. Central Heating Systems and Controls
CENTRAL HEATING RELATING TO BIOMASS APPLIANCES
The boilers discussed here are for central heating. BS EN 303-5:1999 applies to
heating boilers for solid fuels, that are hand or automatically fired with a nominal
output of up to 300kW. This covers appliance properties such as performance,
efficiency, emissions, thermal output, pressure testing, safety measures and testing.
ACCUMULATOR TANK
A combined system for both space
heating and hot water is likely to
generate more energy (heated water)
than is required. Therefore, to get the
most out of the system it is best
practice to store the heated water in a
highly insulated heat storage tank,
called an accumulator. The tank
retains the hot water generated by
your heating system for use when
you need it.
This can be used in log, woodchip or
pellet boilers to deliver both central
heating and domestic hot water to the Typical log burning boiler
house for the rest of the day. A
further advantage is that an accumulator tank can be integrated for use with different
heat sources. For example the water could be pre-warmed by solar panels, or come
from other wood burning stoves, a kitchen range, electric immersion heater or an oil-
fired boiler which could also provide a backup heating system.
Log boiler with thermal store (accumulator) for hot water and central heating
Vent pipe
Cold water feed
Hot water cylinder
Log Boiler
24Low carbon equipment and building regulations Biomass
The heat from these systems cannot be turned off immediately so require inbuilt
safety devices for heat cut-off or ‘heat dump’ to allow them to be used with sealed
type heating systems.
HEAT CONTROLS
Wood biomass heating systems can be fitted with controls that will enable the
generated heat to be directed around the system in accordance with the wishes of
the building occupant.
All new installations must include heat and safety controls to minimise energy
consumption and carbon dioxide emissions and to ensure that the risk of harm to
users of a building is limited. Systems must be installed in accordance with the
building standards requirements of both Section 6: Energy and Section 3:
Environment of the building standards Technical Handbooks.
Controls for wood burning biomass systems have three functions:
• Safety - they provide additional safety in the event of misuse or mechanical failure
• Comfort - to control the comfort level in the building
• Efficiency - to reduce energy consumption.
Safety functions include:
• high limit thermostats to activate the pump if the system gets close to boiling. This
will dissipate the heat
• a low limit thermostat to switch the pump off to prevent condensation of the water
vapour in the products of combustion within the appliance and
• temperature control of stored hot water or water leaving the store.
The other 2 functions of comfort and efficiency are normally considered together and
must satisfy both Section 6: Energy and Section 3: Environment of the Technical
Handbooks.
In the majority of cases the following should be considered:
• the optimum temperature of the water distributed around the system
• control of the temperature of the stored hot water and/or the temperature of the
water leaving the hot taps
• time control for the operation of the heating system
• temperature of the space that is heated and
• zoning of the heating system for larger properties.
TIME AND TEMPERATURE CONTROL FOR HEATING SYSTEM
A full programmer which can be wired to the pump to control the on/off periods for
central heating and hot water is recommended. However, the level sophistication of
any time controls should be selected to be compatible with the appliance. It can also
be linked to temperature sensors to control the level of comfort.
The two main ways of controlling room temperature are:
• Thermostatic radiator valves (TRV) – it is important NOT to fit thermostatic valves
to any ‘heat leak’ radiator as it must be able to operate at all times. It is
recommended that at least one other radiator is NOT fitted with a TRV to allow
heat dissipation in an overheat situation.
25Low carbon equipment and building regulations Biomass
• Room thermostats – an effective control for a wood biomass system is a room
thermostat. This device monitors the temperature in the room and turns the
radiators on and off in order to help maintain the desired temperature and
efficiency.
Schematic layout of semi-pumped system
1. Heating boiler
2. Circulator and isolating
valves
3. Double feed indirect 6
cylinder
4. Hot water draw off 5
5. Cold water storage
cistern
6. Feed and expansion
cistern
7. Heat leak radiator with 7
2 full way lock shield 4
valves
8. Two port normally
open motorised valves 3
controlled by cylinder 3
thermostat (9)
10. High limit pipe thermostat
14
to bring on circulator in
an overheat situation 9
11. Low limit pipe thermostat
to prevent the pump
coming on until a 8
minimum temperature
is reached 12
10 11
12. Programmable room
1
thermostat
13. Injector tee to pump assist 2
the thermosyphon
circuit when the pump
is operating
14. Thermostatic radiator 13
valves to provide
temperature zoning
26Low carbon equipment and building regulations Biomass
8. Building Regulations
BUILDING REGULATIONS AND BUILDING WARRANTS
All wood biomass heating systems serving a building must comply with the
requirements of the Building (Scotland) Regulations 2004. The Building Regulations
are enforced through the building standards system with the Technical Handbooks
providing guidance on how to meet the mandatory building standards.
When installing a wood biomass system a building warrant is not normally required
in a 1 or 2 storey house but may be required for a 3 or more storey house and a flat
or maisonette.
A building warrant may be required for works to a 1 or 2 storey building that alter the
roof, external walls or elements of the structure of the building (further guidance can
be found in clause 0.5.1 of Building Regulation 5).
Building Standards Division in conjunction with the Scottish Association of Building
Standards Managers (principal officers from the Scottish local authorities), have
produced a guidance matrix on combustion appliances and associated work not
requiring a warrant. This matrix has been reproduced in the table to Appendix C, with
information relating to solid fuel appliances highlighted. If you have any doubts you
should seek guidance from your local authority.
PLANNING PERMISSION
Some biomass systems may require the construction of outhouses or areas to store
the wood materials. They may also require the construction of a new means of
access for service vehicles the provision of which may need planning permission.
Guidance is available in Planning Advice Note 45 Annex: Planning for Micro-
renewables.
TECHNICAL HANDBOOKS
Guidance on complying with the building regulations is given in the building
standards Domestic and Non-domestic Technical Handbooks 2011. This chapter will
highlight the issues in each section of the Technical Handbooks that should be
considered when installing a wood biomass heating system.
For further advice on specific projects please contact your local authority’s Building
Standards office. Contact details can be found in the telephone directory, the local
authority website, or from the website of the Scottish Association of Building
Standard Managers.
GENERAL - SECTION 0
Section 0 explains Regulations 1 to 17. It includes guidance on appropriate
standards of durability, workmanship and fitness of materials. A particular
consideration for a wood biomass heating system is that there is sufficient space
surrounding the system to enable access for maintenance and repair (see regulation
8).
27Low carbon equipment and building regulations Biomass
STRUCTURE - SECTION 1
Section 1 aims to ensure that the structure of a building does not pose a threat to the
safety of people in or around buildings.
For biomass heating systems, including associated water storage tanks, the
following issues should be considered to avoid damage to the structure of the
building:
• all loads imposed by the heating system installation on the structure including the
self weight of the appliance, fuel storage units, and radiator units.
• the installation of the chimney or flue may require changes to the building’s
structure, all such changes should be assessed by a chartered engineer or other
appropriately qualified person.
• where a constructional hearth is being installed it may be necessary (for example
where there is a suspended floor) for the trimming of joists and the provision of
additional support, which may require the involvement of a structural engineer or
other appropriately qualified person.
• water storage vessels may change the loads imposed on the structure of the
floors. Changes in loading should be assessed by a chartered engineer or other
appropriately qualified person:
o a supporting floor or attic floor needs to be strong enough, or made strong
enough, to resist the loads imposed by water storage vessels, including the
weight of water;
o for timber floor structures, loads should normally be shared across at least two
joists and it may sometimes be necessary to add members to provide sufficient
support for water storage vessels.
As with any system that distributes water around a building, consideration must be
given to the installation of pipes and ductwork:
• the installation of pipework and ductwork must not weaken the structure of timber
roofs, floors, or walls:
o notches and holes should not be cut in rafters, roof ties, collars or hangers
o notches should not be cut in wall studs, cripple studs or lintels unless a full
structural appraisal has been carried out by a chartered engineer or other
appropriately qualified person
o lightweight trussed rafters should not be cut, trimmed, notched or drilled
o figure 8 below, shows the safe locations and sizes for notches and holes in floor
joists and studs - if in doubt, ask a chartered engineer to check the proposed
installation.
Notches and hole locations in timber floor joists and studs
28Low carbon equipment and building regulations Biomass
FIRE - SECTION 2
Section 2 aims to ensure that the risk of fire is reduced. If a fire does occur measures
must be in place to restrict the growth and spread of fire and smoke to enable
occupants to escape safely and fire-fighters to deal with the fire safely and
effectively.
Where a biomass system is installed the integrity of any fire rated element must be
maintained both during installation and operation of the appliance. Further guidance
can be found in Section 2 of the Technical Handbooks.
It is important to identify if walls and floors are required to have a fire resistance
rating and to ensure that the installation of a wood biomass heating system does not
reduce their required performance (i.e. stability, integrity, and insulation). This should
include:
• separating walls and separating floors which are constructed to prevent the
spread of fire between buildings or between parts of a building, for instance
between flats or attached houses
• walls and floors of compartments, which are parts of non-domestic buildings that
are constructed to prevent the spread of fire to or from another part of the
building
• walls and floors that protect escape routes for example: a protected enclosure or
a protected zone.
For wood biomass heating systems, care should be taken to limit the risk of the
spread of fire through gaps in walls, floors, or ceilings, between cavities, and
between cavities and any other room or space in the building:
• walls and floors that are required to have a fire resistance duration and are made
of combustible materials, for instance timber frame separating walls, must not
contain any pipes or wiring. Pipes, wiring and ducts (other than a ventilating duct)
can pass through walls. However pipes from any solar water system on the roof
of a block of flats must not run down through timber frame walls between flats
• cavity barriers must not be compromised by the pipes or ductwork associated
with the biomass heating system – particular care should be taken not to disturb
cavity barriers separating roof spaces or at the head of a cavity wall, or at the
edges of intermediate floors
• pipes of less than 40mm diameter do not require fire stopping where they pass
through any wall or floor required to have a fire resistance duration (such as
separating walls or floors), or through a cavity barrier. Pipes with diameters of
40mm or more should be fire stopped to preserve the integrity of the wall, floor, or
cavity barrier
• if a roof is required to be fire-rated to prevent the spread of fire from one building
or part of a building to another, the same limitations on pipe work apply.
In any event, consult the relevant Technical Handbook and in particular for more
detailed guidance on service penetrations, see clause 2.2.9 of the Domestic
Technical Handbook and clause 2.1.14 of the Non-Domestic Technical Handbook.
Wood biomass heating systems must not compromise the fire performance of the
roof. For instance, the penetration of the roof by a chimney must not increase the
risk of the external spread of fire.
29Low carbon equipment and building regulations Biomass
ENVIRONMENT - SECTION 3
Section 3 aims to ensure that buildings do not pose a threat to the environment and
that people in and around buildings are not placed at risk from various sources,
including combustion appliances and the effects of moisture.
For wood biomass heating systems particular care should be taken to avoid moisture
damage to the building. To do this:
• any pipes, collectors, or fixings that penetrate the roof should be properly weather
protected to prevent the ingress of rainwater or dampness by using flashings
• any pipes or fixings that penetrate an external wall should be properly weather
protected to prevent the ingress of rainwater or dampness, for instance by
sealing small gaps around pipes
• pipes or fixings that penetrate an external wall should be installed in a way that
does not adversely affect any existing damp proof, waterproof or breather
membranes. If a membrane is damaged or disturbed, it should be repositioned or
repaired.
Safe operation of combustion appliances
Where it is intended to install a wood biomass system the building must also be
designed and constructed in such a way that each fixed combustion appliance
installation operates safely.
Where it is intended to install a wood biomass heating system and extract fan, the
system should be able to operate safely whether or not the fans are running.
However, where possible extract ventilation and biomass systems should not be
installed in the same room.
Where a hearth, fireplace (including a flue box), or system chimney is provided,
extended or altered information essential to the correct application and use of these
facilities should be permanently fixed in the dwelling to alert future workmen to the
specification of the installed system.
Standards applicable to wood biomass systems are:
• BS EN 14785: 2006 'Residential Space Heating Appliances Fired By Wood
Pellets'
• BS EN 12809: 2001 'Residential Independent Boilers Fired by Solid Fuel'
• BS EN 13229: 2001 'Inset Appliances Including Open Fires Fired by Solid
Fuels'
• BS EN 13240: 2001 'Room Heaters Fired by Sold Fuel' and
• BS EN 303 - 5:1999 'Heating Boilers. Heating Boilers with Forced Draught
Burners. Heating Boilers for Solid Fuels, Hand and Automatically Fired, Nominal
Heat output of up to 300kW'.
Chimneys and flues generally
Where a wood biomass heating system is being installed, the building must be
designed and constructed in such a way that any part used for the removal of
combustion gases will withstand heat generated as a result of its operation without
any structural change, that would impair the stability or performance of the system.
The process of burning wood can cause deposits of soot in the flue, chimneys and
flue-pipes therefore should:
30Low carbon equipment and building regulations Biomass
• be swept at least annually where smokeless solid fuel is burnt and more often if
burning wood, peat and/or other high volatile solid fuel such as bituminous coal.
Mechanical sweeping with a brush is the recommended method of cleaning
• have such capacity and be of a height and location with an outlet located so that
the products of combustion are discharged freely and will not present a fire
hazard
• be of a material that will safely discharge the products of combustion into the flue
under all conditions that will be encountered
• have outlet locations that are located for safe and efficient working, taking
account of height, prevailing wind direction and the following minimum
dimensions:
Minimum dimensions to flue outlets`
A 2300mm horizontally clear of the weather skin.
B 1000mm provided A is satisfied or
600mm where above the ridge.
C 1000mm above the top of any flat roof; and
1000mm above any openable rooflight, dormer or ventilator, etc. within
2300mm measured horizontally.
D/E where D is not more than 2300mm, E must be at least 600mm.
Notes:
1. Horizontal dimensions are to the surface surrounding the flue.
2. Vertical dimensions are to the top of the chimney terminal.
3. Specialist advice will be required for roofs with combustible coverings.
Clearance from combustion appliances
A flue pipe that passes through a roof space, partition, internal wall or floor must
have sufficient clearance from combustible material to avoid raising the temperature
of adjacent materials which may result in the possibility of charring or outbreak of
fire. These clearance dimensions may also be defined in the appliance or chimney
manufacturer’s instructions. A 'flue liner' is used to form the flue passage within the
chimney.
31Low carbon equipment and building regulations Biomass
The guidance to Section 3.19 of the Technical Handbooks (Combustion Appliances-
relationship to combustible materials) provides details of acceptable construction
clearance dimensions.
Relationship of metal chimneys to combustible material
manufacturers
declared
distance xx
There should also be a separation distance where the metal chimney runs through or
is in close proximity to combustible material. The separation distance should be
25mm from the outer surface of a single-walled chimney to the combustible material.
A metal chimney should not pass through a separating wall or separating floor.
However, when the chimney or a non-combustible fire rated casing totally encloses
the chimney and is constructed in such a way that, in the event of a fire, the fire
resistance is maintained then a metal chimney can be used.
A flue-pipe serving a wood biomass heating system should be non-combustible and
of a material and construction capable of withstanding the effects of a chimney fire
without any structural change that would impair the stability and performance of the
flue-pipe.
Combustible material should not be located where the heat dissipating through the
walls of fireplaces or flues could ignite it. Generally combustible materials should:
• be located at least 200mm from the surface surrounding a flue in a masonry
chimney
• have a separation distance where a metal chimney passes through or in close
proximity to combustible material.
A fireplace recess should be constructed of solid, non-combustible material in
accordance with the recommendations in Clauses 7 and 8 of BS 8303: Part 1:1994
and to the minimum thickness shown in Figure 2 to BS 8303: Part 3:1994.
All combustion appliances have the potential to cause carbon monoxide (CO)
poisoning if they are poorly installed or commissioned, inadequately maintained or
incorrectly used. Any chimney or flue-pipe installed in a building should:
• be suitable for use with the type of appliance served
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