Housing Retrofit: Wood fuel space & water heating

Wood-fuelled boilers should present an easy, sustainable alternative to gas heating. Though not widespread in the UK, wood fuel has become much more popular in Europe and North America. In Austria, for example where there is a more dynamic economic drive coupled with greater availability of wood, around 80% of new homes are equipped with wood pellet boilers as standard.

In addition to their greater user-base, most of the recent technological development of wood-fueled boilers has been in Austria and Germany - leading to their prevalence in the emerging UK market. But, though there is clearly a growing demand for wood-fuelled appliances in this country, a necessary associate well-functioning market for wood chips and particularly wood pellets has yet to develop.

Another issue that is becoming the subject of growing concern is that of air pollution as a result of burning wood. This has led recently to a suspension, in many cases, of institutional support for mass implementation of biomass fuel until more studies of the problem have been undertaken and possible solutions developed (see also: LB Camden: Planning and air quality).

Advice for the specifier, therefore, must be that before selecting a wood-fuelled boiler, they must consider both the issues of fuel supply and potential air pollution.

wood chip fuel


Design considerations

• Refurbishment provides the opportunity to install a new or replace an existing heating system with a new more efficient type.

• There are a number of factors which need to be taken into consideration when choosing a new system:

• Fuel type:
Commonly: wood chip or wood pellet will in large part determine the type of boiler specified (log fuel is much more demanding and should not be considered unless it is an explicit user preference). Some boilers designed for wood chips can also burn pellets, however boilers designed specifically for pellets cannot generally use wood chips.

• Availability of fuel:
An assured supply will be needed along with guarantees of price and frequency of delivery. Consider an organised group (eg of residents) to optimise supply.

• Access for delivery:
Consider delivery vehicle size, widths and head-heights.

• Boiler size:
The size of the boiler needs to be carefully calculated based on projected hot water and space heating demand – particularly where a building has undergone significant insulation and air-tightness improvements. For well-insulated houses, even the smallest of domestic boilers currently available will be over-sized for the job.

• User needs:
The pattern of use might determine the type of boiler selected. Wood fuel boilers work best whilst under a relatively constant load. If usage is intermittent, the need for hot water storage will have to be addressed.

• Available space:
Selecting wood fuel heating will put extra demands on available space. Installations will require extra space for storage and appliances will probably require more space than an equivalent gas system.

• Boiler location:
Consider carefully the boiler’s location in relation to the thermal / air tightness envelope – putting a wood fuelled boiler together with its feed mechanism within the envelope can lead to significant heat loss due to its contribution to air leakage and thermal bridging.

• Flue size and ventilation requirements:
Wood fuel boilers make substantial demands on ventilation compared with their gas fired equivalents. The flue will have to provide sufficient air movement for the proper operation of the boiler.

• Need for back-up system:
If there is likelihood of a wildly fluctuating load pattern on the boiler, it might be sensible to provide a back-up boiler, whether wood fuelled or gas. The main boiler would, typically, provide 80% of the annual energy need and the back-up appliance would come into operation when the heat load peaks.

• Increased capital cost:
Wood pellet boilers are typically much more expensive (circa. £10 - 15k for an average domestic boiler) than their gas fired equivalent, as is currently their fuel. Wood chip boilers are more expensive again (often double) than pellet boilers.

• Ash storage and removal:
Ash is the inevitable product of wood combustion. A system of storage and removal, usually weekly, needs to be planned as part of the installation.

• Maintenance regime:
Though durable, wood fuelled boilers will produce an amount of residue, which will need cleaning from within the appliance. A regular maintenance schedule, usually weekly, should be identified and implemented.

wood fuel boiler


Wood fuel boilers

• NB:
Before specifying a boiler and heating system, supply a full performance brief to and consult with the manufacturers of the equipment as part of the design process. It is advisable too, to consult a specialist heating engineer.

• Boiler type:
The choice is usually between wood chip and wood pellet (see fuel feeding below). Wood pellet boilers are more like conventional boilers including self-igniting and self-regulating. They are also more efficient, typically burning up to 94% efficiency. When selecting a boiler type, consideration should be paid to the storage space available for fuel. Wood chip fuel needs between 2 and 3 times as much storage volume per unit of output as does wood pellet fuel.

• Operation:
Apart from their fuel, wood fuelled boilers differ from gas boilers primarily in their operation mode. Gas boilers can ‘idle’ for very long periods, or shutdown until required whereas wood boilers are most effective when operating continuously. Most wood boilers will provide a facility for keeping alight at a minimum output. Though effective in keeping the boiler fired in preparation for use, it is clearly energy wasting. A solution is to include a 'thermal store‘/accumulator‘/’buffer’ tank (hot water storage) in the system. Heat generated by burning fuel during periods of no demand, heats water which is then stored for later use. A useful size for an accumulator will be in excess of 800 litres.

• Boiler components:
Appliances commonly comprise of either an integral or external storage hopper that feeds a combustion chamber which in turn heats the water passing through the heat exchanger. For efficiency, fuel burning takes place with small amounts of fuel and at high temperatures.

• Size:
For a small, well-insulated house, the smallest boilers rated at around 15 – 20kW will likely be suitable. For houses performing to near or at Passivhaus standards (typically well under 3,000 kWh/yr), most boilers currently on the market will likely be considerably oversized. Currently, the smallest boiler commercially available is rated at 9.5kW.

• Fuel feeding:
Will usually be automatic via a motorised auger from a manually filled storage hopper. The fuel will be fed into the boiler from above, below or from the side. The design of the fuel feed is what distinguishes the type of boiler. Wood pellets differ from wood chips in their size and uniformity. Pellets are small, dry, uniform and ‘flow’ easily. Consequently the feed mechanisms to specifically pellet-fuelled boilers tend to be more sophisticated in their ability to ‘flow’ the fuel from the store to the firing mechanism. This level of sophistication can provide for a more flexible design when locating the fuel store in relation to the boiler – for example a boiler can be located within a building and can be supplied via a tube from an external store.

• Operating temperature:
Boilers are fitted with thermostats for control of water temperatures and overheating of the system will be protected by safety temperature limiters.

wood pellet boiler and storage hopper
wood pellet boiler and storage hopper


Fuel types


Wood pellet

• The fuel is usually made from compressed sawdust and wood shavings but can be made from most biomass material (eg straw, forestry residues and specially grown energy crops)
• Pellets are usually produced at an industrial scale, though developing technology is enabling smaller-scale production.
• There is not yet a consistent quality of pellet in the UK. Pellets vary according to source.
• Care should be taken when using recycled wood because of the risk of contaminates.
• Pellets are more expensive than wood chip / logs.
• Sources are unlikely to be local – necessitating transportation over long distances – so adding to cost and carbon emissions.
• Pellet quality is crucial – boilers can be ‘intolerant’ to certain sources.


Wood chip

• A local supplier is needed
• Good for district heating operations
• Quality tends to be variable
• Irregular moisture content
• Care should be taken when using recycled wood because of the risk of contaminates such as formica or melamine, plastics and paint.



• More suitable for small operations
• Logs are easy to handle.
• Logs are more often sourced from a known and existing supply chain.
• They can be produced from small scale woodland
• They need to be loaded manually at least once a day


Heating system

The heating system accompanying a wood fuel boiler will be a ‘vented’ or ‘open’ system. The system illustrated below includes a thermal store which acts as an accumulator/buffer tank to the boiler. The Primary water is stored in the cylinder and heated by the boiler (and possible also an ancillary source such as solar thermal). When heated by a boiler the water can also be pumped around the radiators to provide space heating. The Secondary water comes directly from the mains and passes through a heat exchanger which draws heat from the Thermal Store. This water then passes through a thermostatic mixing valve which blends it to the correct temperature before it reaches the taps.

The thermal store illustrated is a 'combined' type, where the expansion tank is integral to the store rather than located separately in the loft. It is important to recognise that when using a store of this type, the expansion tank must be installed at a level higher than the radiators to enable a 'head' of water.

wood fuel boiler heating system

Additional heating sources

The nature of a ventilated heating system is that it can easily include another heat source such as a solar thermal system. In summer, the solar system will provide heat to the hot water storage tank – reducing the demand on the boiler. In Spring and Autumn the boiler and the solar system will work in tandem and in Winter the boiler will provide most of the heat to the store.



The controls regulating a modern heating system provide the user with the capability to minimise energy expenditure. A good and user-friendly control system will only provide heat when and where it is needed. It also ensures that the boiler does not operate unless there is demand for it to do so.
A capable control system will allow a user to:
• Match their occupancy pattern by providing timed water and space heating periods.
• Control individual room temperatures to comfort levels to reduce overheating.

heating zones
• Divide larger houses into zones, each with dedicated time and temperature controls. Zones might reflect usage (eg upstairs / downstairs) or thermal dynamics (eg solar gain areas).


Commonly specified controls will include:

• A time switch which controls space and water heating
• A full programmer which allows the time settings for space and water heating to be fully independent
• Room thermostats which controls the switching on and off of the space heating according to desired temperatures
• Programmable room thermostats perform the above but with the ability to pre-set heating periods.
• A cylinder thermostat, where storage is installed, allows water heating to be switched on and off according to the desired cylinder water temperature.
• A thermostatic radiator valve (TRV) which adjusts the water flow according to room temperature.




Building Regs

• Building Regulations Approved Document ADL2 sets out the requirements for existing buildings.
• The regulations do not oblige the designer to perform DER/TER calculations, but where a new system is installed or an existing system replaced, the installation must follow the requirements of the ‘Domestic Heating Compliance Guide’ (DCLG, 2006).



Domestic Heating Compliance Guide, DCLG, 2006
Domestic Heating Design Guide, CIBSE, 2007



• BS 1566:2002 Copper indirect cylinders for domestic purposes.
• BS 3198:1981, Specification for copper hot water storage combination units for domestic purposes
• BS EN 12897:2006 Specification for indirectly heated unvented hot water storage water heaters
• BS EN 12828:2003 Heating Systems in Buildings. Design for water based systems.
• BS 7671:2001, Requirements for electrical installations, IEE Wiring Regulations, Sixteenth edition.
• BS 7593:2006, Code of Practice for Treatment of water in domestic hot water central heating systems.
• BS 5440:Part 1 and Part 2:2000, Installation of flues and ventilation for gas appliances of rated input not exceeding 60kW (1st, 2nd and 3rd family gases.
• BS EN 483:2000 Gas-fired central heating boilers – Type C boilers of nominal heat input not exceeding 70kW.
• BS EN 12831:2003 Heating Systems in Buildings. Method for calculation of design heat load.
• BS EN 15316-1:2007. Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies. General
• BS EN 15316-2-1:2007 (3 parts) Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies.
• BS EN 15316-3:2007 (3 parts) Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies.
• BS EN 15316-4:2007 (6 parts) Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies.
• The Boiler (Efficiency) Regulations 1993, SI (1993) No 3083, as amended by the Boiler (Efficiency) (Amendment) Regulations 1994, SI (1994) No 3083.
• Domestic Heating Compliance Guide, (Compliance with Approved Documents L1A: New Dwellings and L1B: Existing Dwellings), First Edition, Communities and Local Government.


Further information

• HETAS, the official body of solid fuel domestic heating applications, fuels & services (www.hetas.co.uk)
• Hot Water Association (www.hotwater.org.uk)
• Heating & Hotwater Industry Council (www.centralheating.co.uk)
• Association of Plumbing and Heating Contractors (www.competentpersonsscheme.co.uk)
Thermal Storage



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