Choosing a wood fuel heating system
Wood fuel heating systems are best suited to providing a continuous heat load and burn most cleanly and efficiently when working at their maximum output. Therefore it is best not to over-specify a wood fuel boiler but to choose one which is sized to meet your average heating requirements with additional heating sources to provide extra heat on the coldest days.
A good way to achieve optimum firing is to use an accumulator tank which allows the boiler to operate at maximum output to heat a large reservoir of water that is stored in an insulated container until required.
Unlike oil and gas, wood, in whatever form, does not achieve an optimum burn instantaneously and wood fuel boilers are not designed to continually cycle on and off. Therefore if a continuous, low level heat output is routinely required biomass may not be the best choice.
This can occur, for instance when the boiler is being used for both high level output for winter heating and low level output for summer hot water. A sensible solution to this problem is to install a solar hot water system to provide summer hot water to the water tank, while the wood fuel boiler is switched off.
This is the most environmentally friendly option and means that heating requirements are supplied from 100% renewable sources, and that no wood fuel is required in the summer. In Autumn and Spring the solar system can also provide some water heating (dependent on its size) so reducing the amount of wood fuel required. In winter the wood fuel boiler serves the heat store, allowing full power operation and continuous heat retrieval at any required load.
Wood fuelled heating systems can also be linked with conventional gas, oil or electric water heaters to provide a convenient summer water heating source.
80% of the whole life cost of a heating system is the fuel, and the availability of an affordable, secure supply of appropriate wood fuel nearby (preferably within 20-30 miles) is most important. This should be integral to the planning of any wood fuel heating system and the equipment must be specified accordingly.
Where will the fuel be stored and how will this store be filled?
The storage of wood fuel must be well designed and constructed in order to keep the fuel in a good condition, particularly protecting it from moisture. It must also be possible to deliver the fuel to the boiler conveniently and efficiently.
Wood chip and logs are unable to flow as with oil or natural gas and pellets do not flow as freely. Each type of wood fuel therefore requires a different design of store. For further information see the individual sections on types of wood fuel.
The cost of heating with wood fuels
The following chart shows fuel prices per kWh (excluding VAT) from November 2008 (Provided courtesy of the Forestry Commission)
Prices are for guidance only and can vary significantly with geographical region, order quantities, overall contract size and duration, time of year, delivery distance and time, etc. The efficiency of the heating system is also important when calculating a kWh cost.
Logs are not included in the charts due to a wide variation in prices, from free to those who have access to their own wood, to the cost of logs bought in individual bags. An indicative price of logs purchased in bulk is £80 per 400kg load of well seasoned logs.
(Graph 2 here)
Within each category of boiler (gas, oil, wood etc) there is a wide range of costs depending on size, efficiency, make, model etc.
The following is therefore given as a general indication only of what one might expect to pay for the type of boiler that would heat a typical three bed semi (around 15kW), excluding VAT:
- Condensing Gas or LPG Boiler – £500 to £600
- Condensing Oil Boiler – £1000 to £1100
- Solid Fuel (Coal) Boiler – £1400
- Coal or Wood Stove with back boiler to run 10 radiators – £600 to £700
- Pellet Stove (without back boiler) – £1400
- Log/Pellet Boiler – £4500
In addition there is the cost of the flue and installation-typically in the range of £1000 to £1500 plus VAT depending on the type of heating system installed. In addition there will be the cost of fuel storage to consider.