Equipment
For most installations, logs or pellets are the most suitable type of fuel as the equipment is compact and storage space can be more easily accommodated. You can choose from a wide range of stoves or boilers, depending on your heat demand and usage.
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For a single room, a stove would be the best option. Many people in rural Scotland need a backup source of heating in the event of a power-cut; stoves which can burn logs or briquettes provide the ideal solution and are an attractive focal point in a room. Pellet stoves are also an attractive option but please note that these do need an electricity supply. Stoves are easy to install if a suitable fireplace or chimney already exists, or complete flue systems can be fitted where appropriate.
Stoves can be supplied with back boilers to provide hot water as well as space heating and larger wood-burning can also run central heating systems, if the necessary plumbing arrangements can be made. All central heating systems require electricity to operate the pumps and valves which circulate the water.
Log and pellet boilers for central heating, though compact, are larger than oil or gas boilers, and need to be sited in a separate room or in an outbuilding adjacent to or adjoining your house. Log boilers are fuelled manually once or twice a day. Pellet boilers with an integral hopper can be fuelled manually with pellets every couple of days, or else pellets can be stored in a silo and fed into the boiler by an automatic stoker unit.
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To run an efficient log fired central heating system, the logs must be burnt quickly at high temperatures so that the gasses are burnt completely, eliminating tar deposits and smoke. The heat produced must be stored in order to run a central heating system all day usually in the form of a highly-insulated water tank called an accumulator tank, containing 1500 litres or more. The boiler heats the water to around 95oC which then heats the domestic hot water tank and circulates around the central heating system. Sophisticated control systems ensure that by the end of the day the radiators are still warm, there are a few litres of the hot water left in the accumulator tank and the boiler controller is indicating that it is time to refuel the boiler.
In contrast pellet boilers can operate without an accumulator tank, as the stoking is controlled automatically, though depending upon the heating requirements it may be more efficient to install one.
Storage
When designing a domestic log heating system, you must give some thought to the storage of the logs. A three bedroom house with a boiler of around 15kW could require up to 8-10 m3 of solid wood each year, when split and stacked this would take up around 15-18m3 of storage space. In addition you should allow the same volume to store and season logs for the following year!
A pellet boiler of similar output would require 4-5 tonnes of pellets annually (around 7-8m3), about half the storage space needed for a years supply of logs. Less storage space is required if quarterly deliveries of pellets could be arranged. For domestic users the minimum delivery of pellets is usually one tonne, which can be supplied in 15kg plastic sacks to make handling easier.