Biomass (wood)
Wood is a very versatile fuel and can be burned in many different forms to provide central heating. Until recently wood fuelled heating had the drawback of a lack of controllability. Automatic wood fuelled boilers and many stoves overcome this problem by utilising thermostats which automatically control fuel and air intake with very responsive and programmable temperature settings.
The main fuel options available for central heating with wood are logs, wood pellets and wood chips.
Logs
Logs are the most easily available and common form of wood fuel in the UK. They are often used in wood burning stoves for direct room heating but can also heat water for central heating systems – either in a stove with a back boiler or a log burning boiler designed for burning this fuel. Whereas log stoves have an efficiency of around 65%, 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 and regular deashing. This problem can be overcome by ensuring that a large volume of hot water storage (accumulator tank) is installed. This reduces the frequency of refuelling to around once a day and enables the boiler to operate at optimum efficiency.
Pellets
As with logs, pellets can be used to fuel stoves (with or without back boilers) and pellet burning boilers especially designed for the purpose. Pellet stoves require less attention compared to lo g appliances and are the most convenient wood fuel to use in a domestic setting. Integral fuel hoppers store enough pellets for 1 to 3 days operation and the ash pan only needs emptying between once a month and once a year. Wood pellet boilers are fully automatic and almost as convenient as using gas or oil. They are well suited to meet variable load demands and can be operated on a timer. Pellet stoves and boilers operate at high efficiencies of around 90%. Being a very dense fuel pellets require less storage space than logs or chips.
Wood Chip
As with wood pellets modern wood chip boilers can provide a high level of automation and convenience for wood fuelled space heating. Wood chip systems generally have an output of greater than 20 kW (suitable for a large farmhouse or larger) and are not cost effective or appropriate for typical domestic scale applications. Extensive fuel handling systems and fuel storage facilities are required for automated operation. It is important to be able to source a steady supply of woodchip with a consistent size and moisture content suitable for burning in a boiler, as not all chips are suitable for burning.
Planning permission may be required in conservation areas if a flu is unavailable, but otherwise should not be needed for biomass systems. Although an exemption certificate is required in smokeless zones.
The cost of installation is around £1000 - £1500 with boilers costing between £3000 - £8500 and stoves with back boilers in the region of £600 - £700. There will also be a continuing cost of fuel, which will vary depending on what is required for the system and suppliers. Chimneys will also need to be cleaned every few years.
Most large stove manufacturers also sell stoves with a back boiler, some of which will operate on logs or on multiple fuels including logs. The smallest stoves only have a direct heat output of around 2kW and will often only provide partial hot water. This can be backed up with an electric immersion heater and/or a solar water heating system. They may also have the ability to feed a couple of radiators. Larger stoves are designed to provide a restricted heat output into the room in which they are located, but to provide a much greater heat supply to the back boiler. These will typically provide around 4kW of heat to the room, with 11kW to the back boiler, allowing a full central heating system of up to 10 radiators to be fed as well as meeting normal hot water needs.
