Central Heating with Wood Boilers
|Wood is a very versatile fuel and can be burned in many different forms to provide central heating. Until recently wood fuelled heating has 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.
Fuel and System Options
The main fuel options available for central heating with wood are logs, wood pellets and wood chips.
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.
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 log 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.
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.
Heating output is specified in kilowatts, kW, (metric units) or BTUs (Imperial units) and represents the rate at which the system can deliver heat energy.
Sizing of heating systems should be done by a qualified heating engineer. It is dependent on many factors including levels of insulation and draught proofing of your home, lowest outside temperature of your locality and patterns of use. However the following 'rule of thumb' can be useful for making initial sizing estimates for central heating boilers:
Boiler size (in kW) = volume to be heated (in cubic meters) divided by 34 (for a reasonably well insulated house).
It is important to bear in mind that all biomass boilers burn most cleanly and efficiently when working at their maximum output. Therefore it is best not to over-specify but to choose a biomass boiler which is sized to meet your average heating requirements with additional heating sources to provide extra heat on the coldest days.
All forms of wood heating are suitable for combining with solar water heating to provide domestic hot water during the months when central heating is not needed. This is the most environmentally friendly option and means that heating requirements are supplied from 100% renewable sources.
The most economic and effective system to realise a full solar heating system for single family houses is at present the combination of a wood boiler, a water tank and thermal solar collectors (2-3 m³). In summer the solar system provides hot water and stores it in the tank.
In Autumn and Spring the solar system can also provide some water heating (dependent on its size). In winter the biomass boiler uses the heat store, which allows full power operation of the boiler 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.
Wood as a fuel including retail grading system for woodchip: www.britishbiogen.co.uk/bioenergy/heating/woodasafuel.htm