The main biomass fuel is wood in the form of logs, pellets or wood chips – although there are boilers available that burn a range of cereals. The full range of biomass fuels also include animal, food and industrial waste and high energy crops such as miscanthus, willow, rape and maize.
Biomass boilers tend to be larger than the gas or oil equivalent. They are generally more suitable for people not connected to mains gas who have some space for storage. You will need about 6-7 cubic metres of space near where the boiler is sited to store the fuel (for an average size house). To do a detailed initial assessment of whether or not it’s appropriate, you can download the Carbon Trust’s publication Biomass heating: a practical guide for potential users.
How do biomass boilers work?
Wood is hardly a new fuel for heating houses, but the technology has improved considerably to make it more efficient. Open fires may look lovely, but they are not a good way to heat a room. Most of the heat goes up the chimney and, as the fire draws in oxygen to burn, it creates draughts in the room that can cancel out the benefit of the heat.
Modern wood-burning stoves are a huge improvement on the open fire for room heating. They convert 70 per cent of the fuel into useful heat. If you attach a back boiler, they can also help heat water and supply some radiators.
More efficient still are automatic pellet stoves which operate at 85 to 90 per cent efficiency. They spread the heat through convection, rather than traditional radiation, which means the room is heated more evenly and efficiently using a fan. They are clean and easy to use, with automatic ignition and a thermostatic control. They have an integrated hopper, which automatically tops up the fuel. They generally hold enough fuel for one to three days operation. The ash pan needs to be emptied about once a month. It is also possible to add a back boiler to these.
Biomass boilers can replace oil or gas boilers to heat hot water and radiators (or under floor heating). They burn logs, wood chips, wood pellets or other forms of biomass. The most advanced boilers are fully automatic. They control the amount of fuel and air supplied to the combustion chamber. As a result they are highly efficient and emissions are low.
They are fed with wood chips or pellets from a large hopper sited nearby. If you’ve got space, manufacturers recommend a hopper that’s big enough to hold a year’s supply of fuel. This minimises transport and delivery costs for fuel, as well as work for the owner. Maintenance is minimal – although you will need to clean it and remove the ash about once a month. If that isn’t possible due to space or budget, you can get wood pellet delivered on pallets of 10kg bags, from which you manually fill a smaller hopper.
At the other end of the scale, log-fed boilers are more suitable for people with ready access to a supply of wood, and time to cut it to the right size. These will need more time spent on feeding them with fuel and cleaning out the ash.