Biomass is living and recently dead organic matter which can be used as fuel. When it is burned, carbon that it absorbed when growing is released into the atmosphere. As this happens over a relatively short timescale it is considered carbon neutral, as long as organic matter is planted to replace any burned.

Biomass fuels include animal, food and industrial waste, high energy crops such as miscanthus, willow, rape and maize and a range of wood products. For domestic heating purposes the main fuels used are wood in the form of logs, pellets or chips – although there are boilers available that burn a range of cereals.

Wood fuels can be used in stoves for room heating, and water heating if a back boiler is installed. Alternatively they can be used to power the main boiler, producing hot water and heating for the whole house.

Unlike most renewable energy technologies where there is an upfront investment, then the element that provides the power (ie wind or sun) is free, there is an ongoing fuel cost with biomass heating.

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.

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.

Stoves are suitable for heating any room that has a chimney or a flue. If you are in a smokeless zone you will need to check the appliance before you buy. Some burn the wood so cleanly that they are certified for use in smokeless zones. Some people find the noise of the fans on these stoves annoying, so check how loud it is before you buy.

If you want to attach a back boiler, you will need to check with your installer or plumber whether additional changes to your plumbing are necessary, especially if you’ve got a combi boiler.

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. They require 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 initial assessment tool (it's an Excel file) and/or its publication Biomass heating: a practical guide.

It is important to keep fuel dry, as a higher moisture content in the fuel will reduce the efficiency with which it burns. If wood pellets get wet they turn to unusable mush. Wood pellets can be delivered loose in a lorry and blown into a hopper, or in bags. You need access for a delivery lorry if you don’t want to spend lots of time carrying bags to and fro.

The boiler will need a flue in which the vent material is designed for wood fuel (existing chimneys can be lined). The installation must comply with all relevant building regulations. If you live in a listed building or a conservation area you will need to check with your local planning authority before fitting a new flue.

It’s important that you check that the boiler will work with your existing plumbing, or whether it needs to be altered. Also find out how easy it will be to get the boiler serviced regularly and whether there are local plumbers or engineers who know how to work with it.

You’ll also need to find out if there is at least one, or preferably a choice of, local fuel suppliers, as the cost of fuel varies according to the distance the supplier has to travel. Also check how they can supply the fuel. The lowest maintenance way is in a tanker to a hopper. A pallet full of bags will involve more work feeding the boiler. The National Energy Foundation has an up to date list of wood pellet suppliers. You can search for a local wood fuel supplier at Big Barn.

Wood pellet boilers will need an annual service.

All wood fuels need more storage space than fossil fuels (oil or LPG). The three types of fuel most commonly used are logs, wood chips and wood pellets. Of these, the latter two can be used in automated systems and stored in a hopper. Logs must be manually fed into the boiler and they are less automated, so their efficiency is more dependent on human input. If the wood isn’t adequately seasoned or contains too much moisture, or if the air supply is reduced too early, they will not burn as well and will create smoke and tar.

Wood pellets are made from compressed sawdust and wood shavings and other biomass products and are uniform in size and shape. They have a higher energy content and so take up less storage space than logs or wood chips. Stove and boiler manufacturers specify the size, shape and moisture content their products need to perform well. Wood pellet systems are the smallest, neatest and most like a mainstream boiler and require the least input from the user.

Wood chips are cheaper and abundant. They allow for more mechanisation than logs, but are not as efficient as wood pellets. It’s important that they are pretty uniform in size to work smoothly in a automated domestic system.

If you don’t have a hopper, you will have to load sacks manually into the boiler or stove’s feed system. How frequently you have to do this will depend on the system you choose. Not all suppliers are able to supply loose pellets.

It's important to think through carefully the supply, storage and handling of fuel before you invest in a biomass boiler. There is generally some trade offs between each element, and they will be specific to your site. You need to consider ease of access for the fuel delivery and how you are going to get fuel to the boiler.

To maximise the efficiency of your appliance it’s important to get the moisture content of the fuel right. Ideally logs and wood chips will have a moisture content of less than 25 per cent. Wood pellets need to be dryer – at around 8 per cent moisture. Your supplier should give you information about how to achieve this.

Automated wood pellet stoves 5-7kW in output range from £2 - £4,000
Boilers are in the range £3 - £16k including installation, flue and commissioning. Because of significant differences in price it can be difficult to weigh up the competing claims of different suppliers. Research by Oxford University's Environmental Change Institute found that the average price of a wood-fuelled boiler system installed in the first two years of the LCBP grants was £8,900, with high variability in price, and no correlation between price and the thermal output of the boiler.

When choosing a biomass boiler it’s important to consider the ongoing cost of fuel as well as the initial investment.

As at November 2008 wood chips are the cheapest form of heating fuel at 2.3p per kWh generated according to Biomass Energy Centre. Mains gas and wood pellets both come in at 4.2p per kWh, with oil a bit more expensive at 4.5p. LPG and electricity are more expensive still at 7.1p and 14p per kWh respectively. Prices are around £225 per tonne for wood pellets and £80 per tonne for wood chips. These figures will depend on where you live, as prices for wood pellets and chips vary considerably.

Low Carbon Buildings Programme grants are available for biomass heating. For automated wood pellet-fed room heaters/stoves you can get a grant or up to £600 or 20 per cent of the relevant costs, whichever is lower. There is an overall maximum of £1,500 (or 30 per cent of the costs if that’s lower) on purchases of wood fuelled boiler systems.

There are additional grants programmes for Scotland and Northern Ireland. To find out if there are any local grants available in your area you can ring your local energy advice centre on 0800 512 012 .

Not unless the flue exceeds 1m above the height of the roof, or it is installed on the principal elevation and visible from a road in buildings in conservation areas and world heritage sites. If you live in a listed building check with your local planning department whether consent is needed. More details are available on the planning portal . Make sure that your installation meets the standards of the relevant building regulations on ventilation, noise and safety.

For those of you living in Wales, from the beginning of September 2009, the Welsh Assembly Government announced new planning rules to encourage householders to install renewable energy equipment. A leaflet has been published to explain the changes - Domestic microgeneration permitted development: A guide for householders.

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Sources:

• Biomass Energy Centre
• Centre for Alternative Technology
• Energy Savings Trust
• Logpile
• National Energy Foundation
• The Guardian - Dominic Murphy's experience of a biomass boiler