Wood-fired or Biomass Heating

Biomass describes anything which grows, but most of the time biomass heating refers to burning of wood products, being logs, chips, pellets or briquettes. As long as something grows in the place of a harvested tree, as almost invariably does, there is virtually no adding of greenhouse gas to the atmosphere in the long run, unlike when burning oil, coal or gas (which also require much processing and transportation). This section refers to the most efficient and practical ways to use wood as an alternative to conventional central heating systems.

How does biomass heating work?

Biomass heating systems can be separated into:  A - Room Heaters & B - Central Heating Systems 

Room heaters

  • Log stoves – An open log fire is made hugely more efficient if replaced by a log stove, delivering perhaps 70% of the wood’s heat energy to the room rather than 20-30% and drawing far less cold air into the building as flow up the chimney is very restricted, reducing draughts. Stoves can protrude into the room or be ‘inset’ or ‘cassette’ varieties, flush with the wall.
  • Automated pellet room heaters – these are hand-filled every few days with wood pellets and controlled by timer and thermostat to warm the room as and when required. They have a visible living flame but are very quiet and can also heat radiators and/or a water tank (although they then need refilling much more frequently).

Central heating systems

  • Log, or ‘batch’ boilers – these are filled with very dry logs or strawbales once to three times daily to quickly heat a large tankful of water (a thermal store) which in turn supplies the radiator circuits in response to a timer/programmer and thermostats, for as long as the stored heat lasts.
  • Automated chip or pellet boilers – these have large fuel silos within 15m of the boiler, drawing the fuel in as required and behaving otherwise like a conventional oil or gas boiler. Some can only run on pellets, often fed through flexible pipe using a suction pump, whilst others can use chips or pellets, with a heavy duty auger between silo and boiler.

What are the differences between pellets chips and logs?

The principal pros and cons of the three fuel types are as follows:

Logs – log systems have fewer wearing parts and are simpler and cheaper to install. The downside is that handling the logs requires MUCH more labour time on a daily basis. This is OK if the house to be heated is of small-to-average size, or if you have a constant supply of free labour on site, but it is the reason why many such systems are regretted.

Chips – The advantages of chip systems are that they require much less regular intervention than log systems – they can be left alone for up to a month at a time – and that they can be supplied from local sources, making you independent of world energy markets and price fluctuations. They are ideal for landowners with commercial forestry near their homes as use of low value wood for heating can justify the cost of thinning and other beneficial forestry management operations. Chip systems suit large heat demands such as very large houses and/or groups of dwellings and/or commercial buildings which can be served from a shared boiler in a ‘District Heating Scheme’, as large fuel cost savings are necessary to justify the high cost of the fuel handling equipment.

Pellets – wood pellets are a highly processed product, very dry and dense, which flow a bit like a liquid and take up a quarter of the space of chip with the equivalent heat content. As a result the storage and handling equipment is much cheaper and more compact which makes it more suitable where heat demands are more modest and/or there is less space available for storage or for the manoeuvring of delivery vehicles. A blower delivery lorry must get to within around 15m of the pellet store, whilst a chip trailer needs to be able to tip its full load over a lip into a hopper within a few metres of the boiler house.

Strawbales – Typical straw burners take the very largest round or cuboid bales loaded once or twice per day, so may be suitable for heating farm houses or industrial processes on or adjacent to arable farms where the labour and equipment for regular loading of bales into the boiler is always on hand, and storage of 2-400 bales adjacent to the boiler house is practical.

Briquettes – these are very large pellets, typically the size and shape of a large tumbler, which are a clean-handling and easy-lighting alternative to logs, house coal or anthracite nuts for use in open fires or stoves.  

Is my house suitable?

When considering which biomass boiler system is most suitable for your home, the main issues to consider are: -

  • Fuel supply – The fuel supply must be reliable for the foreseeable future. Logs and chips are too bulky to be transported more than a few miles economically, whilst pellets can be shipped around the world making them a global commodity, without much effect on their bulk cost or environmental impact. Obtain quotes for fuel delivery, chipping, etc, as appropriate, and discuss delivery methods, width or height restrictions, turning space requirement and even cleaning/de-ashing services, before making any purchasing decision. Wood fuel suppliers must guarantee minimum quality standards relating to moisture content, particle size, etc.
  • Heat cost – the relative costs of the different heating fuels is estimated in the table below, compared with equivalent costs, allowing for boiler efficiency and servicing costs for a 3-bedroom detached house in Scotland, from the April ’13 Sutherland Tables.

*Price for radiator use only, not peak tariff backup or hot water heating

Notes

  • Pellet price £190/tonne inc. delivery for min. 5 tonne delivery and service £150 more than gas. Delivery can significantly affect price if you are over, say, 50 miles from the depot.
  • Chip price £100/tonne at 30% moisture content and servicing £500 more than gas
  • Log price £150/tonne at 20% moisture and same service cost as pellet, but NOT allowing any labour cost for daily fuelling
  • Space – Biomass boiler equipment takes up far more space than a conventional boiler system, particularly the fuel store and thermal store which all require. Log systems should be able to store three years of fuel close to the boiler house, chip silos should be at least 30m3 and pellet tanks at least 10m3. A log boiler’s thermal store should hold 50 to (ideally) 100 litres per kW, eg. 40kW boiler à4,000 litre tank. A pellet or chip boiler should have at least 25 litres per kW. Ideally there will be 25m2 of floor space for the boiler installation, plus the fuel store adjacent. A pellet tank can be up to 15m from its boiler
  • Chip store design – there is a variety of ways to lift chips up into a ground-level chip silo using blower or ‘high speed’ augers or conveyors. However, all involve the delivery vehicle spending long periods of up to an hour feeding their load slowly into a trough, which adds to the delivered cost of heat. These systems generally involve a lot of noise and dust. Therefore it is highly recommended that the added investment is made in a buried or partially buried silo (ideally a steeply-sloping site is found) which allows the delivery vehicle to tip its load over a lip and depart quickly. 

How much does it cost?

The cost of a professional biomass boiler installation is dependent on the property, fuel choice and location and ranges from about £600 to £1,200 per kW of peak heat output, excluding the cost of the heat distribution system (eg. underfloor heating).

The installed cost of a typical 40kW system, for example, would be in the region of £36,000 plus the cost of the distribution system. The price per kW gets lower as systems get larger.

What are the advantages?

By harnessing a sustainable resource, a biomass boiler has much lower CO2 emissions. A good system causes at least 90% less CO2 emission than an equivalent oil boiler.

A well-designed system in an energy-efficient dwelling will cost less to run than any other at current fuel prices. An approximate running cost comparison per kWh and per year for a typical 3 bed detached house in Scotland are shown in the table above. (Note that, as explained above, a chip boiler would not be appropriate for such a building on its own due to high fixed capital costs for small installations).

Using a local heating fuel source can insulate you from the sometimes wild fluctuations in heating costs experienced by oil and gas systems, caused by political and other influences on global markets.

The Renewable Heat Incentive

In 2013-14 the RHI pays non-domestic users 2.2-8.6/kWh for each unit of ‘green heat’ generated by accredited non-domestic biomass systems, and is similar to the Feed-In Tariffs already in place for renewable electricity generators. Biomass boilers may be eligible under the domestic RHI when it starts, but possibly not for those with access to mains gas.

The RHI should repay the higher capital cost of a typical biomass system within around 8 years. District Heating Schemes will take a few years longer when the cost of the heat distribution system is taken into account.

For information about the existing non-domestic RHI, the Renewable Heat Premium Payment and proposed domestic RHI schemes, see www.decc.gov.uk/rhi .

Procuring a biomass boiler system involves a number of more sensitive decisions than other heating types and it is worth obtaining good advice independent of any suppliers before committing to any specification, design or supplier.