With the heating of domestic properties accounting for around 14% of the UK’s carbon emissions, there’s a push to replace gas boilers with low-carbon alternatives. Biomass boilers are one of these alternatives, combusting wood pellets or chips to heat rooms and water.
Biomass boilers cost more than conventional boilers, but some of those costs can be offset by the Renewable Heat Incentive (RHI), paying you for every kilowatt-hour of heat your boiler produces for seven years. They’ll also lower your energy bills, but you do have to consider the cost, delivery and storage of the wood fuel.
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Biomass boilers work much like conventional boilers in that they burn fuel to produce heat, which is then used to warm water. But while conventional boilers burn fossil fuels, biomass boilers use organic matter—usually wood pellets, chips or logs but sometimes other plant material. Unlike a stove, which heats a single room, a biomass boiler is connected to a central heating and hot water system, so it can provide heat for an entire home.
Burning biomass releases some carbon dioxide but less than combusting fossil fuel does. Additionally, some argue that burning biomass is actually carbon neutral because the carbon dioxide released was absorbed from the atmosphere while the tree was growing. This claim is controversial, disputed by some environmentalists.
Modern, correctly installed and maintained biomass boilers have an efficiency of between 89-91%, exceeding that of many gas boilers. However, the efficiency depends on the fuel type. Wood pellets, with lower moisture content, are more efficient (around 94%) than wet chips (around 80%).
Biomass boilers have a higher price tag than conventional boilers. A 12kW biomass boiler, suitable for a four-bedroom house, costs around £7,000, while a comparably sized natural gas boiler costs £2,500 with installation.
Costs of biomass boilers vary depending on their size and level of automation. Automatically-fed boilers are more expensive, typically costing at least £10,000, including delivery and installation. Manual boilers, in which you need to insert the wood, are cheaper. But you’re unlikely to find any biomass boiler costing less than £5,000.
The average cost per unit of energy of wood pellets is 4.2p per kilowatt-hour (kWh). This is slightly more expensive than the cost of natural gas, which averaged 3.4p/kWh in December 2020 (but varies depending on your energy tariff and supplier). However, the price of natural gas is expected to increase in the future, as the world transitions away from fossil fuels, while that of wood pellets is expected to fall. Additionally, you can reduce these fuel costs if you have access to a free source of wood or organic waste.
You can reduce the cost of fuel for your biomass boiler by ordering large quantities but you’ll need a place to store them, which may require building a shed to keep the fuel dry and reduce the risk of fire.
So in general biomass boilers are more expensive than conventional boilers. However, some of these costs are offset by the Renewable Heat Incentive (see below), making them competitive with natural gas boilers. Biomass boilers are also expected to decrease in price, alongside their fuel, as they become more popular.
Some of the costs of biomass boilers are offset by the Renewable Heat Incentive (RHI). This is a government initiative paying households with low-carbon heating systems for every kilowatt-hour of heat those systems produce over seven years.
The amount you earn through the RHI will depend on the amount of heat your household needs and the RHI rate paid when your biomass boiler was installed.
From April 2020, the rate paid for domestic biomass boilers is 6.97p/kWh. This rate will apply for seven years for any biomass boiler installed and accredited by 31 March 2022.
| Type of property | Annual heat demand* | RHI earnings per year | RHI earnings over 7 years |
| 1 bed semi-detached | 8,000kWh | £557.60 | £3,903.20 |
| 2 bed semi-detached | 12,000kWh | £836.40 | £5,854.80 |
| 3+ bed detached | 17,000kWh | £1,184.90 | £8,294.30 |
Source: Ofgem, April 2020
Biomass boilers are significantly larger than conventional natural gas boilers and are unsuitable for smaller homes. In those cases, a smaller standalone wood-burning stove would be more appropriate.
Wood burning stoves typically heat one room but can be fitted with a back boiler that uses the heat produced when burning the fuel to heat water, which can then be used as water or for space heating elsewhere in the home.
If you don’t have a space for a biomass boiler or the fuel it requires, you may want to consider another low-carbon heating technology, such as an air or ground-source heat pump or solar thermal panels.
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