Biomass boilers: an introduction
For domestic heating purposes the main biomass fuel is wood in the form of logs, pellets or wood chips. The full range of biomass fuels also include animal, food and industrial waste, and high energy crops such as miscanthus, willow, rape and maize.
Smaller biomass stoves can be used just for room heating, with a back boiler to heat the water, or to produce hot water and heating for the whole house.
In common with most renewable energy technologies an upfront investment is required yet, unlike those technologies where the element (ie wind or sun) that provides the power is free, there is also an ongoing fuel cost with biomass heating. A comparison of heat pumps and biomass boilers can be found here.
Is a biomass boiler suitable for my home?
Wood burning stoves are suitable for heating any room that has a chimney or flue. They come in all shapes and sizes, and can burn logs or wood pellets. The wood pellet stoves tend to be cleaner and easier to maintain, producing much less ash. However, 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 have a combi boiler.
Your stove will need to meet air quality standards if you live in a Smoke Control Area or intend to apply for the Renewable Heat Incentive.
All biomass boiler installations must comply with the Building Regulations.
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 fuel 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.
Ideally the fuel storage area will be under cover, as it is important to keep fuel dry. High moisture content in the fuel will reduce the efficiency with which it burns and if wood pellets get wet they turn to unusable mush.
Your property also needs to be accessible for a delivery lorry. Wood pellets can be delivered loose and blown into a hopper or in bags on a pallet.
The boiler will need a flue designed for wood fuel (existing chimneys can be lined). The installation must comply with all relevant building regulations, particularly if you live in smokeless zone. 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 to 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.
Also, 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. Not all suppliers offer all types of fuel. The lowest maintenance way is in a tanker to a hopper. A pallet full of bags will involve more work feeding the boiler as lifting and emptying the bags is a physical job. The National Energy Foundation has more information on buying and using wood fuel and an up to date list of local wood fuel equipment and fuel suppliers at Log Pile.
Wood pellet boilers will need an annual service.
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 10 kg or 15 kg 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.
Which wood fuel is best for biomass boilers?
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 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 an 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 and remember it requires some physical strength. 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 are 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.
What size / cost for a biomass boiler?
Biomass boilers are eligible for the Renewable Heat Incentive (RHI) which may recoup some of the installation costs.
According to the Energy Saving Trust, an automatically fed pellet boiler costs between £9,000 and £21,000, including installation. Because of significant differences in price it can be difficult to weigh up the competing claims of different suppliers and products.
When choosing a biomass boiler it’s important to consider the ongoing cost of fuel as well as the initial investment. Costs will vary but Carbon Trust has an assessment tool to help you work out the likely costs of using biomass fuels.
For example, wood pellet costs will depend on the size and method of delivery. If you have room for a large fuel store that will accept several tonnes of pellets at a time, delivered in bulk by tanker, you can generally reduce keep the cost, compared to bagged pellets (Energy Saving Trust).
Renewable Heat Incentive
Domestic Renewable Heat Incentive scheme
The domestic renewable heat incentive is a boiler replacement scheme, aiming to bridge the gap between the cost of a replacement oil or LPG boiler and the cost of a renewable heating system. Log-fired boiler stoves are not eligible but biomass boilers and biomass pellet stoves with back boilers are eligible for a tariff rate per kWh, index-linked and paid quarterly in arrears for seven years. The latest tariff rates can be found on the Ofgem website.
Installations that don't meet 100% of the peak space heating load may be eligible, but will need to be metered, as will those where you are not resident for half the year. Biomass technologies must also meet air quality standards. Biomass fuel standards came into force in 2014.
The RHI will be paid on each kWh of heat generated by your biomass boiler or stove. To calculate the payments you will receive, follow the formula in this example:
Home with a heat demand (space heating plus hot water) of 18,000kWh per year. This figure is taken from the EPC and may be different from the figure calculated by your installer.
Sample tariff rate: 4.68p
The calculation is 18,000 x £0.0468 = £842 RHI payment per year x 7 years = £5,896 in total.
In addition, you will make some savings on fuel costs (calculated per kWh) if you are swapping from oil, LPG or solid fuel, but probably not if you are on mains gas.
Non-domestic Renewable Heat incentive
This covers commercial and community installations and pays a generation tariff per kWh produced over a 20 year period. It aims to give a 12% return on capital and is index linked. The heat produced must be measured by a meter. Click here to see the tariff table on the Ofgem website.
The biomass tariff is tiered, to avoid any incentive to generators to generate excess or wasteful heat just to maximise their RHI payments. It is set at ‘a reasonable minimum level of usage it would expect from renewable heat installation providing space heating’, which has been based on the equivalent of running the installation at full capacity for 15% of the year. So, each year you will receive the higher tier 1 tariff for the first 1,314 peak load hours, and the lower tier 2 tariff after that.
Planning permission for biomass boilers
Biomass heating systems are deemed permitted developments unless the flue exceeds the height of the roof by one metre or more. In conservation areas and world heritage sites it is not permitted development if the flue is installed on a wall or roof slope which fronts a highway. 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 clean air, ventilation, noise and safety.
For those of you living in Wales, information on planning rules for small-scale renewable energy schemes is available here.
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