Tim Pullen – HB&R’s eco expert and proud biomass stove owner – explains why it’s a good solution for most homes — but not always cheap or easy.
The rising costs of gas and oil and a desire to be green have seen us look to mainly renewable sources to heat our homes. This decision has typically involved the installation of a heat pump – perhaps with solar thermal – and, to a lesser extent, a biomass boiler or stove. Biomass seems to tick most boxes — the fuel is cheap, sustainable and readily available, and the technology is robust with a long life. And yet there is still a relatively small take-up. So why the resistance?
What is biomass?
Biomass is anything that once grew that can be burned. It includes rapeseed pellet, straw and miscanthus, but is generally taken to mean wood. It’s considered a carbon-neutral fuel, but in reality CO? is emitted in its transportation, and in some cases production. But solar power aside, it’s the greenest fuel we have.
Wood fuel comes in three forms: logs, chips and pellets. The latter is a manufactured product, made from compressed wood dust, and has a heat output of 5,000kW/tonne, compared to 2,000-3,000kW/tonne for logs and chips.
With logs and chips the type of wood used also has an impact on heat output — softwood produces a lot less heat than hardwood. The wood also needs to be thoroughly dried to achieve a moisture content below 25%. The nature of chip boilers and their fuel-delivery systems means that they’re not suited to small applications — 25kW peak load is the minimum.
Log and pellet boilers are the only real options for domestic projects. Pellet boilers are by far the most common as they offer higher levels of convenience, close to that of an oil-fired boiler. Cost will vary from around £5,000 for a fairly basic machine with manual fuel loading, to £20,000 for a sophisticated machine with automated fuel delivery. Like oil-fired boilers an annual service is required, but the only other maintenance is ash removal — usually a monthly activity that provides good nutrients for the garden.
There are a few varieties of log boiler available, but look for gasifying batch boilers. They operate at much higher temperatures, gasifying the logs, increasing efficiency to over 90%. As logs are loaded in batches the boiler only tends to require loading once a day, or perhaps on alternate days. Prices vary from £5,000 to £20,000 plus installation.
The heat output of a log boiler and heat demand of the house is balanced around the size of the hot water cylinder, which is generally much bigger than for a gas or oil system. Ideally a single ‘burn’ of a batch of logs is sufficient to heat the hot water cylinder and that in turn is sufficient to meet the demands of the house until the next day’s burn.
Stoves with back boilers can also be specified to meet whole-house heating and hot water demand. Prices can reach £5,000 for a fairly sophisticated pellet stove with back boiler.
Connecting solid fuel stoves to unvented hot water cylinders can be problematic under the Building Regulations, but models like the Broseley eVolution 8 have solved the problem and provide an effective (if not cheap) whole-house solution.
Is it right for your home?
Biomass has the capacity to meet the needs of most self-build and renovation projects. Retrofitting can be complex in some cases but, if the space is available, it’s generally achievable. And almost without exception biomass is an excellent choice for large renovations, as it provides large quantities of high-temperature water at low cost and produces low CO? emissions.
But it’s ultimately a lifestyle choice. Gas, oil and even heat pumps don’t require too much thought, either at design stage or in their operation. Biomass does — the type of boiler to specify, where to locate it, the type of fuel to use, where to store the fuel, and how biomass heating impacts on the house design, are all issues which need to be addressed properly for a system to be a success.
Fuel Cost Comparison
Heating oil is around 5.9p/kWh*, while natural gas is about 5.2p/kWh (depending on supplier and tariff). The price of logs and chip will vary around the country but as a guide expect to pay £100/tonne for wood chip, which is equivalent to around 3p/kWh. Logs can vary from £30-100/tonne which equates to 1.0p-3.4p/kWh. For wood pellets expect to pay £210/tonne delivered in bulk, or 4.2p/kWh.
Perhaps more interesting is the way prices have changed over time. Logs and chip tend to be very local and only subject to local fluctuations. Wood pellet has more of a national infrastructure and in 2007 the price across the country was averaging £190/tonne. The current price of £210 indicates a year-on-year rise of around 3%, which actually reflects the rise in transport costs rather than a rise in the price of the pellets.
Renewable Heat Incentive
The renewable heat incentive (RHI) is a scheme, similar to the Feed-in Tariff, for heating systems, which started in April 2014. Biomass boilers and stove-boilers will qualify, but stoves will not (because the amount of heat produced by the stove cannot be measured). The tariff rate is 12.2p/kWh, meaning that most homeowners installing biomass should expect to receive between £1,000-2,000 per year in payments depending on your home’s heat demand.
During Autumn 2014, the Department of Energy & Climate Change (DECC) will introduce new sustainability criteria for RHI claimants to ensure their biomass fuel meets the Governmen’ts low carbon and environmental targets. The fuel used will need to be sourced from a trader registered on the Biomass Suppliers List (BSL) which will be available at Gov.uk this year (2014).
A Typical Comparison vs Oil
The following table shows a typical comparison against oil — the kind of thing a heating engineer often does but, as long as you understand it, just the kind of thing you’ll need to get to grips with. It shows how a biomass installation stacks up against using oil and makes various (rather generous) assumptions in favour of oil — so these are really the worst case scenario figures.
We’ve assumed a 3.4p/kWh price of biomass against a 5.6p/kWh price per oil (with oil it’s quite easy to work out your /kWh cost — just divide your litre price (say 56p) by 10. We’ve done two scenarios, one based on a typical heat demand of 30,000kWh (for a large four bedroom house not very well insulated) and a better insulated version needing 20,000kWh. As you can see, the benefits of biomass drop the better the home is insulated. NB: we’ve assumed an £18k installation figure, which would likely reduce for the 20,000kWh scenario. We’ve also factored in the RHI payments.
|Regular old house||Better insulated old house|
|Cost per kWh||5.6||3.4||5.6||3.4|
|Annual use kWh||30,000||30,000||20,000||20,000|
|Total cost £||1,680||1,020||1,120||680|
|Fuel saving £/yr||660||440|
|Total over 7 years||4,620||3,080|
|RHI Payment £/yr||3,660||2,440|
|Total over 7 years||25,620||17,080|
|Cost of system||18,000||18,000|
When I moved into my stone cottage in 2000 my oil bill was £400 per year; by 2006 it had more than doubled. So I took the decision to refurbish the existing oil boiler and install a new stove with back boiler. I didn’t want the inconvenience of lighting the stove each morning, so the oil boiler would do the morning duty and the stove the evening duty.
I calculated that my home had a peak winter heat load of 15kW including hot water, and so installed a 13kW boiler stove which was advertised as giving 4kW to the room and 9kW to the boiler. I soon realised my mistake. The boiler side only reaches 9kW when the stove is full of wood — but too much heat is then given out to the room. Reducing the wood means insufficient heat for the rest of the house. In reality it only happens on the coldest days — but we live and learn.
Even so, oil gives convenience, while the biomass reduces bills. In 2011 I spent £600 on oil and £130 on logs, which combined is less than in 2006, and perhaps £1,000 less than if I’d remained solely on oil today.
This article is sponsored by Niall’s Renewables