Wood-Fired Heating

Wood-Fired Heating

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A wood-fuelled home

For some homes, log or pellet fuel is a great alternative to fossil fuels. The choice of boiler or stove and management of the fuel are both key, for wood-fired heating, because it’s important to burn wood as efficiently as possible.

Modern ‘batch’ log boilers and pellet-fired stoves or boilers offer you convenient and efficient ways to heat with wood fuel. Their high efficiency means that they have much cleaner levels of emissions than traditional stoves or ranges.

CAT has been using wood for heating ever since we started in the 1970s, gradually moving to more efficient appliances as these became available. You can see various modern wood-fuelled boilers on display at CAT, or come one of our short courses to learn more. Our online shop sells many books on heating with wood.

Before switching to any new heating system it’s vital to maximise energy efficiency. Increasing insulation, draught-proofing and lagging pipes will save money on fuel and allow you to choose a smaller boiler.

A low carbon fuel?

Because burning wood still releases carbon dioxide, it’s important that the fuel is sustainably grown. More trees must be planted to replace those being harvested. When burned, biomass fuels release the amount of carbon dioxide that they absorbed when growing. This is unlike the carbon in coal, oil and gas, which was absorbed over millions of years but is being released in the space of a few decades.

The energy used to harvest or extract, and then process and transport different fuels needs to be considered. Very little energy is needed to harvest wood, so when used locally it is a very low-carbon option.

Making sustainable use of biomass as fuel will help us meet our energy needs in a zero-carbon future. It’s a great way of filling in some of the gaps between intermittent types of renewable energy. Growing our own fuel also creates jobs and is ideal for strong, local economies.

Choosing a wood-fired appliance

Advanced log and pellet boilers have been common for many years in mainland Europe. They are as efficient as modern gas boilers – converting about 90% of the fuel into useful heat.

A pellet boiler automatically draws the pellets from a large hopper, making them as convenient as oil or gas heating. For a smaller house, you could fill a pellet boiler-stove manually, using bags of pellets. See for example our case study video. You’d still want one annual bulk delivery of bagged pellets to keep costs down, but only a small storage space would be needed.

You can fire up a ‘batch’ log boiler less than once a day, to heat water in a large buffer cylinder (or accumulator tank). Your thermostat settings will then determine how this hot water is piped around your house.

A simple wood stove is a great improvement over an open fire (which is very poor – most of the heat goes up the chimney). However, some stoves give off more smoke than others – see below.

Combining wood fuel with solar water heating (for the summer), can give you renewably-generated hot water all year round.

Biomass Pellets at CAT
Biomass pellets

No fire without smoke?

Modern log and pellet boilers will emit very low levels of particulate matter and oxides of nitrogen. They need to meet low levels to get a Renewable Heat Incentive (RHI) Emissions Certificate under the current government incentive scheme. Pellets have a much lower moisture content than log or woodchip – less then 10%. It is also easier to run a pellet stove or boiler at a lower output without losing efficiency.

The RHI standard is stricter than is needed for use in a smoke control area. Many standalone (not water heating) log stoves can meet these less strict smoke control standards, but could still be polluting if not operated well. More stringent standards are planned, and you can look for the Ecodesign Ready label on a log stove to find a cleaner burning model. Almost all log stoves (or ranges) that have a back boiler for heating water won’t have smoke control certification, so emissions will be worse.

To burn efficiently, logs must be well seasoned so that they have a low moisture content. Ideally this means storing for almost two years. See the questions below for more on efficient use of a stove.

What’s the installation cost?

The price will vary according to the specification of the boiler. For an average sized house, a basic log boiler might cost about £3,000, while one with a higher standard of controls might be £6,000. A pellet-fired boiler could cost £9,000. For the total installed cost (including flue, accumulator tank, fuel store, etc), add maybe £12,000 to £15,000 to those boiler costs. A smaller pellet stove is perhaps a few thousand pounds.

These high-efficiency appliances are expensive, but you can get support back through the Renewable Heat Incentive (RHI). The amount you get back is based on the heat demand calculated in your home’s Energy Performance Certificate (EPC).

For an eligible biomass boiler, you receive 6.74 pence for each kWh of expected heat demand, paid quarterly for 7 years and linked to inflation. Fuel must be bought from an accredited supplier (or grown onsite), to ensure it was harvested sustainably. See Ofgem’s RHI pages to find out about the eligibility requirements and how to apply.

A log stove could cost between £500 and £1000, with installation costs probably the same again.

Where do I find installers?

When installing a wood-fired heating system, finding a qualified professional installer who can properly size and design the system will be important. Prices can vary, so it’s also key to get a few quotes from different installers.

To receive financial support through the Renewable Heat Incentive (RHI), the installer, equipment and fuel will need to be accredited. There’s a searchable database of installers on the  Microgeneration Certification Scheme (MCS) website.

For biomass equipment not eligible for the RHI (such as a standalone log stove) you could use the HETAS website to find retailers and installers. In addition, some installers will have voluntarily joined other professional bodies that promote high standards.

How do wood fuel costs compare to other options?

Buying pellets one bag at a time could be expensive, but bulk delivery should be similar to or less than fossil fuels. A cost of £250 for a tonne of pellets means about 5.5 pence per kWh, which compares well with other fuels. If you have a large hopper and can buy a few tonnes of pellets at once, then the cost could drop to 4.5p per kWh.

Bought in bulk, log fuel should be cheaper than pellets, but it will depend on your location. Logs are cheapest (and so best used) in rural areas, with locally harvested fuel available. The moisture content is important with logs, because freshly cut wood is much heavier.

Beyond the price per supplied kWh, how efficiently you burn the fuel will then be important. A modern pellet or log boiler has similar efficiency to a modern oil or gas boiler. However, an older or more basic appliance (e.g. a log stove or range, or an old boiler) will not be as efficient, so your effective heating costs will be much higher. A 90% efficient appliance uses about three-quarters as much fuel as a 75% efficient appliance.

Websites that can be helpful when seeking suppliers include HETAS/Woodsure and the UK Pellet Council. The government is putting together a new Biomass Suppliers List, to link in with the Renewable Heat Incentive (RHI).

Further information

For some additional details, see our questions and answers section below, and the books we sell. You can also visit us to look at the biomass systems we have on display here.

Related Questions

How should I store log or pellet fuel?

Logs should be stacked and stored in a covered but well-ventilated place for at least one year, preferably two. This is in order to air-dry the wood to a moisture content below 25 or 30%. An open-sided wood store would be suitable, a garage wouldn’t.

Bringing logs inside for the last week or so improves them to room dryness. Stoves might be specified to cope with 50% moisture content, but the efficiency will suffer – causing worse emissions.

The required storage space depends on how big and how well-insulated your home is. A small cottage is likely to need 8 cubic metres of logs each year, a 3-bedroom house 12 cubic metres, and a large detached house 16 cubic metres.

Wood pellets, made from compressed sawdust, have only about 8% moisture content. Pellets for domestic use should be to the ENplus standard. Pellets have a higher energy content than logs and take up less than half as much space. Compared to heating oil, pellets have about half the energy density so take up twice as much volume of pellets as an oil tank.

Pellets can be piped straight into a hopper, or delived in bags to store in the shed or garage. With a pellet stove using 0.5 to 1.5kg of pellets per hour, a 15kg bag should last a few days.

Larger systems, such as for a school or a block of flats, can use chipped wood. This allows more automation than logs and may be cheaper than pellets. Seasoned wood is delivered, chipped, and stored until it reaches 15% moisture content. For larger schemes, it’s a good idea to have a supply contract with a trusted supplier to ensure a reliable supply of wood.

 

 

 

How can I burn logs efficiently and cleanly?

Manually fed stoves can produce lots of pollutants if operated badly.

Logs should be burned fiercely with lots of air input until they are almost charcoal, after which the stove can be ‘damped down’. Reducing the air supply too early would create lots of smoke & tar. The key is good ‘secondary combustion’ of the high-energy volatile gases given off by burning wood.

Some stoves are fitted with a ‘Lamda’ sensor, to regulate the amount of oxygen added and optimise efficiency.

In traditional log stoves, burning green (fresh, wet) wood leads to corrosion of the liner and stove by acid, and tar coating the interior. Cast iron stoves are popular as they are less susceptible to corrosion from acid from burning damp wood. But this would still mean poor quality emissions. If you’re burning properly seasoned wood a steel stove will be fine.

Avoid burning treated, painted or glued wood, or non-wood waste, as these will give off toxic and polluting gases.

Logs need to be either bought seasoned, or stacked and stored for long enough to reach a suitable moisture content. New green logs must be stored for 2 years to dry enough.

Building regulations require all fuel burners to have a dedicated vent to avoid production of carbon monoxide. The chimney needs an insulated flue to prevent fumes condensing as tar.

With complete combustion, wood burns to a small amount of ash, which is an excellent fertiliser (unlike coal ash).

What does the installed cost of log or pellet heating include?

A more expensive log boiler will be made with higher-grade steel, giving a longer expected lifespan. It will have greater control over aspects such as airflow to the combustion chamber, which means it will operate more efficiently when starting up or winding down. A boiler without as much control (e.g. over air inflow), can suffer from more tar, acid and other by-products of poor combustion. These effects will shorten the life of the boiler & flue, as well as causing a lower quality of emissions.

As well as the boiler itself, the total installation cost will include an insulated flue, accumulator tank, pellet hopper (where needed), pump, controls and labour.

A properly insulated flue, needed for efficient operation of a biomass boiler, can cost around £2,000 to £2,500.

An accumulator (or buffer) tank could cost about £1 per litre. It will probably be sized to between 50 and 100 litres per rated kW for a log boiler. A pellet boiler needs a smaller accumulator, at maybe 20 litres per rated kW. This will enable you to run the pellet boiler flat-out for a longer period, to get the most efficient combustion.

If your house has underfloor heating, the buffer tank can be smaller because you only need to supply water at 30 or 35 degrees Centigrade. To supply radiators with high temperature water, the buffer tank needs to be larger, to keep enough water above 50 degrees.

 

 

Is a back boiler on a traditional log stove a good idea?

In most cases, using a traditional type of radiant stove with back boiler (or water jacket) to feed radiators means will not operate efficiently – causing smoke and using more fuel. An example of the lowered efficiency is that almost no traditional wood burners with back boilers qualify as ‘exempt appliances’ for smoke control areas.

A low efficiency of wood fuel combustion increases the probability of tar condensing in the chimney, and the risks this then poses. The more hot water is required (for example if trying to supply several radiators and a hot water cylinder), the lower the efficiency will be and the harder the stove will be to operate.

There are a couple of boiler-stoves that do meet smoke control standards – the Dunsley Yorkshire Stove & Boiler  and the Broseley eVolution 26 boiler stove. These are both larger heaters, that may be too much for a small home. Higher efficiency appliances move the water heating stage away from the fire box, extracting heat after the gasifying stage of combustion. With this improved design (and also much better controls, for example on air inlets), modern ‘batch’ log boilers or pellet-fired appliances can produce hot water at 90% combustion efficiency.

With traditional back boilers the ratio of water to space heating may not be suitable for a larger home – perhaps 60/40 but often 50/50. If half the output is to space heating it can be difficult to get enough hot water without overheating your main room, so wasting fuel. Pellet stoves have a much better ratio or 80/20 or 85/15 (water to space heating), while log or pellet boilers only produce hot water so the problem does not arise.

Many standalone log stoves (no back boiler) do qualify as exempt appliances and can be run quite efficiently as long as you use dry wood. A standalone stove with no water jacket might be a good supplement to either a heat pump or a log or pellet boiler, as the main heat source can be sized to meet the likely space heating demand, with the stove as top-up for occasional cold snaps. You could look for the Ecodesign Ready label on a log stove to find a cleaner burning model.

 

Can I use biodiesel or other bio-oils to heat my house?

Depending on the boiler it might possible to burn modified vegetable oil in place of heating oil, but there are both environmental and financial reasons why it’s not really an effective solution. If you currently heat your home with oil and are seeking an alternative then a modern wood-fired boiler or perhaps a heat pump is likely to be better.

One of the key issues is that the amount of liquid biofuel we can produce sustainably in the UK is very small, and can meet only a tiny proportion (less than 5%) of the transport fuel demand – let alone further demand from heating. This applies to both oil from crops like oil seed rape, plus the small amount of waste cooking oil available.

To meet a certain energy demand requires far less land for wood fuel than it would for crops like oil seed rape, as the yield in tonnes of fuel per hectare is much higher for wood. In addition, much less processing is needed to make wood into a useable fuel compared to the processing of oil crops (or indeed the processing of waste cooking oil).

If there is a demand for bio-oils beyond what we can meet in the UK, then much more will be imported. This will push up production overseas from, for example, palm oil – with environmental damage from increased deforestation.

Liquid biofuels (like biodiesel or bioethanol) were generally been used for transport purposes in the UK because there was more financial benefit in doing so (due to taxes, although this is no longer really the case). However, there are not usually financial benefits against lower cost heating fuels.

Bulk vegetable oil or commercial biodiesel will both be much more expensive than heating oil. Making your own biodiesel is an involved process and you’ll still need to get the raw material. This is not straightforward – waste cooking oil is generally now collected by companies that pay for it and do the relevant paperwork to show that they’re dealing with it properly. You’ll also need to invest in some modifications to an oil boiler to make it able to burn a biofuel (for some boilers, especially newer ones, this may not be possible anyway).

Given the cost of biodiesel, investing in a modern boiler able to (or even specifically designed to) burn this fuel will still leave high running costs, possibly up to twice as much as an efficient modern wood-fired boiler or a well-designed heat pump system.

Case study: Judith’s pellet boiler stove

Judith has installed a wood pellet boiler. This compact appliance heats her 5 bedroom house using sustainable wood pellet fuel without having to worry about lighting fires. Heating with wood pellets – made from compacted sawdust – is a very controllable way of using wood fuel.
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