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Biomass heating

A biomass boiler may offer an alternative ‘green’ means of space heating and/or hot water in your church or home. This can help us care for God’s Creation.


Biomass boilers are wood-burning stoves, used as boilers to drive central heating and hot water systems.

Small scale biomass heating, using locally and strictly sustainably sourced wood fuel, can be almost net carbon-neutral. This may potentially enable a church to reach net zero, which the Church of England and Diocese of London aim to achieve by 2030.

This technology may be worth considering for some churches, although the difficulties are significant. It is necessary to assess rigorously and realistically whether your building and its location might be suitable. This will be unusual in London.

Even if the building and location seem feasible, there are still important pitfalls to be avoided. The initial cost of a biomass boiler is also substantial.

Different types of wood fuel boilers are available. They usually burn wood chips or pellets. It is important to consider the available options.

Carbon emissions

Biomass and CO2

Biomass is in principle a renewable energy source, composed of recently harvested tree growth or other plant matter. This material is incinerated and the energy is captured and distributed.

Fossil fuels such as coal and oil too are derived ultimately from biological matter, but the carbon was taken out of the atmosphere many millions of years ago; as it gets put back into the air, this changes atmospheric composition, adding large amounts of CO2.

Burning biomass, like other fuels, does still release the greenhouse gas carbon dioxide (CO2) into the atmosphere. But if managed strictly sustainably, biomass crops can in theory be harvested and constantly replenished. So the CO2 should be taken up again by new growth, cancelling out the effect on the climate.

The renewable status of biomass has been strongly contested, especially when used in power stations to generate electricity. In practice, this may causes emissions comparable to those of fossil fuels, because the wood is not harvested sustainably, and/or due to the emissions from international transport.

Therefore, biomass fuel needs to come from regularly pruned trees, self-seeded saplings and excess growth and scrub – not from clear cutting, sourced as locally as possible, and certainly not imported. Alternatively, small scale biomass in the form of wood pellets is likely to come from sawdust collected in saw mills.

Carbon intensity

Assuming the above conditions are fulfilled, the net carbon intensity of biomass should be limited to the manufacture and installation of the hardware, and processing and transporting the fuel.

Biomass fuel

Quality and supply

The advantages of this low carbon technology are to some extent counterbalanced by quality and supply issues.

Fossil fuel products are relatively standardised, generally available, and easy to transport and handle. Whereas wood biomass is highly variable in moisture and ash content, calorific value, density, and availability.

A reliable source of good, consistent quality is essential. Poor quality fuel can turn to dust and jam the boiler.

Choice of fuel

The key factor is proximity:

  • In urban locations such as London, and where longer distance transport is required, pellets are mostly preferred. However, such locations are unlikely to be feasible in most districts of London, if only due to regulatory constraints (see ‘Emissions and regulations’, below)
  • In rural areas, including parts of outer London, the availability and therefore cost of wood chips may be more attractive.

Other points:

  • Wood pellets are generally more consistent than wood chip, with higher density and lower moisture content – making them easier to transport, store and handle;
  • Wood chips can be very variable, particularly in moisture content; this affects transport cost and net calorific value;
  • Chips also need 3-4 times the volume of storage, and a larger boiler;
  • Pellets can be blown from a tanker into the fuel store by hose;
  • However, pellets are about twice the price of chip and require energy for processing;
  • Stored wood pellets may emit significant quantities of carbon monoxide (CO), which is toxic. This requires storage to be either sealed or else ventilated. Wood chip may also emit some CO, but is relatively easy to manage from this point of view;
  • The correct fuel must be maintained throughout the boiler’s life; an alternative source could risk damaging the boiler;
  • Maintenance is a significant issue, eg some pellet boilers require cleaning once a month.


The source must be located, and the fuel processed as close as possible to the church, to limit the distance before the fuel is burned. Most of the carbon emissions from carbon emissions come from procurement and transport.

Bear in mind a firm although local might be buying remotely. This needs to be avoided.

Never use wet or green wood, or accept wood gathered or donated by others, however tempting or well meaning. Emissions from wet or green wood are toxic, even worse than coal. The effects of such pollution on health can be serious.

Delivery and storage

Unlike other renewable technologies, wood can be stockpiled in suitable conditions and used when needed. Different fuel sources and boiler types will require their own storage conditions.

Many pellet boilers have large enough silos to stockpile fuel for 2-12 months. Most urban churches will need a large, indoor, sealed space.

Many churches were built to accommodate solid fuel systems. A disused oil boiler room and tank space may be suitable. Or an area once used to store coal or coke may be available.

The storage location should be easily accessible for deliveries, and close enough to the boiler (typically up to 15m for both).

Some biomass boilers can be manually fed, but this is a chore.

More normally, a feed auger (automatic arm) can take fuel from the silo to the boiler. The auger must be correctly installed and properly maintained.

System design

A wood fuelled system may be integrated into an existing central heating and hot water system.

If an old oil or gas boiler exists, then a new biomass system might be installed straight into the main central heating pipe work.

A profile of continuous warming, not intermittent heat, is important. Solid fuel boilers do not generally like being turned down. They work best when switched on then left to run undisturbed, at the same continuous load.

Therefore a wood-fired boiler can be ‘teamed’ with a gas boiler, so that the wood-fired unit can provide for the base load, with the gas boiler cutting in at peak times. But of course this is more complex, with yet more cost and more to go wrong – as well as, obviously, not eliminating the use of fossil fuel.

Nevertheless you may be able to install a biomass system while your old gas boiler is still running, then keep this boiler running for much longer, running below its capacity. That should improve efficiency.

A substantial thermal store, usually a water tank, will also be needed.

It is essential for your engineer to make measured drawings of equipment layout. Sufficient allowance should be made for any back up boiler, for access and maintenance, including withdrawing and replacing components.

Boiler housing and sizing

A biomass boiler can be bulky. Biomass involves more equipment than a traditional oil or gas system. An outbuilding or disused oil tank and boiler house may be suitable.

Boiler location needs to be optimal. This is critical to performance. Unsuitable siting may result in failure, eg due to furring up of pipework.

Boiler running

Traditional buildings including historic churches commonly benefit from a background level of heating at 8-15 degrees C.

The church is then raised to a pre-set temperature of between 16 – 19 deg C for services or other activities.

However the system should be sized for temperatures up to at least 20 deg C. This may be required during very cold weather, as there will still be cold spots.

Boiler flue

Biomass boilers, like other boilers, require a flue. It needs to be carefully sited, and correctly lined.

If there isn’t a suitable flue already present then planning permission may be required. It is essential to check this with the Local Authority at feasibility stage.

Bear in mind that the boiler flue may emit smoke when starting up, until it reaches a steady state. Therefore the flue should not, for example, be pointing towards someone else’s windows (or even near openable windows in the church itself).

Smell and noise

During steady running, a biomass boiler should not smoke or smell offensively, if specified and installed correctly.

Boilers can sometimes smell a little woody. Under normal running, what appears to be smoke may more likely be water vapour.

There is a periodic whirr from the auger that feeds the combustion chamber.


Biomass boilers require a significant amount of maintenance.

It is advisable to enter into a maintenance contract at time of installation.

Legal requirements

Planning permission

Planning permission may be required from your Local Authority, if there is any significant change to the outside appearance of the building.

This might include a new flue which appears different to any flue that was already there.

Building regulations approval may also be required.

It is essential to check this with the Local Authority at feasibility stage.

Faculty permission

A faculty is also needed for any installation in a church.

Emissions and regulations

Combustion of biomass can produce emissions that affect local air quality and are subject to regulation.

Biomass is subject to the Clean Air Act 1993 and the Smoke Control Areas (Authorised Fuels, England) Amendment (No.2) Regulations 2011; usually also the Waste Incineration Directive (WID).

Within smoke controlled areas, wood chip and wood pellets may only be burned in an exempted appliance that has been specifically tested and approved. Non-exempt appliances require approved smokeless fuels. Both lists should be on your local authority’s website.

As of January 2023, additional regulation is being introduced to cover locations outside Smoke Control Areas.

The Pollution Prevention & Control Regulations may apply in addition to the requirements of the WID.

In addition, particular local areas may be designated as Air Quality Management Areas (AQMAs), therefore subject to special controls. This is likely to rule out a biomass boiler.

Some local authorities are also restricting use of a biomass boiler, where planning permission (see above) is required due to a change in the building’s external appearance.

The economics of biomass heating

Cost and funding

The next questions are cost, and how to raise funds.

Typically, running costs for wood pellets are less than for electricity or LPG, and comparable with oil or gas. However, the capital cost may be high.

The following factors need to be considered:

  • Capital cost compared to, say, a gas boiler;
  • Additional maintenance;
  • Differential fuel costs, less –
  • Any grant or subsidy such as the Boiler Upgrade Scheme (small premises only).

Biomass boilers can offer local benefits. Using local wood creates local jobs.

Information and advice

Engineers and consultants

It is important to appoint competent and experienced consultants and installers.

Installers should be certified under the Microgeneration Certification Scheme (MCS).

Problems can occur where different roles in a team are imprecisely defined, or where too many cooks spoil the broth. Make sure everyone knows who is responsible for what, and that this is in writing.

Your insurers

You should inform your building insurers of your proposed biomass installation, before it is installed – it involves health and safety issues.


Parish Property Support Team
Head of Environment and Sustainability.

Getting a faculty.

Generating your own Energy
Climate Action Projects
Climate Action Finance.

Boiler Upgrade Scheme
Microgeneration Certification Scheme (MCS).

Environment and Sustainability, front page.

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