How do I size a biomass boiler?

Biomass boiler capacity should match the property heat loss, typically calculated at the design outside temperature. For domestic properties, common sizes are 15-30 kW. Unlike heat pumps, biomass boilers can provide high flow temperatures (60-80C) so existing radiators work without modification.

How much fuel storage space do I need?

Wood pellets have an energy content of approximately 5000 kWh per tonne with a bulk density of around 650 kg/m3. A typical 20 kW domestic system uses 4-6 tonnes per year, requiring around 6-9 m3 of storage. Ideally, you should store enough fuel for at least 2-3 months of winter use.

What are the flue requirements for biomass boilers?

Approved Document J requires the flue terminal to be at least 1m above the ridge of the roof. Minimum flue diameter is typically 150mm for boilers up to 25 kW. The flue must be twin-wall insulated stainless steel or a lined masonry chimney. Flue draught of 12-20 Pa is typical.

Do I need a buffer vessel with a biomass boiler?

Yes, a buffer vessel (thermal store) is strongly recommended. It prevents the boiler from short-cycling during low-demand periods and allows the boiler to run at optimal efficiency. A typical size is 20 litres per kW of boiler output, so a 25 kW boiler needs a 500-litre buffer.

Are biomass boilers eligible for grants?

Biomass boilers are eligible for the Boiler Upgrade Scheme (BUS) grant of £5,000 in England and Wales. The boiler must be MCS-certified and installed by an MCS-accredited installer. The property must have an EPC with no outstanding loft or cavity wall insulation recommendations.

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Biomass Boiler Calculator

Size a biomass boiler from heat loss, calculate fuel storage requirements, flue sizing per Document J, and compare running costs with gas.

Property Heat Loss

From heat loss survey

Property Type

Fuel Type

Pellets are most automated

Fuel Cost

Current delivered fuel price

How We Calculate This

This calculator sizes a biomass boiler based on your property heat loss and estimates annual fuel consumption, storage requirements, and running costs compared to gas.

Fuel energy content

Wood pellets: ~5,000 kWh/tonne (EN Plus A1 grade, <10% moisture)
Wood chip: ~3,500 kWh/tonne (at 25-30% moisture content)
Logs: ~4,200 kWh/tonne (at 20% moisture, seasoned 2+ years)

Flue sizing

Flue diameter and height are determined per Approved Document J and the boiler manufacturer's specifications. Adequate draught is essential for efficient combustion and emissions compliance.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.