Biomass Boiler Calculator

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 (60-80°C) so existing radiators work without modification.

Wood pellets have a net calorific value of about 4,800 kWh per tonne (Forest Research: 17 GJ/tonne = 4.8 kWh/kg at 10% moisture; ENplus A1 guarantees at least 4,600) with a bulk density of around 650 kg/m³. A typical 20 kW domestic system uses roughly 5-10 tonnes per year, needing around 8-15 m³ of storage. Ideally, store enough fuel for at least 2-3 months of winter use.

Flue diameter is set by the appliance manufacturer (commonly 150mm for boilers up to about 25 kW) — Approved Document J does not specify diameters. Doc J and the BFCMA cite a chimney height of around 4.5m (and a flue pressure of about 10 Pa) as a rule of thumb for adequate draught. The terminal must also satisfy the outlet geometry: at least 1m above the roof where within 2.3m horizontally of the pitch, and at least 600mm above the ridge if it is within 600mm of the ridge. The flue must be twin-wall insulated stainless steel or a lined masonry chimney. Always follow the manufacturer instructions and Approved Document J.

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.

Biomass boilers are eligible for the Boiler Upgrade Scheme (BUS) grant of £5,000 in England and Wales (air-source and ground-source heat pumps get £7,500 under the same scheme). The boiler must be MCS-certified and installed by an MCS-accredited installer, and BUS-funded biomass is restricted to rural properties off the gas grid (with an emissions certificate). Since 28 April 2026 a valid EPC is no longer required and the previous rule about no outstanding loft or cavity wall insulation recommendations has been removed (insulation is still strongly recommended). Always check the current rules on Ofgem before applying.