How do I size a biomass boiler?

Size the boiler to meet the design heat loss of the property (kW). For domestic properties, this is typically 10-25 kW. Biomass boilers should not be oversized as they operate inefficiently at low loads. A buffer tank helps manage part-load conditions.

How much fuel does a biomass boiler use?

Wood pellets (ENplus A1) contain approximately 4.8 kWh/kg. At 92% efficiency, a property using 25,000 kWh/year needs about 5,700 kg (5.7 tonnes) of pellets per year. Wood chips (30% moisture) contain about 3.5 kWh/kg and logs (20% moisture) about 4.0 kWh/kg.

What flue size does a biomass boiler need?

Approved Document J Section 2 specifies minimum flue sizes for solid fuel appliances. For biomass boilers up to 20 kW, the minimum is typically 125mm. 20-30 kW requires 150mm, and 30-50 kW needs 175mm. The flue must be at least 4.5m tall and terminate 1m above the roof ridge.

Do I need a buffer tank with a biomass boiler?

Yes, a buffer tank is strongly recommended (and often required by MCS for RHI eligibility). Biomass boilers cannot modulate as quickly as gas boilers, so a buffer tank stores excess heat during low demand periods and prevents the boiler from cycling. Typical sizes are 500-2000 litres.

What ventilation does a biomass boiler room need?

Approved Document J requires permanent ventilation for solid fuel appliances. The minimum is 550 mm² of free area per kW above 5 kW of rated output. A 20 kW boiler needs approximately 8,250 mm² of ventilation — equivalent to a 100mm diameter duct.

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Biomass Boiler Calculator — Fuel & Flue Sizing

Calculate biomass boiler sizing, fuel consumption (pellet kg/kWh), fuel store volume, flue sizing per Approved Document J, and buffer tank requirements.

Heat Demand

Design heat loss of property

Fuel Type

Type of biomass fuel

Annual Heat Demand

Total heating + DHW demand per year

How We Calculate This

This calculator sizes biomass boiler installations including fuel consumption, fuel store volume, flue sizing per Approved Document J, and buffer tank requirements.

Key calculations

Fuel (kg/year): Annual kWh ÷ (kWh/kg × efficiency)
Fuel store: 4 weeks' supply ÷ bulk density
Flue: Minimum diameter per kW (Doc J Section 2)
Ventilation: 550 mm² per kW above 5 kW
Buffer tank: Sized to prevent boiler cycling

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.