How this biomass boiler calculator works
Wood is a variable fuel. A tonne of fresh-cut logs and a tonne of pellets differ in energy by a factor of two, mostly because of water. Buying, storing and burning wood well therefore starts with the moisture content. This calculator converts your annual heat demand into the mass and volume of logs, chips, pellets or briquettes you need, then sizes the boiler, buffer tank and fuel store and compares the running cost with gas, oil and a heat pump.
The steps:
- Fuel energy = heat demand ÷ seasonal boiler efficiency.
- Net calorific value of the wood as received, from its moisture content (ISO 17225 relationship).
- Mass = fuel energy ÷ NCV; volume from the bulk density of the fuel type and wood species at that moisture.
- Boiler output from full-load hours, buffer tank from litres per kW, store volume from the number of coldest months it must cover.
- Costs compared with a 92% gas boiler, an 88% oil boiler and a heat pump, plus payback of the extra investment against a gas boiler.
The formula
Worked example: a family home on pellets
The defaults describe a house needing 20,000 kWh of heat a year with an ENplus A1 pellet boiler (8% moisture, 91% seasonal efficiency):
- pellet NCV = 18.8 × 0.92 − 2.443 × 0.08 = 17.1 MJ/kg = 4.75 kWh/kg;
- fuel needed = 20,000 / (0.91 × 4.75) ≈ 4.6 tonnes a year, about 7.1 m³ at 650 kg/m³;
- at €330/t that costs about €1,530 a year, or 7.6 cents per kWh of heat, versus 10.9 cents from a gas boiler at €0.10/kWh;
- the boiler should be about 13 kW; a pellet store holding a year’s supply needs around 10.5 m³ gross, because only about two-thirds of a sloped-floor store is usable;
- including maintenance, the saving against gas is about €520 a year, so a €16,000 pellet system (vs €4,000 for a gas boiler) needs roughly 18 years to recover the difference — faster with grants, higher gas prices or carbon taxes.
Switch the fuel to logs and the same house needs over 6 t (about 19 stacked m³ of softwood) but at a lower cost per kWh; try the wet logs preset to see how 35% moisture inflates both the tonnage and the emissions.
How to read the results
- Fuel needed per year: tonnes as received, with the equivalent stacked or loose volume to compare suppliers’ units.
- Net calorific value is the energy in each kilogram you buy; compare price per kWh, not per tonne.
- Buffer tank: mandatory in practice for log boilers; optional for modulating pellet and chip boilers, but it reduces cycling.
- Fuel store volume is the gross volume, including the unusable share of the store.
- Cost comparison chart shows fuel plus maintenance for each heating option.
Typical values
| Fuel | Moisture | NCV | Bulk density | Volume per MWh of fuel |
|---|---|---|---|---|
| Wood pellets | 6–10% | 4.7–4.9 kWh/kg | 600–700 kg/m³ | 0.32 m³ |
| Wood briquettes | 8–10% | 4.6–4.8 kWh/kg | 650–750 kg/m³ stacked | 0.30 m³ |
| Seasoned logs, hardwood | 15–20% | 4.0–4.3 kWh/kg | 450–520 kg per stacked m³ | 0.5 m³ |
| Seasoned logs, softwood | 15–20% | 4.0–4.3 kWh/kg | 300–360 kg per stacked m³ | 0.75 m³ |
| Wood chips, softwood | 25–35% | 3.2–3.7 kWh/kg | 200–250 kg per loose m³ | 1.3 m³ |
| Green wood | 45–55% | 2.0–2.5 kWh/kg | — | — |
Tips
- Buy dry, buy by energy. Ask for moisture content and compare the price per kWh; cheap wet wood is often the most expensive.
- Season logs for at least two summers under cover with airflow; softwood dries faster than oak.
- Size the store for delivery logistics. Larger pellet or chip deliveries are cheaper per tonne.
- Insulate first. Every kWh of heat demand saved reduces fuel handling, store size and boiler size.
- Service yearly. Clean heat exchangers keep efficiency high; a 1 mm layer of soot can cut efficiency by several percent.
- Compare all heating options side by side with the heating cost comparison calculator.
Limitations
Calorific values and bulk densities are typical; species, chip size, log length and stacking change them by ±10–20%. Seasonal efficiency depends strongly on boiler sizing, hydraulics and use pattern. Oversized boilers that cycle on and off perform worse than the rated value. The monthly profile is a generic temperate-climate heating curve. Wood combustion CO₂ is treated as biogenic, which assumes sustainably managed forests; local air-quality regulations and particulate emissions are not modelled.
Method & assumptions
Fuel energy = heat demand ÷ seasonal efficiency. Net calorific value as received: NCV = 18.8 × (1 − w) − 2.443 × w MJ/kg (w = moisture, wet basis), ÷ 3.6 for kWh/kg. Fuel mass = fuel energy ÷ NCV. Volume = mass ÷ bulk density, where bulk density = dry mass per m³ ÷ (1 − w). Boiler size = demand ÷ full-load hours (+10%). Buffer tank for log boilers ≥ 55 L per kW (EN 303-5 practice). Store volume covers the selected number of coldest months, divided by the usable share of the store.
Frequently asked questions
How many tonnes of wood pellets do I need per year?
How much heat is in a kilogram of wood?
Why does moisture content matter so much?
What size biomass boiler do I need?
Do log boilers need a buffer tank?
Is heating with wood cheaper than gas?
Sources & references
- ISO 17225: Solid biofuels: Fuel specifications and classes (pellets, chips, logs)
- EN 303-5: Heating boilers for solid fuels, manually and automatically stoked, nominal heat output up to 500 kW
- ENplus: quality certification scheme for wood pellets
- Biomass Energy Centre / Forest Research (UK): Typical calorific values of fuels
- IEA Bioenergy: Task 32 Biomass Combustion
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