Design a wood boiler for a village loop around three numbers: a fuel rate of 37.8 kg an hour at the 120 kW peak, a heat store large enough to carry the night (about 13.8 m³ of water for 480 kWh over a 30 K swing), and a safe plan for pump failure, because with 1 m³ of water in the boiler circuit and the fire at full output the water rises from 80 °C to boiling in 11.6 minutes. Run the boiler open to the atmosphere, protect its return from cold water, and burn wood seasoned to 20% moisture.
Part of District Heating
Sizing, safety and operation of a wood-fired hot-water boiler for a village heating loop, including the heat store and the pump-failure case.
Fuel Rate And Firing Pattern
At 120 kW peak and 80% efficiency, wood at 14.3 MJ/kg (20% moisture) needs 37.8 kg/h (derivation in Heat Source); at the 48 kW season average it needs 15.1 kg/h. A batch of 100 kg lasts 100 ÷ 37.8 = 2.6 h at full output and 6.6 h at the average. So the stoker’s job changes with the weather: several loads a day at peak, one or two in mild weather.
Wood moisture matters. Coppicing & Fuel Management gives 10.1 MJ/kg at 40% moisture against 14.3 at 20%, so wet wood needs 14.3 ÷ 10.1 = 1.42 times the mass, and a firebox that cannot hold it will starve the load. Seasoning & Drying gives the way to get to 20%.
Heat Store: Let The Fire Burn Hard, Not Long
A wood fire burns best hot and steady. A store lets the boiler run at its best point and the loop draw what it needs.
Inputs: night load of 60 kW for 8 h (assumed, above the 48 kW average because it is the coldest part of the day); water swing 30 K; c = 4.18 kJ/kg·K.
- Energy = 60 kW × 8 h = 480 kWh = 1,728 MJ.
- Mass of water = 1,728,000 kJ ÷ (4.18 × 30) = 13,780 kg, about 13.8 m³.
This agrees with Seasonal Storage, which gives 1 m³ of water over 40 K as 167 MJ (46 kWh); over 30 K it is 125 MJ, and 1,728 ÷ 125 = 13.8 m³. A tank of that size is a major build (see Hot Water Tanks and Heat Storage Systems). A smaller store cuts the number of hours the boiler can be off.
Pump Failure: The Case That Injures
If the circulation pump stops while the fire is burning, the boiler’s heat has nowhere to go.
Inputs: 1 m³ of water in the boiler circuit (assumed); fire at 120 kW; start at 80 °C; open vent at 100 °C.
- Heat to reach boiling = 1,000 kg × 4.18 × 20 K = 83.6 MJ.
- Time = 83,600 kJ ÷ 120 kW = 697 s = 11.6 min.
The burning fuel bed holds more heat still, so it will not stop at 120 kW when the pump stops. Three defences, none from the corpus and all engineering reasoning:
- A gravity safety loop from the boiler to a tank or radiator above it, which circulates without a pump (Thermosiphon Systems gives the principle).
- An air damper that closes by gravity or spring when a thermostat or a fusible link releases it, so the fire starves.
- An open vent that lets the boiler boil gently into a safe place, instead of building pressure, so it cannot burst.
Only an open or properly relieved system survives this; see Safety Valve.
Boiler Layout
The carrier page offers a water-jacket firebox plus fire tubes, with a firebox of 60 × 60 × 100 cm for 30 buildings. A 0.6 × 0.6 m grate is 0.36 m², so 120 kW over it is 333 kW/m². This page cannot confirm that figure from the corpus; test-fire the boiler and measure the real output (the flow rate and the temperature rise across the boiler) before connecting the village. Fire-tube construction follows Fire-Tube Boiler and Boiler Construction; test the shell with water, not steam (Pressure Testing).
Return Protection
A solid-fuel boiler fed with cold return water can condense tar and acid on its heat-exchange surfaces. Commonly quoted minimum return temperatures are 55 to 60 °C (recalled, not corpus). A mixing valve that blends hot boiler outlet water into the return protects the boiler while the network return runs cold.
Common Mistakes
- No plan for pump failure. It is the most dangerous single event.
- Sizing only for peak and burning at 20% output all season. A boiler that smoulders at part load tars and smokes; the store avoids this.
- Burning unseasoned wood. More fuel, more tar, less heat.
Safety
Never isolate a wood boiler from its vent
A closed valve between a fired boiler and its open vent or relief turns it into a pressure vessel with a live fire under it. Have no valve between boiler and vent, check the vent is clear before every firing, keep carbon monoxide detection in the boiler house, and make sure the person on duty knows what to do within 10 minutes of a pump stop.
What’s Next
- Learn next: Expansion & Venting — the open vent and the expansion volume
- Combine with: Pump Requirements — the pump the failure plan depends on