Estimate a heating season’s fuel by adding the heat the houses use, the heat lost in the pipes and the boiler’s inefficiency: 30 houses at 4 kW design and 40% average load over 4,800 h need 230.4 MWh, the 400 m network loses 53 MWh, and an 80% boiler burns 354.5 MWh of fuel, which is 89 t of wood at 20% moisture, 126 t at 40%, 99 t of peat, or 43 to 53 t of bituminous coal. Individual stoves for the same houses would burn 145 t at 40% efficiency, 105 t at 55% and 83 t at 70%, so the saving from district heating is large only against poor stoves.
Part of District Heating
A step-by-step energy balance for a village heating season: demand, network loss, fuel by type, comparison with stoves, and the woodland it takes.
The Energy Balance
All quantities are for the reference case of this series: 30 houses, 4 kW design each, 4,800 h heating season (200 days), 400 m of trench with 50 mm of wool-class insulation, 80 / 50 °C supply and return.
- Heat demand. 4 kW × 0.40 (assumed average load as a share of design) × 4,800 h = 7,680 kWh per house; × 30 = 230.4 MWh = 829 GJ.
- Network loss. 27.7 W per metre of trench for the pair (from Insulation Methods) × 400 m = 11.1 kW; × 4,800 h = 53.2 MWh = 191 GJ. The loss does not fall with the load unless the supply temperature is lowered in mild weather.
- Heat the boiler must make. 230.4 + 53.2 = 283.6 MWh.
- Fuel energy. At 80% boiler efficiency: 283.6 ÷ 0.80 = 354.5 MWh = 1,276 GJ.
Fuel Quantities
Tonnes = fuel GJ ÷ GJ per tonne. Heating values: wood and peat from BTU Content Compared to Wood (net 14.3 MJ/kg at 20% moisture wet basis; 10.1 at 40% from Coppicing & Fuel Management; peat 12.9 at 30%); coal from Coal Types & Quality.
| Fuel | GJ per tonne | Tonnes per season |
|---|---|---|
| Wood, 20% moisture | 14.3 | 89 |
| Wood, 40% moisture | 10.1 | 126 |
| Peat, 30% moisture | 12.9 | 99 |
| Bituminous coal | 24 to 30 | 53 to 43 |
| Sub-bituminous coal | 15 to 20 | 85 to 64 |
| Lignite | 10 to 15 | 128 to 85 |
Working, wood at 20%: 1,276 ÷ 14.3 = 89.2 t. The carrier page’s “80 to 120 tonnes” for wood and “50 to 80” for coal agree with the table. It does not say which moisture basis its tonnes are on, and a tonne of wet wood holds far less heat than a tonne of seasoned wood.
Sensitivity To The Average Load
The 40% load factor is an assumption. The loss is fixed at 53.2 MWh.
| Average load | Demand | Fuel at 80% | Wood at 20% |
|---|---|---|---|
| 30% | 172.8 MWh | 282.5 MWh | 71 t |
| 40% | 230.4 MWh | 354.5 MWh | 89 t |
| 50% | 288.0 MWh | 426.5 MWh | 107 t |
Check your own load factor from a season’s fuel log: weigh a sample of each load of wood, count loads, and divide the heat by the hours.
Against Individual Stoves
The same 230.4 MWh at stove efficiencies of 40, 55 and 70% (the carrier’s 40 to 70% range) is 576, 419 and 329 MWh of fuel: 145, 105 and 83 t of wood at 14.3 GJ/t.
| Stove efficiency | Stove wood | District wood | District saving |
|---|---|---|---|
| 40% | 145 t | 89 t | 38% |
| 55% | 105 t | 89 t | 15% |
| 70% | 83 t | 89 t | −7% (district burns more) |
The break-even stove efficiency is 230.4 ÷ 354.5 = 65%. The carrier page’s “150 t against 80 to 100 t, a 30 to 50% saving” matches the 40% stove case and not the 55% one, so the figure depends on how bad the stoves are. Fuel Budgeting gives 5 to 10 tonnes of air-dried wood per household per year for cooking, heating, hot water and workshop together, or 150 to 300 t for 30 households; district heating replaces only the heating share.
Woodland Needed
The corpus yield of coppice is 3 to 15 t per hectare per year (Yield Calculations per Acre), a range whose tonne basis the page itself says is unstated. At 89 t a year that is 89 ÷ 15 = 5.9 ha to 89 ÷ 3 = 30 ha. The carrier page’s 10 to 20 ha is inside that range. Measure your own coppice yield on the same moisture basis as your fuel.
Common Mistakes
- Mixing tonne bases. Green, air-dried and oven-dry tonnes differ by large factors; always say which.
- Forgetting the pipe loss. At 23% of the demand it is not a rounding error.
- Using the design load for energy. The average load over the season is what counts.
Safety
A fuel stack for 90 tonnes is a fire load
A season’s wood, coal or peat is a very large store of combustible material. Keep it well apart from the boiler house and dwellings, stack it stably, keep coal and peat piles from heating spontaneously by not piling them deep and wet, and have a rule for who may enter the store. A boiler house full of unburnt fuel is also a carbon monoxide risk when the fire smoulders.
What’s Next
- Learn next: Pump Requirements — the power the pump needs on the same loop
- Combine with: Expanding the Network — how the fuel changes as houses are added