Extend a village heating network by adding houses close to the existing main, because a spur’s loss is fixed by its length, 133 kWh per metre per season at 27.7 W/m: a house on a 5 m spur loses 9% of the 7,680 kWh it uses, one on a 50 m spur 87%, and one on a 100 m spur 173%, more than it receives. Growing from 30 to 40 houses on the same main takes the plant from 120 to 160 kW and the pump from 31 to 51 W of hydraulic power, so plan the boiler staging, the heat store and the pipe size before the houses arrive.
Part of District Heating
How to add houses to an existing village heating network: what each addition costs in heat, pumping and plant, and how to prepare the system for growth.
What A Spur Costs
The pair of pipes loses 27.7 W for each metre of trench (the reference case: 50 mm wool-class insulation, 80 / 50 °C, derived in Insulation Methods). Over a 4,800-hour season that is 27.7 × 4.8 = 133 kWh per metre. A house using 7,680 kWh a season (4 kW design, 40% average load):
| Spur length | Loss per season | Loss as share of the house’s 7,680 kWh |
|---|---|---|
| 5 m | 665 kWh | 9% |
| 10 m | 1,330 kWh | 17% |
| 20 m | 2,660 kWh | 35% |
| 50 m | 6,650 kWh | 87% |
| 100 m | 13,300 kWh | 173% |
Working, 100 m: 100 × 133 = 13,300 kWh; ÷ 7,680 = 1.73. The loss, spent all season whether or not the house calls for heat, is 1.73 times what it receives. A far house is better served by its own stove, or by a cluster that shares one spur. The break-even against stoves, which is where the whole network stops saving fuel, is worked in Central Plant vs Distributed Sources.
Adding houses to the main, with short spurs, improves the ratio. Ten new houses on 5 m services add 50 m of trench: 50 × 133 = 6.65 MWh a season against their 76.8 MWh of demand, 8.7%, much better than the original network’s 23%.
What More Houses Do To Pump And Boiler
Assumptions: the main stays the same nominal 50 mm steel pipe and the same 250 m, each house draws 4 kW design, 30 K drop, the exchanger drop stays at 20 kPa. Pressure from the method in Pump Requirements (tapered flow along the main).
| Houses | Peak load | Flow | Velocity | Friction at full flow | Head, farthest house | Hydraulic power | Electric at 40% |
|---|---|---|---|---|---|---|---|
| 30 | 120 kW | 0.98 L/s | 0.45 m/s | 49 Pa/m | 31 kPa | 31 W | 76 W |
| 40 | 160 kW | 1.31 L/s | 0.60 m/s | 86 Pa/m | 39 kPa | 51 W | 128 W |
| 50 | 200 kW | 1.64 L/s | 0.76 m/s | 132 Pa/m | 50 kPa | 81 W | 203 W |
| 60 | 240 kW | 1.96 L/s | 0.91 m/s | 188 Pa/m | 62 kPa | 122 W | 305 W |
Going from 30 to 40 houses raises the trunk friction by 86 ÷ 49 = 1.76 times and the electrical power by 128 ÷ 76 = 1.7 times. The 50 mm main works to 60 houses on paper, but the pump power quadruples; a larger pipe on the first stretch of the main reduces it. Re-do the pump calculation for each stage.
The plant must grow too:
- Boiler: 160 kW burns 160 × 3.6 ÷ 11.44 = 50.4 kg of wood an hour (80% efficiency, 14.3 MJ/kg).
- Heat store: scales with the night load, 13.8 m³ for 120 kW, so about 18.4 m³ at 160 kW.
- Expansion tank: grows with the water volume; re-check it (Expansion & Venting).
- Staging: two boilers of 80 to 100 kW each let one carry the mild-weather load and give a backup.
Method: Preparing For Growth
- Build the first main with stubs. At every planned branch fit a tee and an isolating valve, and cap the stub. A capped stub costs little in heat and removes a hot-tap job later.
- Choose the main for the end state. A pipe one size up costs little more than the trench; the pump is cheaper to run for it.
- Survey and mark future building plots; see Contour Mapping and keep records (Record Keeping).
- Connect nearest first. The nearest houses cost least in heat per kWh.
- After each addition, re-balance the throttles, vent the system and check the return temperature; a new house starves the far end if it is not balanced.
- Log the plant headroom at the coldest weather before saying yes to the next house.
Common Mistakes
- Treating a far house as free to connect. The spur’s loss can exceed the house’s demand.
- Adding houses until the pump struggles. Re-size when the head doubles.
- Forgetting the store and tank. They scale with load.
- Hot-tapping a live main. Shut and drain the section.
Safety
Never break into a live main
A connection made by cutting into a main at 80 °C and 120 kPa releases scalding water. Use the capped stubs, close the valves each side, drain and let the section cool, and re-test the new joints at 1.5 times working pressure before opening up.
What’s Next
- Learn next: Operation & Scaling — logging and the yearly routine
- Combine with: Pump Requirements — the pump calculation for each stage