A district heating system is a central heat source, an insulated supply pipe, a return pipe, a connection at each building and a circulating pump, and it works or fails on three numbers: the heat each building draws, the heat lost per metre of buried pipe, and the power to pump the water. For a village of 30 houses at 4 kW each, 400 m of trench with 50 mm of mineral-wool-class insulation loses about 11.1 kW at 80 °C supply and 50 °C return, which is 9% of the 120 kW peak and 23% of the heat delivered over a 200-day season.
Part of District Heating
The overall design of a village hot-water network and the numbers that decide whether it is worth building.
Five Parts
- Central plant: a boiler (or several sources) that heats the water. See Heat Source.
- Supply pipe: carries hot water out, typically 70 to 90 °C.
- Return pipe: brings the cooled water back, typically 40 to 60 °C.
- Building connections: a heat exchanger or direct connection at each house. See Building Connections.
- Circulation pump: moves the water. See Pump Requirements.
Everything else (valves, vents, an expansion tank, a heat store) supports these five.
The Reference Case Used In This Series
| Input | Value | Source |
|---|---|---|
| Houses | 30 | assumed |
| Design heat per house | 4 kW | within the 3 to 5 kW on the District Heating page |
| Peak plant output | 30 × 4 = 120 kW | derived |
| Trench length | 250 m main + 30 services × 5 m = 400 m | assumed layout |
| Pipe | nominal 50 mm steel: 60.3 mm outside, 52.5 mm inside diameter (handbook dimensions, recalled); one supply, one return | assumed |
| Insulation | 50 mm, conductivity 0.041 W/m·K | wool value in Natural Insulation Materials |
| Temperatures | 80 °C supply, 50 °C return, ground 8 °C | assumed |
| Heating season | 200 days, 4,800 h at 40% average load | assumed |
The Three Numbers For That Case
- Demand. 4 kW × 0.40 × 4,800 h = 7,680 kWh per house; × 30 = 230 MWh per season.
- Loss. About 27.7 W per metre of trench for the pair of pipes (derived in Insulation Methods); × 400 m = 11.1 kW; × 4,800 h = 53 MWh. That is 53 ÷ 230 = 23% of what the houses take, and 11.1 ÷ 120 = 9% of the peak.
- Pumping. About 70 W of hydraulic power as an upper bound (derived in Pump Requirements), around 175 W of electricity.
The loss is the number that decides the project. Pumping is small; boilers are big but known. The heat that leaks from the pipes into the ground depends on how many metres of trench each house needs, which is 400 ÷ 30 = 13.3 m per house here.
What The Architecture Chooses
- Central plant or several sources — Central Plant vs Distributed Sources.
- Pipe layout — Supply & Return Piping Layout.
- Temperatures and flow — Design Temperature & Flow Rate.
- The distribution network itself — Distribution Network.
Common Mistakes
- Designing the plant and ignoring the pipes. The plant is a one-time cost; the pipe loss is paid every hour of every season.
- Counting trench in streets, not metres per house. A 400 m network for 30 houses is compact; the same houses spread over 1.2 km lose three times as much per house.
- Stating a number of households served without the pipe run. A figure for houses with no trench length behind it has no loss behind it either.
Safety
The loop is 80 to 90 °C water under pressure
Water at 80 °C scalds in seconds, and a leak in a trench can bring hot water to the surface. An open-vented system can reach at most 100 °C at the vent before it boils over; a sealed one can store enough energy to rupture. Fit a vent or safety valve, do not isolate the boiler from its vent, and keep a pump-failure plan: see Safety Valve and Wood-Fired Boiler Design.
What’s Next
- Learn next: Central Plant vs Distributed Sources — when a single plant pays back its pipes
- Combine with: Insulation Methods — the derivation of the loss number