The distribution network, the buried supply and return pipes between the plant and the houses, decides how much of the fuel reaches the radiators: for 400 m of trench with a 60 mm steel pipe at 80 °C supply and 50 °C return, the loss is 108 kW bare, 11.1 kW with 50 mm of wool-class insulation, and 72.5 kW if that insulation is wet. Choose a pipe that survives 80 °C, insulate and keep it dry, bury it, leave it room to expand, vent the high points, and pressure-test it at 1.5 times its working pressure before filling the trench.
Part of District Heating
What a village hot-water pipe network consists of, what each part contributes to loss or failure, and the build and commissioning sequence.
The Four Parts And What Each Decides
- Pipe — temperature rating and joints: Pipe Materials (Clay, Steel, Plastic).
- Insulation — the loss: Insulation Methods.
- Trench and burial — protection and frost: Burial Depth & Trenching.
- Expansion and venting — keeping the water and the pipe from damaging the system: Expansion & Venting.
The Heat Budget Of The Network
Reference: 400 m of trench with a 60 mm steel pipe, supply 80 °C and return 50 °C, ground 8 °C, 4,800 h season, 230 MWh delivered to 30 houses. From the full derivation in the insulation article:
| Case | Pair loss, W per metre of trench | 400 m, kW | Season, MWh | Share of 230 MWh |
|---|---|---|---|---|
| Bare steel | 270.5 | 108 | 519 | 225% |
| 50 mm wool-class insulation | 27.7 | 11.1 | 53 | 23% |
| 50 mm, wet (k = 0.5, assumed) | 181.3 | 72.5 | 348 | 151% |
The same 400 m of pipe is therefore worth building or worthless depending on whether it is insulated and dry. Pumping cost is small by comparison; see Pump Requirements.
Build And Commissioning Sequence
- Survey the route and levels (Elevation Measurement); mark the high points for vents and the low points for drains.
- Dig the trench to the depth and width in the burial article, with a sand bed.
- Lay and join the pipe, with expansion allowances and an isolating valve at each branch.
- Pressure-test with water at 1.5 times working pressure before insulating. Salvaged Modern Pipe gives the 1.5-times rule for salvaged pipe, and Pressure Testing explains why to test with water and not steam. For a working pressure of 120 kPa (about 5 m of static head plus 72 kPa from the pump), test to 180 kPa.
- Insulate and case the pipe, sealing the ends and joints, then backfill in layers with sand around the pipe.
- Fill from the low point and vent the air from every high point.
- Heat in steps. Raise the temperature gradually, checking joints, expansion bends and supports at each stage, and watch the trench for steam or wet ground.
- Record the as-built route with distances to fixed landmarks; see Record Keeping.
Common Mistakes
- Insulating before the pressure test. A leak under wet insulation is invisible.
- Choosing the pipe by pressure rating alone. A plastic pipe rated for 20 °C is the wrong pipe at 80 °C.
- No documentation of the route. Trench maps are the only way to find a leak.
Safety
Hot pressurised water in a trench
A pipe failure at 80 °C scalds anyone standing near the leak, and a pressure test with air or steam stores energy that a water test does not. Test with water only, keep people clear of the trench during the test, and shut and drain a section before it is opened.
What’s Next
- Learn next: Pipe Materials (Clay, Steel, Plastic) — which pipe survives
- Combine with: Building Connections — where the network meets the houses