Lay out a village heating network as a two-pipe tree, one supply and one return in the same trench with a branch to each house, because every house then sees the full supply temperature and can be valved off alone. A single-pipe series loop passing 5 houses that each take 4 kW from a 20 kW flow cools the water by 6 K at every house, so the fifth house gets 56 °C against 80 °C at the first and needs radiators about twice the size.
Part of District Heating
The topologies for a village hot-water network (two-pipe tree, reverse return, series loop), what each does to temperature, flow balance and trench length.
Three Layouts
Two-pipe tree (standard). One supply pipe and one return pipe run side by side from the plant along the main, with a short two-pipe branch to each house. Each house is in parallel, takes water at the full supply temperature, and returns it to the return pipe. The carrier page calls this the most flexible, and each building is independent.
Reverse return. A variant that makes every circuit the same length, so each house gets the same pressure difference without valves.
Single-pipe series loop. One pipe passes through all the buildings in turn. Each takes heat from the water, so the temperature falls along the loop. The carrier page limits it to 5 to 10 buildings with low extraction.
What The Series Loop Does To Temperature
Inputs: 5 houses, 4 kW each, 20 kW total, supply 80 °C and return 50 °C (a 30 K drop around the loop), c = 4.18 kJ/kg·K.
- Flow = 20 ÷ (4.18 × 30) = 0.159 kg/s.
- Each house takes 4 kW, so it cools the water by 4 ÷ (0.159 × 4.18) = 6.0 K.
- Water arrives at house 1 at 80 °C, house 2 at 74, house 3 at 68, house 4 at 62, house 5 at 56, and leaves at 50.
Radiators give less output at lower water temperature (see Radiator & Underfloor Heating): with the output rising as the 1.3 power of the water-to-room difference, house 5 (mean water 53 °C, difference 33 K) delivers (33 ÷ 57)^1.3 = 0.49 of what the same radiator gives at house 1 (mean 77 °C, difference 57 K). So the last radiator must be about twice as large, and any house that turns its radiator down changes the temperature of everyone downstream. The series loop is good for a cluster of 5 identical houses with one operator; a village should use a tree.
Direct Return Versus Reverse Return
In a tree where the return pipe simply runs back along the supply, the first house has the shortest circuit and the last the longest. For N houses at spacing s:
- Circuit length for house k: 2 × k × s. With N = 10, s = 15 m: 30 m for the first, 300 m for the last, so the first house pulls most of the flow unless something throttles it.
- Reverse return: the return pipe is routed so that the first house’s return connects furthest from the plant. All circuits are then equally long, but the total pipe goes from 2 × N × s to 3 × N × s: 300 m to 450 m for the example, 50% more pipe (and a third pipe in the trench).
The village solution is direct return with a throttling valve at each branch, set at commissioning by measuring flow or return temperature. See Building Connections. Reverse return suits a very long main with many equal branches.
Planning The Route
- Survey the levels along the route. Use Elevation Measurement and Contour Mapping to mark the high points (where air collects, so vents go) and the low points (drain valves). See Expansion & Venting.
- Measure the metres of trench per house with Distance Measurement. This is the number that sets the loss; see Central Plant vs Distributed Sources.
- Run the main where the houses are densest and put short spurs, not long ones, to outlying houses. A 100 m spur with a 4 kW house loses 2.8 kW all season; see Expanding the Network.
- Put an isolating valve at the head of every branch and at every junction on the main, so one section can be shut for repair.
- Keep supply and return in one trench: one excavation, and the shared ground warms between them.
- Slope the mains toward a drain valve at the lowest point, and place a vent at each high point.
- Mark every valve, junction and bend on a map and in the ground. See Record Keeping.
Common Mistakes
- Using the series loop for a village. The downstream houses get cold water, and one house’s valve affects the rest.
- Crossing supply and return. One transposition between plant and house reverses flow and reduces output.
- No isolation valves. One failure then shuts the whole village.
Safety
Isolation valves and hot pipes
A closed section of pipe full of hot water heats and expands; a valve that isolates it between two points without a relief can burst it. Close isolating valves only on a section that is vented or has a relief, and drain and cool a section before opening it. Water at 80 °C scalds instantly.
What’s Next
- Learn next: Design Temperature & Flow Rate — how the 30 K drop sets the pipe size
- Combine with: Distribution Network — the pipe, insulation and trench that the layout needs