Connect each building to a village heating main through a heat exchanger and a set of isolating valves if you can, and directly only if you must, because in a direct connection a leak inside one house drains the whole network: a 15 mm hole at 120 kPa passes about 1.7 L/s and empties a 0.75 m³ expansion tank in 7 minutes, leaving the boiler short of water. Every connection needs two isolating valves, a strainer, a throttle to balance the flow, a thermometer in the supply and return, a drain and a vent.
Part of District Heating
How a house is hooked up to the village hot-water main: the connection types, the parts of the substation, and how to set it up.
Three Ways To Connect
| Type | How | Advantage | Risk |
|---|---|---|---|
| Direct | District water flows through the house radiators | No exchanger; simplest | A leak or dirt in one house affects all; house pressure equals network pressure |
| Indirect, exchanger | A heat exchanger separates the district loop from the house loop | A house fault stays in the house; the house loop can have its own vent and pressure | Needs an exchanger and a small pump or thermosiphon in the house loop |
| Indirect, tank | A coil in the house’s storage tank | Doubles as hot water storage | Larger coil area, and slow recovery |
The carrier page describes all three and calls the direct connection “simpler but a leak in any building drains the whole system”.
What A Leak Does
Inputs: a 15 mm round hole in the house loop; pressure 120 kPa; water density 975 kg/m³; discharge coefficient 0.62 for a sharp-edged opening (a recalled handbook value); expansion tank of 0.75 m³ (sized in Expansion & Venting).
- Area = π ÷ 4 × 0.015² = 1.77×10⁻⁴ m².
- Ideal jet speed = √(2 × 120,000 ÷ 975) = 15.7 m/s.
- Flow = 0.62 × 1.77×10⁻⁴ × 15.7 = 1.72×10⁻³ m³/s = 1.7 L/s.
- Time to drain the tank = 0.75 ÷ 1.72×10⁻³ = 436 s = 7.3 min.
The head falls as the tank drains, so the real figure is a little longer, but it is minutes, not hours. After that the plant loses water at the leak, air enters, and the boiler runs short of water. A float make-up valve and a low-level alarm on the tank are required for a direct-connected network, and a worthwhile addition to any.
The Substation, Part By Part
- Two isolating valves on the supply and the return at the building, so the house can be cut off. Close both before touching anything.
- A strainer on the supply. Rust and scale from salvaged radiators block exchangers and valves.
- A throttle (balancing) valve on the return, set at commissioning so each house gets its share (see Supply & Return Piping Layout).
- A thermometer on the supply and one on the return. The return temperature tells you how well the house uses the heat; the heat drawn is m × c × (supply − return).
- A drain and a vent at the low and high points of the house loop.
- The heat exchanger or coil (Heat Exchanger Design).
- Emitters (Radiator & Underfloor Heating) and a hot water tank (Domestic Hot Water Takeoff).
Why The Return Temperature Matters
A house that returns water at 65 °C when it could return at 45 °C takes the same 4 kW with more flow: at a 15 K drop the flow is 4 ÷ (4.18 × 15) = 0.064 kg/s, against 4 ÷ (4.18 × 35) = 0.027 kg/s at 35 K, 2.3 times as much. The extra flow raises the network’s return temperature, wastes pumping power and heat, and starves houses further along. Log each house’s return temperature and fix the worst first.
Commissioning A Connection
- Flush the house loop and strainer before opening to the network.
- Test the house loop with water at 1.5 times its working pressure (the rule in Salvaged Modern Pipe); salvaged radiators often leak.
- Open the supply and return slowly, venting at the high point.
- Set the throttle so the return temperature is as low as the radiators allow while the rooms stay warm.
- Record the settings against the house.
Common Mistakes
- Connecting a radiator system with unknown pressure rating directly. Test it first.
- No strainer. One piece of scale stops a throttle valve.
- Setting the throttle once and leaving it. Check return temperatures through the season.
Safety
A leak in a direct connection can run a boiler dry
A house leak draining the tank can leave a fired boiler short of water, and a boiler that heats with no water in it can fail catastrophically. Fit a low-level alarm and a float make-up, and teach every household where the isolating valves are. Water at 70 °C or more scalds instantly; shut both valves and let a house loop cool before opening a joint.
What’s Next
- Learn next: Heat Exchanger Design — sizing the unit that separates the loops
- Combine with: Pump Requirements — why high return temperatures raise the pump load