Choose the supply and return temperatures first, because they fix the flow through m = Q ÷ (c × ΔT): a 120 kW load at a 30 K drop needs 0.96 kg/s (0.98 L/s) in a nominal 50 mm steel pipe, losing 49 Pa of pressure per metre, while a 40 K drop needs 0.72 kg/s and about half the pumping power, and a 20 K drop needs 1.43 kg/s and nearly three times as much. Cooler supply water also cuts the pipe loss in proportion to its excess over the ground temperature, but it needs larger radiators.
Part of District Heating
Choosing the supply temperature, return temperature and flow rate of a village heating loop, and what each choice does to flow, pumping and heat loss.
The Flow Equation
The heat carried by water is Q = m × c × ΔT, with m the mass flow in kg/s, c = 4.18 kJ/kg·K and ΔT the supply-minus-return temperature difference. So m = Q ÷ (c × ΔT). The carrier page puts supply at 70 to 90 °C, return at 40 to 60 °C and ΔT at 20 to 30 °C, and says a larger ΔT means smaller pipes but larger heat exchangers.
Worked Example: The 120 kW Reference Case
Inputs: Q = 120 kW; water density 975 kg/m³ and viscosity 0.00044 Pa·s at about 65 °C (handbook values, not from the corpus); pipe nominal 50 mm steel, 52.5 mm inside diameter (area 2.165e-3 m²; handbook dimension, recalled), roughness 0.1 mm (assumed for steel); friction from the Darcy–Weisbach equation with the Haaland friction factor (recalled, an engineering approximation).
| ΔT | Mass flow | Volume flow | Velocity | Friction per metre | Hydraulic power, upper bound |
|---|---|---|---|---|---|
| 20 K | 1.43 kg/s | 1.47 L/s | 0.68 m/s | 108 Pa/m | 194 W |
| 30 K | 0.96 kg/s | 0.98 L/s | 0.45 m/s | 49 Pa/m | 70 W |
| 40 K | 0.72 kg/s | 0.74 L/s | 0.34 m/s | 29 Pa/m | 37 W |
Working, the 30 K row: m = 120 ÷ (4.18 × 30) = 0.957 kg/s; volume flow = 0.957 ÷ 975 = 0.98 L/s; velocity = 0.98e-3 ÷ 2.165e-3 = 0.45 m/s; Reynolds number = 975 × 0.45 × 0.0525 ÷ 0.00044 = 52,700; friction factor 0.0259; pressure drop = 0.0259 × 975 × 0.45² ÷ (2 × 0.0525) = 49 Pa/m. The hydraulic power column assumes the full flow through all 800 m of pipe, plus 30% for fittings and 20 kPa across the house heat exchangers, so it is an upper bound; derivation in Pump Requirements.
Going from ΔT = 20 K to 40 K halves the flow and cuts the friction per metre by 108 ÷ 29 = 3.7 times.
Temperature Against Heat Loss
Pipe loss is proportional to the pipe’s excess over the ground, so a cooler loop leaks less. With 50 mm of wool-class insulation on 60 mm pipe, buried 0.8 m in 8 °C ground (derived in Insulation Methods):
| Supply / return | Loss for the pair, W per metre of trench | Over 400 m |
|---|---|---|
| 80 / 60 | 30.1 | 12.0 kW |
| 80 / 50 | 27.7 | 11.1 kW |
| 80 / 40 | 25.3 | 10.1 kW |
| 65 / 40 | 21.6 | 8.6 kW |
Running 65 °C supply and 40 °C return instead of 80 and 50 cuts the loss by (27.7 − 21.6) ÷ 27.7 = 22%.
What Pushes The Other Way
- Radiators. They give less at lower water temperatures; see Radiator & Underfloor Heating. Existing salvaged radiators set the minimum supply temperature.
- Heat exchangers. A larger ΔT on the district side needs a lower primary return temperature, so the exchanger must be larger; see Heat Exchanger Design.
- Boiler return temperature. A boiler that receives return water that is too cold can condense water and tar from the flue gas onto its heat-exchange surfaces. Commonly quoted minimum return temperatures for solid-fuel boilers are in the range of 55 to 60 °C; that figure is recalled, not from the corpus, and a mixing valve that blends some hot outlet water into the boiler return protects the boiler while the network return stays cold.
- Domestic hot water. Hot water for washing needs 60 °C or more stored; see Domestic Hot Water Takeoff.
Method: Setting The Design Point
- Set the supply at the temperature your radiators need at the coldest design day (70 to 90 °C is the carrier’s range).
- Choose the largest ΔT the house heat exchangers and boiler return protection allow; 30 K is a sensible start.
- Compute the flow for the plant’s peak output, size the main for a friction of a few tens of Pa/m, and check the velocity.
- Run the supply colder in mild weather: loss falls with (T − T_ground) and the flow falls with the load.
Common Mistakes
- Reading the temperatures off a radiator’s nameplate. Test it; see the radiator page.
- Designing for the largest ΔT only. The exchanger and boiler may not allow it.
- Running at design temperature all season. The loss is paid every hour.
Safety
Supply above 90 °C is a boiling margin, not a target
An open-vented loop boils at 100 °C at the vent. Keep the design supply at 90 °C or below, a high-limit thermostat or relief on the boiler, and a heat sink for pump failure; see Wood-Fired Boiler Design.
What’s Next
- Learn next: Distribution Network — pipe, insulation and trench for the chosen temperatures
- Combine with: Pump Requirements — the pumping power in full