Size the circulation pump from the flow and the pressure drop of the most distant house: the 120 kW village loop at a 30 K drop needs 0.98 L/s and about 31 kPa (3.2 m of head) once the flow tapers along the main, which is 31 W of hydraulic power and about 76 W of electricity at an assumed 40% overall efficiency, with 71.4 kPa and 70 W as the upper bound if the full flow ran through every pipe. Gravity circulation gives only 159 Pa per metre of boiler-to-radiator height, so it cannot drive the village loop and is marginal even for a 5-house pilot.
Part of District Heating
How to calculate the flow, head and power of the circulation pump for a village hot-water loop, whether gravity can replace it, and how to power it.
Flow And Pressure Drop
Hydraulic power is flow times pressure rise, P = Q × Δp. The flow for the reference case is set by the heat and the temperature drop (see Design Temperature & Flow Rate): 120 kW at 30 K needs 0.957 kg/s, 0.98 L/s. In the nominal 50 mm steel main (52.5 mm inside diameter) that is 0.45 m/s with a friction loss of 49 Pa per metre at full flow (Darcy–Weisbach with the Haaland friction factor; water properties and pipe roughness are handbook values, recalled).
Upper bound. Full flow through all 800 m of pipe, plus 30% for fittings, plus 20 kPa across the house exchangers (assumed): 49.4 × 800 × 1.3 = 51.4 kPa, plus 20 gives 71.4 kPa, and 0.98 L/s × 71.4 kPa = 70 W.
Tapered estimate. The flow in the main falls as houses draw off their share. With 30 equal draws along the main, friction at distance x scales roughly as (1 − x/L)^1.9, whose integral over the length is about 0.345 of the full-flow value (an approximation). For the farthest house:
- Supply and return, 250 m each: 2 × 250 × 49 Pa/m × 0.345 = 8.5 kPa.
- Fittings, +30%: 11.1 kPa.
- House exchanger, 20 kPa (assumed).
- Service pipes: a 10 m run of 20 mm pipe at one house’s flow loses about 0.1 kPa (negligible).
- Total, about 31 kPa, or 3.2 m of water head (31.1 ÷ 9.81).
Power = 0.98×10⁻³ m³/s × 31.1×10³ Pa = 30.5 W, call it 31 W of hydraulic power. At 40% overall efficiency (pump and motor, an assumed value) the electrical power is 31 ÷ 0.40 = 76 W. The carrier page’s range for the pump, 50 to 200 W, brackets this. The valves at the nearer houses throttle the surplus head away: the pump must satisfy the worst circuit.
Gravity Instead Of A Pump
A thermosiphon pushes with the density difference of the hot and cold columns. With water densities of 971.8 kg/m³ at 80 °C and 988.0 kg/m³ at 50 °C (handbook values, recalled; the corpus uses the same method for 20 and 60 °C in How Thermosiphon Works), the driving pressure per metre of height between heater and cooler is 16.2 × 9.81 = 159 Pa/m. The village loop needs 31,100 Pa, which would take 195 m of height. Not possible.
A 5-house pilot (20 kW). Flow at 30 K is 0.159 kg/s (0.16 L/s) in the same pipe, 0.076 m/s, with a friction of about 1.8 Pa/m. For 100 m of pipe plus 30% that is 237 Pa. A boiler 2 m below the radiators gives 2 × 159 = 318 Pa, a margin of 1.3, and 1.5 m gives 239 Pa, a margin of 1.0. The radiators’ own resistance is not included, so gravity circulation in a pilot is marginal and needs a pipe that is large, short, smooth and continuously sloped. The corpus Hot Water Systems page makes the point that a short rise turns over a loop of short, wide pipe and no more. Keep a small pump on hand to test.
Cube Law At Part Load
For a centrifugal pump, flow goes with speed, head with the square of speed and power with the cube (affinity laws, recalled). If the flow follows the load at a constant temperature drop and the system friction follows flow squared, the power goes as the cube of the flow. At the season-average load of 40%, the hydraulic power is 0.4³ = 0.064 of full: 31 W becomes about 2 W. A pump that runs at full speed all season wastes most of its power, which a throttled bypass or a stepped drive (belt ratios) avoids.
Powering It
- Electric pump from a micro-hydro set. The corpus Hydro Generator page gives watts = head × flow × 9.81 × efficiency, with a 3 m, 5 L/s stream netting about 44 W (an overall efficiency of about 0.30). For 76 W: head × flow = 76 ÷ (9.81 × 0.30) = 26 m·L/s, about 3 m at 8.6 L/s. For the 175 W upper bound it is 59 m·L/s, 3 m at 20 L/s.
- Mechanical pump from a water wheel, which avoids the generator losses. See Micro-Hydro Turbine.
- Battery and small pump, as the carrier page suggests for the pilot.
Method: Choosing And Installing
- Calculate the flow and head as above, for your actual pipe lengths.
- Pick a pump whose curve (or plate) gives that flow at that head, or a little more; salvaged pumps rarely have curves, so test with a bucket and gauge.
- Fit two pumps in parallel with isolating valves, one running, one spare; fit a non-return valve on each.
- Put the pump on the return, at the point where the expansion tank connects (Expansion & Venting) and vent it.
- Wire it so that the boiler’s fan or damper reacts to a pump stop; see Wood-Fired Boiler Design.
Common Mistakes
- Pump sized for the upper bound and run flat out. Wastes power and noise.
- Relying on gravity for a long loop. 159 Pa per metre gets nowhere.
- No standby. The pump is the single part whose failure heats the boiler to boiling in minutes.
Safety
A pump stop is a boiler emergency
With a fired boiler and 1 m³ of water, a pump stop takes the boiler from 80 °C to boiling in about 12 minutes (see the boiler article). Wire the damper, thermosiphon safety loop or open vent so that a pump failure is not a pressure event, and train the operator to respond within minutes.
What’s Next
- Learn next: Expanding the Network — how pump head grows with more houses
- Combine with: Building Connections — the exchangers’ share of the head