Circulating Pump Calculator — Flow & Head

Flow rate (L/s) = System kW ÷ (4.186 × ΔT). For a 15 kW system with 11 °C ΔT: 15 ÷ (4.186 × 11) = 0.326 L/s = 19.5 L/min. The pump must deliver this flow rate at the required head pressure. Use our Boiler Size Calculator to determine your system kW, and our Heating Balancing Calculator to set the correct ΔT.

The index circuit is the longest pipe run from the pump through the system and back — typically to the furthest radiator and back. This has the highest resistance and determines the pump head requirement. All other circuits will have lower resistance.

Pipe fittings (elbows, tees, valves) add resistance beyond the straight pipe length. A common approach is to multiply the measured pipe length by 1.3 (30% allowance for fittings). This is an approximation — precise calculations use equivalent pipe lengths for each fitting type.

Modern pumps are often variable-speed or three-speed. Start on the lowest speed that achieves the correct ΔT across the system (11-12°C). Too high a speed wastes electricity and can cause noise. Variable-speed pumps (e.g. Grundfos Alpha, Wilo Yonos) adjust automatically.

Typical resistance for 22mm copper pipe at domestic flow rates is 150-250 Pa/m. For 15mm microbore, resistance is significantly higher at 400-800 Pa/m. Exact values depend on flow velocity and can be found in CIBSE pipe sizing tables.

For copper central heating pipe, aim for a water velocity of about 0.3-1.0 m/s, with roughly 1.5 m/s as the absolute maximum. Above 1.5 m/s you risk flow noise and erosion-corrosion at bends and fittings; well under 0.3 m/s the pipe may be oversized and prone to debris settling. The velocity figure uses the true internal bore from BS EN 1057 (former BS 2871) Table X copper tube — e.g. 22mm OD has a 20.2mm bore, 15mm OD a 13.6mm bore. If the velocity flags as high, step up a pipe size.

It is calculated as mass flow from the heat-transfer relation Q = m × Cp × ΔT (Cp = 4.186 kJ/kg·K) and reported directly as a volume flow in L/min. At central-heating mean temperatures hot water is slightly less dense (about 0.97-0.98 kg/L), so the true volumetric flow is marginally higher, but the difference is under 3% and well within pump-selection margins. This is the standard CIBSE convention for domestic low-temperature hot water (LTHW) systems.