How do I calculate the system flow rate?

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.

What is the index circuit?

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.

How do I account for pipe fittings in the resistance calculation?

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.

What speed should I set the pump to?

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.

What pipe resistance value should I use?

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.

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Circulating Pump Calculator — Flow & Head

Calculate total system flow rate, index circuit resistance, and pump duty point for central heating circulating pump selection.

System Size

Total heating system output

System ΔT

Flow-return temperature difference

Index Circuit Length

Longest pipe run (flow + return)

Pipe Diameter

Primary pipework diameter

How We Calculate This

This calculator determines the circulating pump duty point by calculating the total system flow rate and the pressure loss through the index (longest) circuit.

Method

Flow rate: kW ÷ (4.186 × ΔT) = L/s
Effective length: Index circuit length × fittings factor (1.3)
Resistance: Effective length × Pa/m resistance
Pump head: Total resistance ÷ 9807 = metres head
Duty point: Flow rate (L/min) @ head (m)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.