Most homeowners never think to ask this question. The heat pump is running, the radiators are warm enough and the installer signed it off, so it must be fine. Except that in a significant number of installations across the UK, the system is underperforming and the culprit is something that was decided before a single pipe went in: flow rate.
A heat pump is rated at its output (5kW, 8 kW, 12 kW, 16 kW, etc.) under specific conditions. But that rated output is only achievable if the system can actually move enough hot water around to deliver it. Get the pipe sizing wrong, add a restrictive component in the wrong place or simply default to whatever the last boiler installation used and your heat pump ends up throttled. It runs harder, costs more and still can’t keep the house warm on the coldest days.
Use the calculator below to work out the flow rate your system needs to deliver its rated output. Then ask your installer how they intend to achieve it.
Is Poor Flow Quietly
Throttling Your Heat Pump?
Flow rate formula: L/min = kW ÷ (ΔT × 0.0698). Velocity: Q/(π·r²) × 1000/60. Pressure drop uses the Darcy-Weisbach equation with friction factor estimated via the Colebrook-White approximation for smooth pipes. Pipe analysis covers straight runs only — add 20–30% for fittings, valves and bends. Target velocity below 1 m/s. Built by Renewable Heating Hub.
