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 the ground: flow rate.
A heat pump is rated at its output (5 kW, 8 kW, 12 kW, 16 kW) 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.
The calculator below is designed specifically for the primary pipe run… the pipework connecting your outdoor unit to the house, typically the buried or external section. This is where undersizing does the most damage and where problems are hardest to fix after installation.
Internal distribution pipework (the circuits running from your plant room to individual radiators or zones) is a different calculation entirely, because each branch carries only a fraction of the total flow. A 22mm pipe serving one half of your ground floor is carrying far less than a 22mm pipe carrying the full system output. We’ll be publishing a separate branching circuit calculator for that in due course.
For now, use this tool on your primary run. Then take the numbers to your installer before the pipework goes in… not after! This is particularly important if your heat pump is set away from the house.
Is Poor Flow
Throttling Your Heat Pump?
This section covers the primary pipe run between your outdoor unit and the house… typically the buried or external section. Internal distribution pipework branches and carries progressively less flow per circuit, so a separate calculation applies to those runs.
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.
