Heat Pump Pipework Calculator

Flow rate (L/s) = Heat output (kW) / (4.186 x delta-T). Heat pumps typically operate at delta-T 5C (compared to 11-20C for gas boilers), resulting in much higher flow rates. A 10 kW heat pump at dT5 needs approximately 0.48 L/s (28.7 L/min), compared to 0.22 L/s for a boiler at dT11.

Adding glycol (typically 25% propylene glycol for frost protection) lowers the volumetric heat capacity by roughly 5-10% and increases viscosity. As an indicative installer correction, this calculator raises the required flow rate by 10% for 25% glycol and 15% for 33% glycol, and increases friction loss (about 10% at 25% glycol, 20% at 33%). Always confirm against your glycol manufacturer’s fluid-property data. Pipe sizes may need to increase one size.

For heat pump primary circuits, target 0.5-1.0 m/s, with around 0.9 m/s ideal for copper. MCS and CIBSE Guide B1 (2016) Table 1.A1.4 set 1.0 m/s as the maximum for copper pipes 15-50mm, because copper is prone to erosion-corrosion above this speed. Below 0.5 m/s, air may not be carried to separators. The 1.5 m/s figure sometimes quoted is a general plumbing limit, not the heat-pump limit. The higher flow rates of heat pumps often require 28mm or 35mm primary pipework. CIBSE Guide C friction-loss target is 100-300 Pa/m, with 350 Pa/m the maximum.

Heat pumps operate at a lower temperature differential (delta-T 5C vs 11-20C for boilers), so the flow rate is 2-4 times higher for the same heat output. A 10 kW ASHP typically needs 28mm primary pipework, whereas a 10 kW boiler uses 22mm. Undersized pipes cause excessive pressure drop and reduced efficiency.

The maximum pipe run depends on the available pump head and the pipe size. Most domestic heat pump circulators provide about 4-6m of head (40-60 kPa). This calculator assumes a 50 kPa (~5 m) circulator and allows only about half of that for straight-pipe friction, because fittings, the emitter or manifold, any buffer or low-loss header, and the heat pump’s own internal resistance consume the rest. On that basis a 28mm primary at heat-pump flow rates typically allows runs of around 20-40m. Longer runs may need larger pipe or a higher-head pump.