How is heat pump flow rate calculated?

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.

How does glycol affect pipework sizing?

Adding glycol (typically 25% propylene glycol for frost protection) reduces the specific heat capacity by approximately 10% and increases viscosity. This means flow rates must increase by 10% to deliver the same heat, and pressure drops increase by approximately 15%. Pipe sizes may need to increase one size.

What pipe velocity should I target?

Target pipe velocity is 0.5-1.5 m/s. Below 0.5 m/s, air may not be carried to air separators. Above 1.5 m/s, noise and erosion become concerns. For heat pump primary circuits, 0.8-1.2 m/s is ideal. The higher flow rates of heat pumps often require 28mm or 35mm primary pipework.

Why do heat pumps need larger pipes than boilers?

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.

What is the maximum pipe run for a heat pump?

The maximum pipe run depends on the available pump head and the pipe size. As a guide, most domestic heat pump circulators can handle 4-6m of head (40-60 kPa). With 28mm copper at 1.0 m/s, runs of 20-30m are typical. Longer runs may need larger pipe or a higher-head pump.

TradeCalculator

Heat Pump Pipework Calculator

Calculate flow rates, pipe sizing, and pressure drop for heat pump installations. Includes glycol flow adjustment and maximum pipe run length.

Heat Pump Capacity

Rated heat output

Design Delta-T

HP typically 5C, boiler 11C

System Fluid

Glycol increases flow rate

Pipe Material

How We Calculate This

This calculator determines the correct pipe sizing for heat pump primary circuits based on the required flow rate, target velocity, and glycol adjustment.

Flow rate calculation

Flow rate: Q = kW / (4.186 x delta-T) in L/s
Glycol adjustment: +10% flow for 25% glycol, +15% for 33%
Pipe selection: Target velocity 0.5-1.5 m/s
Pressure drop: Estimated from velocity and pipe length

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.