Why Pipe Sizing Matters
Heat pumps require a minimum water flow rate through the heat exchanger to operate efficiently and prevent damage. Undersized pipework restricts this flow, causes the heat pump to modulate down or stop, and can trigger low-flow error codes.
Minimum Flow Rate Requirements
Most heat pump manufacturers specify a minimum flow rate through the primary circuit. Typical requirements:
- 5 kW unit: 0.8–1.0 l/min per kW = 4–5 l/min minimum
- 10 kW unit: 10–12 l/min minimum
- 16 kW unit: 16–20 l/min minimum
Pipe Diameter Guidance
As a general guide for heat pump primary circuits: 22mm copper is suitable for up to 8 kW, 28mm for 8–14 kW, and 35mm for larger units. Long pipe runs require larger diameters to minimise resistance.
Common Installation Mistakes
- Using 22mm throughout when 28mm+ is required
- Excessive bends increasing pipe resistance
- Failing to balance the system after installation
Sources: MCS certification and Vaillant's heat pump range.
