Sizing the pipework and the circulating pump
Heat pumps run with a small temperature drop across the system, typically 5 K, where a boiler might run at 20 K. Moving the same heat with a quarter of the temperature drop needs four times the flow, so pipes that were fine for a boiler are often too small for a heat pump.
Here is how to work out the flow rate, choose pipe sizes, and size the circulating pump.
Step 1: the flow rate
Flow rate (litres per second) = heat (kW) ÷ (4.18 × ΔT)
4.18 is the specific heat capacity of water in kJ/kg K. For an 8 kW heat pump at a 5 K ΔT, the flow rate is 8 ÷ (4.18 × 5) = 0.38 l/s, or about 23 litres a minute.
| Heat pump output | Flow at 5 K ΔT |
|---|---|
| 5 kW | 0.24 l/s (14 l/min) |
| 8 kW | 0.38 l/s (23 l/min) |
| 12 kW | 0.57 l/s (34 l/min) |
Step 2: pipe size and velocity
Keep water velocity low enough to avoid noise and excessive pressure loss. Around 0.5 to 1.0 m/s is a common range for domestic copper. At those speeds, copper pipe carries roughly:
| Copper pipe | Heat carried at 0.5 m/s, 5 K ΔT | At 1.0 m/s, 5 K ΔT |
|---|---|---|
| 15 mm | about 1.5 kW | about 3.0 kW |
| 22 mm | about 3.3 kW | about 6.7 kW |
| 28 mm | about 5.6 kW | about 11 kW |
So an 8 kW heat pump usually needs 28 mm primary pipework, and branches to rooms are sized for the heat each one carries. Check the manufacturer's minimum flow rate and pipe size too.
Step 3: the pump
The circulating pump must deliver the design flow rate against the pressure loss of the index circuit, the route with the highest resistance, usually the longest run to the furthest emitter. Add up the friction loss along that route plus the losses through fittings, valves and the heat pump itself.
- Friction loss per metre rises steeply with velocity, so a pipe one size bigger can cut the pump head needed dramatically.
- Many heat pumps have a built-in pump. Check its available head at your flow rate before adding another.
- Avoid unnecessary low-loss headers and buffers where the system design doesn't need them, as they can lower efficiency.
Do it with the free tool
The pipe planner in the free tool works out flow rates, pipe sizes and friction losses for each run as you draw it, and finds the index circuit for you.
Frequently asked questions
Why do heat pumps need bigger pipes?
Because they run with a small temperature drop, about 5 K, compared with 20 K or so for a boiler. To move the same heat with a quarter of the temperature drop, you need four times the water flow, and so bigger pipes.
What size pipe does a heat pump need?
It depends on the output and flow rate. As a rough guide at 5 K ΔT, 22 mm copper carries up to about 6 to 7 kW and 28 mm up to about 11 kW. Always check the manufacturer's requirements.
How do you calculate flow rate for a heat pump?
Flow rate in litres per second = output in kW ÷ (4.18 × ΔT). An 8 kW heat pump at a 5 K ΔT needs about 0.38 l/s.
What is the index circuit?
The circuit with the highest pressure loss, usually the longest run to the furthest radiator. The pump is sized to push the design flow through it.
Do I need a buffer tank with a heat pump?
Not always. Some manufacturers require a minimum system volume or a buffer for defrost, but many systems run better without one. Follow the manufacturer's instructions.
Related guides
Sources: CIBSE Guide C (pipe pressure loss data); manufacturer installation manuals; MCS MIS 3005-D. Free Heat Pump Design follows the MCS method but is not an MCS product and is not endorsed by MCS. Check designs against MCS MIS 3005-D and the official MCS Heat Pump Calculator.