Heat Loss and Design · 5 lessons

Do radiators work with a heat pump?

2 min read · self-test time extra

Yes, if they're big enough. A heat pump works best with cooler water, so a radiator gives out less heat than its label says, and some rooms need a larger one.

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Why might larger emitters help a low-temperature design?

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Why cooler water

The cooler the water a heat pump has to make, the less electricity it needs for each unit of heat. So heat pump systems run at lower flow temperatures than most boilers, and the radiators have to do the same job with cooler water.

Reading a radiator's output

Radiators are rated at a temperature difference, ΔT, between the mean water temperature and the room. With 45 °C flow and 40 °C return, the mean water is 42.5 °C; in a 20 °C room that's ΔT 22.5 K. A radiator rated 2,000 W at ΔT 50 gives roughly 2,000 × (22.5 ÷ 50)^1.3, about 708 W, at that point.

Underfloor heating

Underfloor heating warms a whole floor gently, so it can heat a room with water cooler still. It suits heat pumps well, though floor coverings and the room's heat loss still decide whether it's enough.

Weather compensation

Weather compensation turns the flow temperature down on milder days and up on cold ones, following a curve set at commissioning. The house stays comfortable and the heat pump runs as efficiently as the weather allows.

Key numbers

ΔT 22.5 K: Mean water 42.5 °C (45 °C flow, 40 °C return) in a 20 °C room.
about 708 W: A radiator rated 2,000 W at ΔT 50, working at ΔT 22.5 K.

Common mistakes

Using the flow temperature instead of the mean water temperature to work out ΔT.
Assuming a radiator gives its label output at heat pump temperatures.
Turning the flow temperature up to cover a small radiator, which costs efficiency.

The lessons this guide comes from

Why low flow temperatures?
Radiators at low temperature
Sizing a radiator
Underfloor heating
Weather compensation