Guide · Troubleshooting
Radiator sizing for heat pumps
Why the same radiator gives you less heat on a heat pump — and what to change.
Because a heat pump runs at 40–45°C rather than a boiler’s 70°C, a radiator delivers roughly 40–50% of its rated output, so heat pump systems need radiators around two to two-and-a-half times larger in output terms. Upgrading a radiator typically costs £250–£500 fitted, and most UK retrofits need only a handful changed.
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How output derating works
Radiator outputs are published at a 50°C temperature difference between water and room. At heat pump flow temperatures the difference is far smaller — often 20–25°C — so actual output falls to roughly 40–50% of the catalogue figure.
A room needing 1,200 W therefore needs a radiator rated around 2,400–3,000 W at catalogue conditions.
Ways to add output without a bigger wall
- Switch a single panel to a double panel double convector of the same length.
- Increase height rather than length where the wall is short.
- Use low-temperature fan-assisted radiators in awkward rooms.
- Add a second smaller radiator on a different wall.
What good practice looks like
- Every room checked individually against its heat loss.
- Pipework capacity checked — microbore often limits flow.
- System balanced at commissioning so flow rates match design.
- Radiator schedule provided in writing with the quote.
Frequently asked questions
- Do I need bigger radiators for a heat pump?
- Some rooms usually do. At 45°C flow a radiator gives roughly half its catalogue output, so under-sized rooms need larger emitters.
- How much does it cost to upsize a radiator?
- Typically £250–£500 fitted per radiator, depending on size and pipework changes.
- Can I keep my microbore pipework?
- Sometimes, but microbore limits flow rate and often needs replacing on the runs serving the largest emitters.
- Energy Saving Trust — Heat pump efficiency, running cost and SCOP benchmarks (UK).
- MCS Certified — Installer certification scheme required for grant eligibility.
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