Why do radiators need re-sizing for heat pumps?

Radiators are rated at a standard delta-T of 50C (BS EN 442: flow 75C, return 65C, room 20C). Heat pumps operate at much lower flow temperatures (35-45C), which dramatically reduces radiator output. At 45C flow with a typical 5K heat-pump return spread and 21C room, a radiator produces approximately 35% of its BS EN 442 rated output (ΔTm ≈ 21.5K, factor = (21.5/50)^1.3).

What is the de-rating factor?

The de-rating factor is the proportion of a radiator rated output available at the heat pump flow temperature. Computed live from (ΔTm/50)^1.3 per BS EN 442, with default 5K return spread and 21C room. Typical values: 35C flow ≈ 0.17, 40C ≈ 0.25, 45C ≈ 0.35, 50C ≈ 0.48, 55C ≈ 0.62. A 1000W rated radiator produces only ~350W at 45C flow.

Do I always need to replace radiators?

Not always. If your existing radiators were originally oversized for the rooms (common in older installations), they may still provide adequate output at lower flow temperatures. Radiators in well-insulated rooms with low heat loss are also more likely to be adequate.

What type of replacement radiator is best?

For heat pump systems, Type 22 (double panel, double convector) or Type 33 (triple panel) radiators provide the most output for their footprint. Fan-assisted convectors are also excellent as they force air across the fins, significantly boosting output at low flow temperatures.

Can I use a higher flow temperature instead?

Yes, but higher flow temperatures reduce heat pump COP. At 45C flow, ASHP COP is approximately 3.0. At 55C, COP drops to approximately 2.5. Running at lower flow temperatures is more efficient and cost-effective. It is generally better to upsize radiators than increase flow temperature.

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Heat Pump Radiator Calculator

Re-size radiators for heat pump flow temperatures. Apply de-rating factors to existing radiator output at 45C vs 70C flow.

Room Heat Loss

From room-by-room heat loss calc

Current Radiator Output (BS EN 442)

Rated output at delta-T 50C

Heat Pump Flow TemperatureLower is more efficient

Number of Similar Rooms

For total deficit estimate

How We Calculate This

This calculator applies the BS EN 442 de-rating factors to your existing radiator output to determine if they are adequate for heat pump flow temperatures.