Why do radiators need re-sizing for heat pumps?

Radiators are rated at a standard delta-T of 50C (flow 75C, return 65C, room 20C). Heat pumps operate at much lower flow temperatures (35-45C), which dramatically reduces radiator output. At 45C flow, a radiator only produces approximately 52% of its BS EN 442 rated output.

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. At 35C flow it is 0.30 (30%), at 40C it is 0.40, at 45C it is 0.52, at 50C it is 0.65, and at 55C it is 0.80. A 1000W rated radiator produces only 520W 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.

TradeCalculator

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 Temperature

Lower 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.

De-rating factors (% of rated output)

35C flow: 30% of rated output (delta-T 12.5C vs standard 50C)
40C flow: 40% of rated output
45C flow: 52% of rated output
50C flow: 65% of rated output
55C flow: 80% of rated output

Frequently Asked Questions

Related Calculators

Air Source Heat Pump Sizing Calculator

Size air source heat pumps from whole house heat loss with defrost derating.

Ground Source Heat Pump Calculator

Calculate ground source heat pump borehole depth or trench pipe loops per kW.

Heat Pump COP Calculator

Calculate heat pump COP and seasonal COP with running cost comparison vs gas.

Solar PV Array Calculator

Calculate solar PV array size, orientation yield factor and annual kWh by UK region.

Solar PV Inverter Calculator

Size solar PV inverters from array kWp with string voltage check.

Last updated: March 2026

All calculations are estimates. Verify with your supplier.