Will my radiators work with a heat pump in Woking?
Almost certainly not at the size they are. A radiator gives its catalogue output at 75/65 into a 20 °C room; a heat pump runs far cooler, and output falls faster than the temperature does. Against Woking’s design temperature of −6.4 °C, here is what that costs.
What a radiator delivers at each flow temperature
Share of the catalogue rating, and the catalogue rating a room would need to stay warm. EN 442 rates radiators at 75/65 into a 20 °C room.
| Flow / return | Mean ΔT | Output | Rating needed per kW of demand |
|---|---|---|---|
| 75/65 | 50 K | 100% | 1.00 kW |
| 65/55 | 40 K | 75% | 1.34 kW |
| 55/45 | 30 K | 51% | 1.94 kW |
| 50/40 | 25 K | 41% | 2.46 kW |
| 45/35 | 20 K | 30% | 3.29 kW |
| 40/30 | 15 K | 21% | 4.78 kW |
| 35/30 | 13 K | 16% | 6.06 kW |
What that means room by room
Heat demand for a room in this climate, and the catalogue radiator rating needed to meet it at each flow temperature. For an insulated 1980s house; change the construction in the calculator.
| Room | W | 65/55 | 55/45 | 50/40 | 45/35 | 40/30 | 35/30 |
|---|---|---|---|---|---|---|---|
| Living room | 1,183 | 1,581 | 2,298 | 2,912 | 3,893 | 5,658 | 7,171 |
| Bedroom | 690 | 922 | 1,340 | 1,699 | 2,271 | 3,301 | 4,183 |
| Kitchen | 591 | 790 | 1,149 | 1,456 | 1,946 | 2,829 | 3,586 |
| Bathroom | 296 | 395 | 574 | 728 | 973 | 1,415 | 1,793 |
Check the radiator on your wall
Take the output printed on the radiator or in its datasheet — it will be quoted at ΔT50 — and the floor area of the room it heats.
Why output falls so fast
Radiator output follows Q = Q₅₀ × (ΔT/50)^1.3, where ΔT is the mean water temperature minus the room temperature. The exponent is what does the damage: halving the temperature difference does not halve the output, it cuts it to about 40 per cent.
- Measured: the design temperature, the degree days and the monthly means. All of it from one station, Farnborough, over 30 complete years.
- The catalogue outputs offered here are typical mid-range values for common European panel types and vary by roughly ten per cent between manufacturers. Use the figure for your own radiator if you have it.
- The exponent 1.3 is the standard value for steel panel radiators. Column and cast iron radiators sit slightly lower; fan-assisted units behave differently and are not covered.
- Lower flow temperatures make a heat pump more efficient, which is the whole reason this matters. Every step down the table buys efficiency and costs radiator output.
- This is a calculation, not a survey. Before spending money, have the building assessed.