Heat pump radiator sizing
Radiators are sold with an output measured at ΔT50, the difference between the average water temperature and the room when the water runs at about 75 °C. A heat pump runs much cooler than that, so the same radiator gives out far less heat.
Here is how much less, the factors to use, and how to work out whether an existing radiator is big enough.
Why output drops
A radiator's output depends on how much hotter its surface is than the room. The catalogue figure (BS EN 442) is at a mean water temperature of 70 °C in a 20 °C room, a ΔT of 50 K. At a heat pump flow of 45 °C with a 5 K drop, the mean water temperature is 42.5 °C, so in a 21 °C room the ΔT is only 21.5 K.
Output doesn't fall in a straight line. It follows the radiator's exponent, typically about 1.3:
Output at your ΔT = Output at ΔT50 × (ΔT ÷ 50)^1.3
Correction factors at common flow temperatures
Assuming a 5 K drop across the radiator and a 21 °C room:
| Flow temperature | Mean water temp | ΔT | Output vs ΔT50 | Size up by |
|---|---|---|---|---|
| 55 °C | 52.5 °C | 31.5 K | 55% | 1.8 × |
| 50 °C | 47.5 °C | 26.5 K | 44% | 2.3 × |
| 45 °C | 42.5 °C | 21.5 K | 33% | 3.0 × |
| 40 °C | 37.5 °C | 16.5 K | 24% | 4.2 × |
| 35 °C | 32.5 °C | 11.5 K | 15% | 6.8 × |
Bedrooms designed at 18 °C have a bigger ΔT, so their radiators lose a little less. Always use the room's own design temperature.
Worked example
- The living room heat loss is 1,200 W at a 21 °C design temperature.
- The design flow temperature is 45 °C, so the factor is 0.334.
- Required ΔT50 output = 1,200 ÷ 0.334 = 3,593 W.
- The existing radiator is rated 1,800 W at ΔT50, so it gives about 600 W at 45 °C. It needs replacing or adding to.
This is why a heat pump job often means changing some radiators. The lower the design flow temperature, the more efficient the heat pump, but the bigger the radiators.
Do it with the free tool
The free tool sizes every radiator at your design flow temperature from the room heat losses, and shows which existing radiators can stay.
Frequently asked questions
Do you need bigger radiators for a heat pump?
Often, yes. At a 45 °C flow temperature a radiator gives about a third of its catalogue output, so rooms whose radiators were sized for a boiler may need larger ones. Some existing radiators are already oversized and can stay.
What is ΔT50?
The temperature difference of 50 K between the radiator's average water temperature and the room, used to rate radiator outputs under BS EN 442. A boiler system running at 75 °C flow and 65 °C return in a 20 °C room is at ΔT50.
What flow temperature should I design radiators for?
As low as is practical, because the heat pump gets more efficient as flow temperature falls. Many designs settle between 40 °C and 50 °C as a balance between efficiency and radiator size.
Can I use underfloor heating with a heat pump?
Yes, and it suits heat pumps well, because its large surface area works at low flow temperatures, often 30 °C to 40 °C.
What radiator exponent should I use?
Use the manufacturer's figure where given. Panel radiators are typically around 1.3, which is what the table above uses.
Related guides
Sources: BS EN 442 (radiator outputs at ΔT50); manufacturer correction data; MCS MIS 3005-D. Free Heat Pump Design follows the MCS method but is not an MCS product and is not endorsed by MCS. Check designs against MCS MIS 3005-D and the official MCS Heat Pump Calculator.