Short answer: a radiator’s output depends on how much hotter the water inside it is than the room. Catalogues quote output at ΔT50, which a boiler at about 75 °C flow achieves. A heat pump runs cooler, typically 45 to 55 °C, and at those temperatures the same radiator gives half or less of its catalogue figure. So a room that needs 1,000 W needs a radiator sold as about 2,000 W. That is the whole reason heat pump quotes come with a long list of radiator changes.
The relationship is not linear, and knowing the actual factors turns a vague “you will need bigger radiators” into a number.
What ΔT means
ΔT (delta T) is the difference between the radiator’s mean water temperature and the room temperature. The standard test condition, and the one every catalogue output is quoted at, is 75 °C flow, 65 °C return and a 20 °C room: mean water 70 °C, room 20 °C, ΔT50.
Run the same radiator at 50 °C flow with a 20 °C room and the mean water temperature is somewhere around 47 °C: ΔT is now about 27 to 30, and the panel is putting far less heat into the room. Not 30/50 of it, either. Less.
The correction
Radiator output follows the EN 442 relationship:
output at ΔT = output at ΔT50 × (ΔT ÷ 50)^n
where n is about 1.3 for panel radiators. The exponent is why the fall-off is steeper than the temperature ratio. The formula and exponent here are the ones implemented in the BTU Calculator’s heating system model:
| System | Typical flow temperature | ΔT | Output factor | Catalogue output needed per 1 W of room requirement |
|---|---|---|---|---|
| Conventional boiler | ~75 °C | 50 | 1.00 | 1.00 |
| Low-temperature boiler | ~65 °C | 40 | 0.75 | 1.34 |
| Heat pump, high | ~55 °C | 35 | 0.63 | 1.59 |
| Heat pump | ~50 °C | 30 | 0.51 | 1.94 |
| Heat pump, low | ~45 °C | 25 | 0.41 | 2.46 |
Read the last column as the multiplier on the room’s requirement. At ΔT30 you need a radiator whose catalogue output is nearly twice the room figure. At ΔT25, two and a half times.
Worked example
Take the modern lounge from our radiator sizing guide: 4 m × 5 m, one external wall, double glazed, concrete floor. Its requirement is 1,063 W (3,627 BTU/hr).
| System | Factor | Catalogue output to buy |
|---|---|---|
| Boiler at ΔT50 | 1.00 | 1,063 W |
| ΔT40 | 0.75 | 1,421 W |
| ΔT35 | 0.63 | 1,690 W |
| ΔT30 | 0.51 | 2,065 W |
| ΔT25 | 0.41 | 2,619 W |
On the boiler, a single-panel radiator around 1,100 W does it. On a heat pump at ΔT30, you are looking for a panel sold as about 2,100 W: a double panel, double convector Type 22, or a longer or taller radiator, or both.
For the 1930s semi version of the same room, 1,856 W on the boiler, the ΔT30 figure is about 3,600 W. That room’s answer is probably insulation and glazing before it is a radiator, because a 3.6 kW panel is a large object.
What this means in practice
The requirement does not change; the radiator does. The room loses the same heat whatever is heating it. A heat pump does not need “more heat”; it needs more radiator surface to deliver the same heat at a lower temperature.
Existing radiators may already be big enough. Boiler-era radiators were often oversized, sometimes by a lot. A room whose radiator was chosen at 1,600 W for a 1,063 W requirement is already covered at ΔT40, where it gives about 1,200 W, and falls only a little short at ΔT35. The calculation is done room by room precisely because some pass and some do not.
Type matters as much as size. A Type 22 double panel with two convector fins gives roughly twice the output of a Type 11 single panel of the same height and length. Swapping type in the same wall space is often the least disruptive fix.
Running the pump hotter is the expensive option. A heat pump’s efficiency falls as flow temperature rises. Raising the flow to avoid changing two radiators is a cost paid on every bill for the life of the system; the right radiators at a low flow temperature are what make a heat pump cheap to run.
Reduce the loss first. Every watt of heat loss removed is nearly two watts of catalogue radiator not needed at ΔT30. Loft insulation, glazing and draught-proofing move the radiator list more than anything else.
Using the BTU Calculator for this
The BTU Calculator’s room requirement is the figure in the left-hand column: the heat the room loses, which is independent of the heating system. To find the catalogue output to buy, divide that figure by the factor for your system’s ΔT from the table above, or multiply by the last column.
The figure to compare against the catalogue is the corrected one. Comparing a ΔT50 catalogue output to an uncorrected room requirement is how heat pump installations end up with cold rooms and a flow temperature turned up to compensate.
Who does the real design
A heat pump installation is a full-system design. Installers working to the MCS standard carry out a room-by-room heat loss calculation to BS EN 12831 and size the emitters, the pump and the flow temperature together. This guide, and the app, are for understanding that design and checking it looks sane, not for replacing it. Our guide on how a heat loss calculation works explains where the simplified method stops.
The EN 442 exponent (n = 1.3) and the ΔT bands with their flow temperatures are as implemented in the BTU Calculator's heating system model; the worked figures were computed from that formula on 11 September 2026. Flow temperatures are typical, not fixed, and a specific system's ΔT should be taken from its design. Plan CD is not affiliated with any heating or heat pump manufacturer, MCS or any certification body.