Usually only in some rooms. Radiators are rated at ΔT 50 K (75/65 °C into a 20 °C room), and at a heat pump flow of 45 °C a panel delivers about 35% of that rating — but many existing radiators were sized generously for the boiler and many homes have been insulated since, so a room-by-room survey typically finds two or three rooms that need a larger panel. Some houses need no change at all.
Why does the same radiator give less heat with a heat pump?
Because a radiator's output depends on the temperature difference between its water and the room, not on the boiler behind it. European radiator ratings follow EN 442 at ΔT 50 K — 75 °C flow, 65 °C return, 20 °C room. A heat pump runs 40–50 °C flow with a narrow 5 K drop, so the driving temperature difference roughly halves, and output falls faster than the temperature does. Output scales with ΔT to the power of about 1.3.
How much bigger do heat pump radiators need to be?
| Flow / return (20 °C room) | Mean ΔT | Output vs ΔT 50 rating | Radiator size needed |
|---|---|---|---|
| 75 / 65 °C | 50 K | 100% | 1.0 × |
| 55 / 50 °C | 32.5 K | 57% | 1.8 × |
| 50 / 45 °C | 27.5 K | 46% | 2.2 × |
| 45 / 40 °C | 22.5 K | 35% | 2.8 × |
| 40 / 35 °C | 17.5 K | 26% | 3.9 × |
| 35 / 30 °C | 12.5 K | 16% | 6.1 × |
Read the table in both directions. It explains why 35 °C is an underfloor number rather than a radiator number, and why the practical retrofit window is 45–50 °C: below that, radiators become physically impossible in most rooms. Each 5 °C you claw back is worth roughly 10–12% on running cost, so the emitter budget buys efficiency permanently. See what flow temperature should I run my heat pump at?
Do I have to replace every radiator?
No. Replace the radiators that fail, not the set. Work room by room from a proper heat load calculation: for each room, compare required watts against what the existing panel delivers at your target flow temperature. Typically two or three rooms fail — usually the largest room and any room with a small panel behind furniture.
The escalation ladder is cheaper than wholesale replacement:
- Same size, more convection: single panel (K1, Type 11) to double panel double convector (K2, Type 22) roughly doubles output in the same wall space.
- Longer, not taller: extending a panel horizontally is often the only geometry a room allows.
- Fan-assisted radiators: reach full output at 40–45 °C in the same footprint, at the cost of a small electrical load and some noise.
- Underfloor in one problem room: usually the kitchen or bathroom extension where wall space has run out.
Remember that the worst room sets the flow temperature for the whole house. Fixing five easy rooms and leaving one bad one saves nothing.
What about pipework, microbore and flow rate?
Heat pumps move much more water than boilers because they run a 5 K temperature drop instead of 20 K — roughly four times the flow rate for the same kilowatts. That exposes undersized pipe. In practice 10 mm microbore serves small radiators on short runs; 8 mm rarely does and is normally replaced. Check the main circuit is 22 mm or 28 mm to the manifold, open every lockshield, and fit a magnetic dirt separator: sludge that a boiler tolerated will throttle a heat pump's flow and trigger short cycling.
Is underfloor heating a better answer?
For efficiency, yes; for retrofit cost and disruption, usually not. Underfloor heating lets the system run at 30–35 °C, which is worth roughly 20–40% on running cost compared with a 45–55 °C radiator system — but it means lifting floors or losing 15–20 mm of height. Most successful retrofits mix the two. The trade-offs are set out in heat pump with underfloor heating vs radiators.
Frequently asked questions
Can a heat pump use the same radiators as my gas boiler?
Often, in most rooms. Radiators sized generously for the boiler, or rooms insulated since they were fitted, frequently cope at 45–50 °C; a room-by-room heat loss check identifies the two or three rooms that typically need a larger panel. At 45 °C flow a radiator delivers roughly 35% of its ΔT 50 rating, which is what that check tests against.
What percentage bigger should heat pump radiators be?
For a 50 °C design flow temperature, expect roughly a 120% uplift in rated output; for 45 °C, around 180%. Installers often quote a 20–50% uplift, which only holds where the existing radiators were already generously sized.
Why did my survey say some radiators should be smaller?
Because emitter sizing follows the room's calculated heat loss, not the previous radiator. A room that was over-radiatored by a boiler installer, or that has since been insulated, can legitimately end up with a smaller panel than before.
Do I need bigger pipes as well as bigger radiators?
Often the tails and the main circuit, rarely everything. A heat pump runs about four times the flow rate of a boiler for the same output, so 8 mm microbore usually has to go, while 10 mm frequently survives on short runs to small radiators.