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Do you need new radiators for a heat pump? (usually not, here's why)
The standard claim is that a heat pump means replacing every radiator. In most houses it means replacing three or four. Here is how to work out which ones, and what it buys you.
The fear is understandable and mostly wrong. A radiator sized for 75 °C water at design conditions delivers about 63 % of that output at 50 °C, and about 51 % at 45 °C. So the question is not “are my radiators big enough for a heat pump”, it is “which of my radiators has less than twice the output its room needs”.
In most British houses the answer is: a small number of them. Radiators are rarely sized precisely. They are sized from a catalogue, in the sizes that were on the van, and the result is that half of them are already oversized by 30–50 % for the room they sit in. Those are fine at 45 °C without being touched.
How to find the limiting room
Work out each room's heat loss, then divide each radiator's catalogue output at 75 °C by that room's loss. A ratio above 2.0 works at 45 °C; above 1.6 works at 50 °C; above 1.3 works at 55 °C. The lowest ratio in the house sets the flow temperature for the whole system, because one weather-compensation curve serves all of it.
That is why the fix is usually cheap: upsize the two or three worst rooms and the whole house drops 5–10 °C of flow temperature. Our radiator versus underfloor calculator does this arithmetic and shows the running-cost difference at each temperature step.
What 5 °C is worth
Going from 55 °C to 50 °C moves the realistic SCOP from about 2.8 to about 3.0 — a 7 % cut in heating electricity. On a house using 16,000 kWh of delivered heat that is around 380 kWh a year, or roughly £99 at the current cap. Over a fifteen-year system life, upsizing three radiators at £450 each pays for itself comfortably, and the comfort improvement is immediate rather than deferred.
When you do need to do more
- Microbore pipework. 8 mm and 10 mm microbore limits flow rate, and a heat pump moves more water than a boiler at a smaller temperature difference. This is a real constraint and bigger radiators do not solve it.
- Solid-wall rooms with one small radiator. A north-facing solid-wall room with a single panel is the classic limiting case, and sometimes there is no wall space for a radiator twice the size.
- Towel rails counted as heating. A bathroom heated only by a towel rail almost never works at low flow temperature. Budget for a separate emitter.
What not to accept
“You'll need all new radiators” without a room-by-room calculation is a sales position, not an engineering conclusion. So is “your existing radiators will be fine” without one. Both answers require the same piece of work, and you are entitled to see it.
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Questions people ask
Do I have to replace all my radiators for a heat pump?
Usually not. Radiators are commonly oversized for their rooms already. In most houses only the two or three worst rooms limit the flow temperature, and upsizing those is enough.
How much does upsizing radiators improve efficiency?
Dropping the design flow temperature by 5 °C typically improves seasonal efficiency by about 0.2, which is a 6–7 % cut in heating electricity.