The short answer
Not necessarily all of them — but some may need upsizing. A heat pump runs at a lower flow temperature than a gas boiler, so each radiator is cooler and gives out less heat than before. To keep a room warm at that lower temperature, the radiator must have enough surface area. The MCS heat loss survey checks each room's heat demand against the output its current radiator can deliver at the design flow temperature. Radiators that are already large enough — often the case in rooms with oversized or double-panel radiators — can usually stay. Those that are too small are upsized (taller, wider, or replaced with double or triple-panel types) or, where present, underfloor heating can be used. Better insulation lowers a room's demand and can reduce the radiator changes needed. So the answer is property-specific, decided by the survey rather than a blanket rule.
The radiator question worries a lot of people considering a heat pump. The reality is more nuanced than "replace everything" — the survey decides, and good insulation can shrink the list.
Radiator facts
- Replace all radiators?No — some can usually stay
- Why some need upsizingLower flow temperature = cooler radiators
- Decision made byMCS heat loss survey
- Upsizing optionsLarger / double-panel radiators
- Insulation effectLowers demand, fewer changes
Where the radiator worry comes from
The fear that a heat pump means ripping out every radiator in the house is one of the most common — and most overstated — concerns about switching. It comes from a real fact dressed up as a worse one. The real fact is that a heat pump runs cooler than a boiler, so radiators give out less heat than they used to. The exaggeration is the assumption that this means wholesale replacement of every emitter in the home.
In practice the picture is far more nuanced. Many radiators in UK homes are already larger than they strictly need to be, because households tend to fit generous radiators and because some rooms have modest heat demand. Those radiators can often stay. Only the ones that fall short at the lower flow temperature need attention, and even that list shrinks if insulation is improved first. The decision is not a blanket rule applied to the whole house; it is a room-by-room calculation done during the heat loss survey, which turns guesswork into a documented answer. Understanding that distinction is the difference between dreading the change and planning it sensibly — for most homes, some radiators are kept, a few are upsized, and the result heats the home comfortably at an efficient low flow temperature.
Why a heat pump changes the radiator maths
A radiator gives out heat in proportion to how much hotter it is than the room. A gas boiler running at around 70 °C makes a radiator very hot, so even a modest radiator emits plenty of heat. A heat pump deliberately runs at a lower flow temperature — commonly 45 to 50 °C — to stay efficient, which makes the same radiator cooler and reduces its output.
To deliver the same warmth at the lower temperature, a radiator needs more surface area. That is why heat pump installations consider radiator size carefully. It is not that the old radiators stop working — they simply emit less at a lower flow temperature, so some rooms need bigger emitters to compensate.
There are several ways to add that surface area without necessarily replacing a radiator outright. A single-panel radiator can often be swapped for a double- or triple-panel unit of the same width, which fits the same wall space and pipe connections but emits far more heat. A taller or wider radiator achieves the same end where there is room. In some cases a fan-assisted radiator, which blows air across the fins, can lift output without a larger physical size. And where a room has underfloor heating, its very large surface area means it already suits the low flow temperature with no change at all. The point is that upsizing is usually a targeted, room-by-room adjustment rather than a wholesale rip-out, and the survey decides which approach each room needs.
How the survey decides which radiators change
The MCS heat loss survey does the calculation room by room. For each room it works out the heat demand at the cold design temperature, then checks whether the existing radiator can deliver that output at the design flow temperature. The result is a clear list of which radiators stay and which are upsized.
Often a meaningful number can be kept — particularly in homes with large or double-panel radiators already, or in rooms with lower heat demand. Where a radiator is too small, options include a taller or wider unit, a double or triple-panel replacement, or underfloor heating if it is being installed. The survey turns the radiator question from guesswork into a documented, room-by-room answer.
| Situation | Likely outcome |
|---|---|
| Existing radiator already oversized | Usually kept |
| Standard radiator, low room demand | Often kept |
| Small radiator, high room demand | Upsized |
| Underfloor heating present | Ideal — works at low flow temp |
Illustrative outcomes — the survey decides each case. Sources: Energy Saving Trust; MCS.
The underfloor heating option
Where underfloor heating is present or being installed, it is the ideal partner for a heat pump. Its very large surface area means it can heat a room while running at a low flow temperature — often around 35 °C — which gives the highest efficiency. Homes with underfloor heating throughout typically need no radiator work at all.
Retrofitting underfloor heating across an existing home is disruptive and costly, so it is usually only considered during major renovation or in new extensions. For most retrofits, the practical route is keeping the radiators that are big enough and upsizing the few that are not, rather than tearing up floors. The survey will set out the most sensible combination for your home.
A practical middle path some homes take is to combine the two: underfloor heating on a ground floor with a solid or screeded floor, where it can be laid during other works, and upsized radiators upstairs where lifting floors would be far more disruptive. This gives the efficiency benefit of a very low flow temperature downstairs while keeping the upstairs work simple. The key point throughout is that none of these choices is a blanket rule — the heat loss survey weighs each room's demand, the existing emitter, the floor construction and the practicality of change, then sets out the least disruptive combination that lets the whole house run at an efficient low flow temperature.
Frequently asked questions
Will I have to replace every radiator?
Almost never. The heat loss survey checks each radiator against its room's demand at the design flow temperature; many are already adequate and can stay. Only the radiators that are too small to deliver enough heat at the lower temperature are upsized. Improving insulation can reduce the number that need changing.
Why are heat pump radiators bigger?
Because a heat pump runs at a lower flow temperature than a boiler, each radiator is cooler and gives out less heat. A larger radiator has more surface area, so it can deliver the same warmth at that lower temperature. Bigger radiators are what allow the system to run efficiently at a low flow temperature.
Is underfloor heating better than radiators with a heat pump?
It pairs especially well, because its large surface area lets it heat a room at a very low flow temperature, giving the highest efficiency. But radiators sized correctly for a heat pump also work well. Retrofitting underfloor heating across an existing home is disruptive, so most retrofits keep adequate radiators and upsize the rest.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific home. They are guidance, not a quotation or guaranteed saving.