The radiators you actually have to change
19 May 2026
Not all of them, and not none of them. Both of those answers are sales positions rather than engineering ones.
A radiator emits less heat the cooler the water inside it is. A heat pump runs at around 45 degrees where a boiler ran at 70, so a given radiator delivers roughly half of what it used to. The fix is more surface area, which usually means a deeper radiator in the same space rather than a bigger one.
Which radiators need that depends on the heat loss of the room they are in, not on their age or their looks. On a typical three or four bedroom house we change between two and five. The living room and the coldest bedroom are the usual candidates.
The firms that tell you every radiator must change are selling radiators. The ones that tell you none will need to change are hoping you will not notice the cold room until after the final payment. The calculation names them, individually, before you commit to anything.
Why most heat pumps are sized wrong
Oversizing is the industry default, and it is the reason the unit is loud, cycles constantly and costs more to run than the brochure said.
What the grant actually pays for
It is £7,500, it goes to the installer rather than to you, and there are two conditions people are regularly surprised by.