Do you need new radiators for a heat pump?

Do you need new radiators for a heat pump? Sometimes, but far less often than most people are told. I’ve fitted plenty of heat pumps onto the same radiators that were already on the wall, and the house heats fine.

The line you hear is “you’ll need to replace every radiator in the house.” It’s said so often it’s practically a script. And it isn’t wrong, exactly, it’s just lazy. A heat pump wants to run at a lower flow temperature than a boiler, somewhere around 35–45°C instead of 65–70°C, and a radiator sized for the hotter water will put out less heat at the cooler setting. That part’s true. What’s missing is the next question: how much less, and does it actually matter in your rooms?

I did a heat-loss survey on a 1930s semi in Sevenoaks last winter, nine radiators, and the installer before me had quoted a full replacement, all nine, before he’d even measured the house. I went room by room instead. Two of the bedrooms had radiators that were already oversized for the space, probably fitted by someone erring on the side of too big years ago. Running them ten degrees cooler still left plenty of output. The lounge radiator was borderline, and the smallest box room genuinely needed a bigger one. Out of nine, I upsized two. The homeowner had budgeted close to £3,000 for a full re-rad and ended up paying under £400 for the two radiators that actually needed it.

That’s the pattern more often than not. Most houses have some radiators that are already generous for the room, some that are marginal, and one or two that were undersized even for the boiler and were always going to struggle. A proper survey finds the difference. A rough guess, or a sales pitch dressed up as a survey, doesn’t, and it either scares people out of a heat pump they’d have been fine with, or sells them nine new radiators they didn’t need.

Where the “replace everything” advice does hold up is in a badly under-radiated house to begin with, small panel radiators picked to save money on a new-build twenty years ago, or a Victorian terrace where someone swapped in undersized rads at some point and never noticed because the boiler just ran hotter to cover for it. Fit a heat pump to that house without checking anything and you’ll get cold rooms and a homeowner who tells everyone heat pumps don’t work. They’d have been right to be annoyed. The heat pump wasn’t the problem.

The other thing nobody mentions is that upsizing a radiator is not the drama it sounds like. It’s a morning’s work, not a house-wide refit, and it’s cheap compared to the cost of guessing wrong on flow temperature for the next fifteen years. I’d rather spend an extra hour with a heat-loss calculation and swap two radiators than fit nine new ones nobody needed, or leave three too small and have the customer ringing me in January.

So if someone tells you flatly that you’ll need to replace every radiator before you can have a heat pump, ask what the survey found, room by room. If there wasn’t one, that’s your answer. Most houses need a couple of radiators looked at, not a wall of new ones, and the only way to know which is which is to actually measure.