Why most heat pumps are sized wrong
14 August 2026
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.
The quickest way to quote a heat pump is to look at the size of the house, look at the old boiler, and pick a unit somewhere above both. It takes ten minutes, it is never obviously wrong on the day, and it produces most of the systems people complain about two winters later.
The problem with an oversized unit is that a heat pump is at its best running slowly and continuously. Oversize it and it reaches temperature quickly, shuts off, cools, and starts again. Each of those starts costs energy and wear, and the seasonal performance you actually get drifts well below the figure on the datasheet.
A room-by-room heat loss calculation takes a couple of hours and produces a number in watts for every room in the house at a design outdoor temperature. That number tells you the unit size and, separately, which radiators can deliver that heat at 45 degrees rather than 70.
It is not a clever technique. It is the standard method, it has existed for decades, and the only reason it is a differentiator is that quoting without it is faster.
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.
The radiators you actually have to change
Not all of them, and not none of them. Both of those answers are sales positions rather than engineering ones.