Why flow temperature changes the radiator question
A gas boiler sends water to your radiators hot; a heat pump runs cooler, because low flow temperature is where its efficiency lives. A cooler radiator gives off less heat, so a room that was comfortably served at boiler temperatures sometimes needs more radiator area at heat pump temperatures. That is the entire issue: not compatibility, just surface area.
In practice many radiators are already oversized for their rooms, fitted generously when the house was built or refurbished, and carry on fine. The room-by-room heat loss survey identifies the exceptions, and swapping a handful of radiators for larger or higher-output ones is routine work that belongs itemised on the quote. Underfloor heating suits heat pumps beautifully, but as an option, not an entry requirement.
Where insulation genuinely helps
Insulation is not an eligibility hurdle for the grant, but it changes the arithmetic in your favour twice. A house that loses less heat can run a smaller heat pump, which costs less to buy, and it can run at a lower flow temperature, which costs less per hour. Loft top-ups and cavity wall insulation are the cheap, high-impact end of that lever.
Done before the survey, insulation can take a size out of the unit rather than adding cost on top, which is the right order to do things in. Solid-wall external insulation is a much bigger project and rarely the make-or-break: plenty of older homes run heat pumps well with sensible emitter sizing instead. The survey shows where your house sits on that spectrum.
Compare heat pump quotes
Up to three trusted, MCS-certified installers quote for the work, grant applied where you qualify. Free, no obligation.
Compare heat pump quotesWhat the survey decides, and what to ask
Everything above funnels into the heat loss calculation: unit size, flow temperature, which radiators change and whether a cylinder needs housing. It is why two neighbouring houses can get quite different quotes, and why a figure produced without a survey is a guess. The costs it feeds into, and the £7,500 that comes off them, are covered in the grant explained.
When quotes arrive, ask each installer for the design flow temperature and which emitters they propose changing. A low number with few changes means a well-matched house; a high flow temperature with no changes is a warning sign, because it quietly gives away the efficiency you are buying. Compare quotes from MCS-certified installers on those answers as much as on price.
Compare heat pump quotes
Up to three trusted, MCS-certified installers quote for the work, grant applied where you qualify. Free, no obligation.
Compare heat pump quotes