Reference / Heat pumps / Overview
Heat pumps
A heat pump is not a like-for-like boiler swap. The survey, the radiator sizing and the flow temperature are the entire story. The hardware itself is mostly interchangeable once you have those right.
The most common failure mode is an undersized radiator system that leaves the heat pump running at a higher flow temperature than it should, eating its efficiency advantage.
The heat loss survey
A proper room-by-room heat loss calculation is the single most important document in a heat pump install. It decides the size of the pump, the flow temperature you can use and which radiators (if any) need replacing.
ASHP vs GSHP
Air source heat pumps (ASHP) suit nearly every UK home that has the outdoor space for the unit. Ground source (GSHP) wins on long-run efficiency but adds capital cost and disruption that rarely pays back at domestic scale.
Radiator sizing
Most existing radiator systems were sized for an 80°C flow temperature. A heat pump wants 45-55°C. The maths usually requires larger radiators in at least the worst-case rooms.
BUS grants
The Boiler Upgrade Scheme takes £7,500 off an ASHP install for an eligible property. The eligibility detail is worth reading before assuming you qualify.