Low flow temperatures change everything
A gas boiler runs happily at 70–80°C. A heat pump is efficient at 35–45°C. That single difference drives every other decision: emitters must be larger, pipework must be sized for greater flow, the fabric must be improved, and the system must be allowed to run steadily rather than in short bursts. Retrofitting a heat pump to a system designed around a boiler, without revisiting the emitters, is the most common way to end up with a house that is cold and expensive.
Why underfloor heating pairs well
Water-based underfloor heating is a large, low-temperature emitter, which is exactly what a heat pump wants. The screed or panel system, the pipe spacing and the floor covering all affect output: a thick rug or an engineered board with a high thermal resistance can reduce the heat reaching the room by a quarter. Floor finishes therefore need to be decided before the underfloor design is finalised, which is earlier in the interior design programme than most clients expect.
Floor build-ups and levels
In a refurbishment the build-up is the constraint. A traditional wet screed system may need 65–75mm plus insulation; low-profile systems reduce that to 20–30mm at some cost in output and responsiveness. Every millimetre affects door heights, threshold details, stair risers and the junction with retained floors elsewhere in the house. We set out floor levels across the whole plan at the start, in one drawing, rather than resolving them room by room as the work proceeds.
Plant space, noise and the neighbours
The external unit needs airflow, defrost drainage and a location that will not become a nuisance. On terraced and semi-detached London sites, acoustics usually govern: sound pressure at the neighbour’s window, and any permitted development limit that applies, are checked before a position is fixed. Internally, the cylinder, buffer where used, and controls need real space — an unvented cylinder with its safety discharge arrangement is not a cupboard afterthought.
Controls and commissioning
Heat pump systems reward simplicity. Weather compensation, sensible zoning and a control strategy the household understands beat a complicated schedule that gets overridden every evening. Commissioning records — flow temperatures, flow rates, actual outputs against design — are handed over with the operating instructions, because a system that has never been balanced will never perform as designed.