Underfloor Heating, Radiators and Heat Pumps

Underfloor heating and heat pumps — and what to do with radiators.

Heat pumps deliver heat gently, at low flow temperatures, so they pair beautifully with large emitters like underfloor heating. But you do not always need to rip out radiators. The real question is whether your emitters are big enough to heat each room at a low flow temperature — and that is something to measure room by room.

Why emitter size is critical for heat pumps

A radiator's heat output depends on the difference between the water temperature inside it and the room air around it — the 'delta-T'. A radiator sized for a 70°C boiler emits far less heat at a heat pump's 45°C flow temperature, because that temperature difference is much smaller. Roughly, a radiator outputs less than half its boiler-era heat when run at heat-pump temperatures. To keep a room warm, the emitter must be larger, or the room's heat loss must be lower — or both.

The emitter options, from best to most limited

Underfloor heating: a huge emitting surface that works beautifully at 30–40°C — the ideal low-temperature emitter
Upsized radiators: larger or higher-output (e.g. type 22 or triple-panel) radiators that deliver enough heat at a low flow temperature
Fan-assisted radiators: compact units that boost output at low temperatures where space is tight
Existing standard radiators: may work in rooms with low heat loss, especially after fabric improvements

Retrofitting underfloor heating — when it makes sense

Underfloor heating is superb for heat pumps, but retrofitting it is disruptive and adds floor height. It makes most sense when you are already renovating a floor, building an extension, or replacing a ground-floor slab. Low-profile retrofit systems exist for adding underfloor heating over an existing floor with minimal build-up. In upstairs rooms, upsized radiators are usually the more practical choice.

Don't forget the floor insulation underneath

Underfloor heating over an uninsulated floor pushes heat downward into the ground or a cold void instead of up into the room. Any underfloor heating retrofit must sit above proper floor insulation, or much of the heat — and the efficiency — is lost. This is exactly the kind of detail a readiness assessment catches before the work is done.

How RetrofitIQ assesses your emitters

Measure room-by-room heat loss at the design temperature
Compare each emitter's output at the proposed low flow temperature against that room's heat loss
Identify which rooms need larger emitters, and where fabric improvement is the better lever
Advise where underfloor heating is worthwhile versus upsized radiators
Provide the emitter findings and a room schedule to your heat-pump designer

Common questions from clients.

Do I have to install underfloor heating for a heat pump?

No. Underfloor heating is ideal but not essential. Correctly sized radiators — helped by lower heat loss after fabric improvements — run well on a heat pump in most homes.

Will my existing radiators work with a heat pump?

Some will, some will need upsizing. It depends on each room's heat loss and the radiator's output at the lower flow temperature, which we assess room by room rather than assuming.

Is it worth retrofitting underfloor heating?

In the right rooms — especially during a floor renovation or in an extension — it can be excellent for heat-pump efficiency and comfort. Over an existing floor it is more disruptive, so we advise where it genuinely pays off.

Can I mix underfloor heating and radiators on one heat pump?

Yes, and it is common — for example underfloor heating downstairs and radiators upstairs. The system is designed around the highest flow temperature any circuit needs, so it pays to keep the radiators well sized.

Why does underfloor heating need floor insulation?

Without insulation beneath it, underfloor heating loses heat downward into the ground or void instead of warming the room. Insulating the floor first keeps the heat where you want it and protects the system's efficiency.