A heat pump asks the radiators to do more with less

Output falls faster than temperature does

A radiator gives out heat in proportion to the difference between the average temperature of the water inside it and the temperature of the room, and the relationship is a little steeper than a straight line because convection strengthens as the surface gets hotter. Halve the difference and you lose rather more than half the output.

That single fact governs the whole subject. A system designed to run hot delivers plenty from a modestly sized panel. Drop the water temperature towards the range where a heat pump becomes efficient and the same panel delivers a fraction of what it did, in a room whose heat loss has not changed at all.

So the emitter, not the appliance, is usually the binding constraint in a retrofit. A heat pump installed onto radiators sized for hot water either runs hotter than it should, which costs efficiency, or fails to heat the room on the coldest days, which costs goodwill. Both outcomes get blamed on the technology rather than on the installer’s sums.

Underfloor heating is simply a very large radiator

Spread the emitting surface across an entire floor and the area becomes enormous compared with a panel on a wall. A large area can move the required heat at a small temperature difference, which is exactly the condition a heat pump wants, and it’s why the two are so often described together.

There is a ceiling on this, though, and it is set by comfort rather than by physics. A floor much above skin temperature is unpleasant to stand on, and that limit caps how much heat a square metre of floor can deliver. A poorly insulated room with large glazing can exceed that cap, in which case the floor alone will not heat it.

The order of work therefore matters. Reduce the heat loss first and the emitter required shrinks accordingly; install the emitter first and you may have sized it for a building you are about to change. Insulation makes low-temperature heating possible in a way no emitter choice can.

Retrofitting a floor is where the cost lands

In a new build, pipes go into a screed before anything else happens and the cost is marginal. In an existing house it means lifting floors, finding somewhere for the additional height, adjusting door leaves and thresholds, and living around the disruption. Low-profile overlay systems reduce the depth and not usually the disturbance.

Which is why, for most retrofits, the sensible answer is larger radiators rather than underfloor heating. A panel with more surface area, or a deeper one, or a fan-assisted unit, achieves the same low-temperature output for a fraction of the upheaval. It is a less elegant answer and generally the better one.

That is worth saying plainly because underfloor heating is often presented as a requirement for a heat pump. It is not. It is one way of obtaining a large emitting surface, and it competes with other ways that are cheaper in a building that already exists.

Response time is a genuine trade rather than a detail

A screed floor holds a great deal of heat, so it warms slowly and cools slowly. Asked to raise a room’s temperature quickly, it will not. Left running steadily against a weather-compensated control, it is superb, and steady running happens to be the mode in which a heat pump performs best anyway.

Radiators do the opposite. Low mass, quick response, easy to control room by room, and correspondingly worse at holding a temperature when the heat source cycles. Neither behaviour is better in the abstract, and neither is the one you’d choose for every room; they suit different patterns of occupancy, and a room used for two hours a day is a poor match for a slow floor.

Fan coil units sit between the two, using forced air over a coil to obtain a large effective surface in a small volume. They respond quickly, they work at low water temperatures, and they make a noise, which is why they appear more often in commercial buildings than in bedrooms.

The pipework is part of the emitter

Delivering more heat at a lower temperature means moving more water, because the energy carried depends on both the flow rate and the temperature drop across the circuit. A system originally designed to run hot with a large drop may have pipework too narrow to carry the flow a low-temperature design needs.

Small-bore and microbore distribution is the common obstacle in older installations. So are undersized circulating pumps, restrictive valve arrangements and a system that has silted up over decades. None of this is visible from the boiler cupboard, and all of it changes what an emitter can actually deliver.

The corrective is a proper room-by-room heat loss assessment rather than a rule of thumb about floor area, followed by emitter sizing against the intended flow temperature. Some rooms will need a change and some will not. That survey is the part worth paying for, and it belongs to a qualified designer working to the standards in force locally.