Commonly called a ‘radiator’, this simple steel ‘envelope’ of hot water will actually convect more heat than it radiates. Warm air rises from it, and some heat radiates to anything in its sight.
Radiators are often the only option simply due to the lower installation cost and minimal upheaval.

In the old days of un-insulated houses, we generally used big boilers to blast in the heat at specific times. Radiators were not ideal, and often a lot of the heat rose to the ceiling. However, energy-efficiency was not such a high concern then.
When heat pumps came on the scene, it became apparent that what boilers could do easily – produce very hot water, heat pumps could struggle, and some expensive-to-run heat pumps resulted.

Things have moved on a pace since then and we are better equipped to match heat pumps to the good old radiator.
For well insulated buildings, heating is easy, and a relatively small radiator can give out adequate heat at a relatively low lukewarm energy-efficient temperature. This is ideal for heat pumps.

But for old uninsulated buildings, the problems remain that in general, radiators may need to be hotter than the ideal energy-efficient range for heat pumps.
In order to reduce the temperature of a radiator, it can simply be left on for longer. This is all very well, but the house will now be warmer than needed for longer, thus potentially wasteful. However due to the fact the heat pumps are more energy-efficiency at low temperatures, then there can be a net energy gain if, for example, you change your heat pump’s setting and start time from say 7am and 50°C, to 5am and 40°C. This is a thing you would need to try out over a period of time, and watch the running costs.

TRV Thermostatic Radiator Valves

These valves are now almost universally fitted to every radiator. They are cheap, simple and effective, and automatically ‘turn down’ the flow when the room reaches the set temperature (or setting 0 to 5).
However, for some systems, it can be a mistake to reduce the settings thinking that you are improving the economy of the system. It may be better to set the main room TRVs to high, and then to turn down the setting on the heat pump unit. (E.g. the weather-compensation heating curve). TRV valves are most useful in bedrooms and other rooms where the temperature may need limiting.
Fan Radiators
For harder-to-heat older buildings, fan-assisted convectors could be used. The modern designs are a lot better and much quieter than they used to be. However, beware of the controls fitted into some that try to stop them blowing cool-feeling air E.g. if a heat pump is set to a nice low and efficient water temperature, the circulating water could be colder that the convector will accept, so the fan stays off. The detail of the ‘minimum water temperature’ should be checked.