Why are my radiators only lukewarm?

Updated 29 September 2026

Short answer: if the rooms are reaching the temperature you set, nothing is wrong. A heat pump sends cooler water round the radiators than a boiler, for longer.

A boiler heats radiators until they're too hot to touch, then stops. A heat pump runs them just warm, often most of the day, because cooler water is what keeps its COP (heat out divided by electricity in) high. With weather compensation — the heat pump setting the water temperature from the weather outside — the water gets warmer on cold days and cooler on mild ones, so how warm the radiators feel changes with the weather.

The test is the room, not the radiator. If the room is at its target, the radiator is doing its job.