Heat pump not heating the house

What it usually means

A heat pump does not heat a house the way a boiler does, and almost every complaint of this kind begins there. A boiler makes water at seventy degrees and blasts a room warm in twenty minutes. A heat pump makes water at thirty-five or forty-five and keeps a house at temperature by running gently for most of the day. Efficiency falls sharply as the water it produces gets hotter, so the design intent is a long, low, boring run — which means a heat pump that is on all the time is not necessarily struggling. That is what it is meant to do.

The consequence catches people out in a specific way. Set a heat pump back overnight by four degrees the way you would a boiler, and it has to recover the whole house in the morning using an appliance with deliberately limited output. It cannot, so the house sits cool until early afternoon and then overshoots. The same logic applies to fiddling: every adjustment restarts a slow process, and a system that is switched off and on twice a day will underperform one left alone at a steady setting. Most systems also use weather compensation, which raises the water temperature as it gets colder outside — so on a mild damp day the radiators feel barely warm, and that is correct behaviour rather than a symptom.

When something genuinely is wrong, the pattern changes from lukewarm to stopping. Heat pumps depend on water moving through them at a decent rate, because they extract only a few degrees from each pass. Too little flow and the appliance protects itself: it will report a flow fault or cycle repeatedly on and off, and the usual causes are close at hand — a dirty filter or strainer on the primary circuit, air in the system, a valve part closed, or a circulating pump running slowly. Low system pressure produces the same result from a different direction, and manufacturers publish thresholds for it.

Two things are worth ruling out before any of that. Water heating usually takes priority over space heating, so a system reheating a cylinder — or running a weekly legionella cycle — will stop heating the house for an hour, by design. And a heat pump defrosts its outdoor unit periodically in cold damp weather, which pauses heat output and produces a cloud of vapour that looks like smoke and alarms people every winter.

Then there is the fault with no code at all, and it is the important one: heat coming out of the radiators but a bill that has jumped. Every heat pump has an electric immersion or backup heater for the coldest days and for the legionella cycle, and when a control setting is wrong or the heat pump cannot meet demand, that backup quietly takes over. The house stays warm and the running cost triples. If your house is warm and your electricity use has climbed, that is the fault to chase, and it is a settings investigation rather than a broken part.