Why a heat pump runs all the time

Most complaints about heat pumps come from people running them like boilers. It is an easy mistake, because a boiler is what everyone has had for forty years, and the two want opposite things.

Powerful and occasional, or gentle and constant

A gas boiler is enormously powerful — 24 kilowatts is normal — and it works in bursts. It blasts heat in for twenty minutes, stops, lets the house cool a little, and fires again.

A heat pump might be 5 kilowatts. It cannot blast anything. What it does instead is trickle heat in continuously, at roughly the rate the house is losing it, so the temperature simply never moves.

Both keep a house at 21 degrees. They just get there differently, and the second one is much cheaper if you let it work the way it wants to.

Why the radiators feel lukewarm

This is the thing that alarms people first, and it is the system working correctly.

A heat pump gets more efficient the cooler the water it has to produce. So a well set up one makes water at perhaps 40 degrees, where a boiler used 70. A radiator at 40 degrees feels warm to the hand, not too hot to touch.

It is still putting the same heat into the room. It is doing it slowly and constantly over a bigger surface, instead of quickly and occasionally over a hot one. If the room is at the temperature you set, the radiator is doing its job, however unconvincing it feels.

Why turning it off costs money

The instinct is obvious: if it is off, it is not using anything, so it must be cheaper. The first half is true and the conclusion is not.

A house loses heat in proportion to how much warmer it is than outside. Turn the heating off and the house cools slowly, so for the first hour or two you save very little — the house is still nearly as warm, so it is still losing heat nearly as fast.

Then you have to get it back. Recovering several degrees quickly needs the heat pump to run hot, and running hot is precisely the expensive mode. You have swapped several hours of cheap gentle running for an hour of expensive hard running.

This barely mattered with a boiler, which is equally inefficient whatever it does. A heat pump's efficiency depends on how hard it is working, so the recovery costs real money.

What weather compensation does

A properly set up heat pump has a sensor outside, and it adjusts the water temperature to the weather. Mild day, cooler water. Cold snap, warmer water. It is aiming to match the rate the house is losing heat, continuously, without ever being asked.

That is why a heat pump does not really want a room thermostat clicking on and off. The controller is already doing something cleverer, and interrupting it stops it settling.

If your installer has set this up, the best thing you can do is leave it alone for a fortnight and see where the house lands.

What this means in practice

• Set a temperature and leave it. Not a schedule of on and off, just the temperature you want the house to be.
• A small overnight setback is fine. One or two degrees. Anything more and you pay to recover it.
• Do not close radiators in unused rooms. It reduces the surface the heat pump has to give its heat away through, which pushes the water temperature up for the whole house.
• Do not turn the thermostat up to warm the house faster. It will not. The heat pump has one speed, roughly, and turning the dial to 25 only means it overshoots.
• Give it a fortnight before judging. A house has a lot of thermal mass and a system running continuously takes days to settle.

If the house genuinely will not get warm

Then something is wrong with the design, not with how you are running it, and turning things up will not fix it.

The usual cause is a radiator too small for its room, which forces the whole system to run hotter than it should, or a heat pump sized for a different house. Both are things to take back to whoever installed it, with the room-by-room heat loss calculation in hand.