Heat pumps: common questions

Yes. Physics does not care how old a house is; it cares how hot the water in the radiators has to be.

What changes with an older house is the cost. Poorly insulated rooms with small radiators need hotter water, and hotter water means a higher bill. Bigger radiators or better insulation bring it down.

Usually only some. Many existing radiators are already generously sized because they were fitted before the house had insulation and double glazing.

Replacements are often a matter of depth, not length — a deeper radiator on the same stretch of wall can nearly double the output.

Because they are meant to. A heat pump runs gently and continuously at a low water temperature instead of in hot bursts, and that is the efficient way to do it.

If the room is at the temperature you set, it is working. There is more in why a heat pump runs all the time.

No. Turning it off saves very little, because a warm house loses heat slowly, and getting the temperature back needs the heat pump to run hot, which is the expensive mode.

Set a temperature and leave it. A one or two degree overnight setback is fine.

Yes. Air source heat pumps operate down to about −20 °C, colder than almost anywhere in Britain gets.

Output does fall as it gets colder, which is exactly what the sizing allows for. A correctly sized system holds the house on the coldest normal day without any backup heater.

About as loud as a fridge, and quieter a few metres away. A modern unit is around 50 to 60 decibels at the machine itself.

There is a legal limit of 37 decibels at a neighbour's window, and where the unit is placed matters far more than the figure on the datasheet.

Against oil, LPG or electric heating, almost certainly. Against mains gas, it depends on the design.

Electricity costs about four times what gas does per unit, and a boiler loses some of the gas it burns, so a heat pump has to produce roughly three and a half units of heat per unit of electricity just to break even. A well designed system beats that; one running hot may not.

Yes, if you currently have a combination boiler. A heat pump cannot heat water on demand, so it heats a tank in advance.

Finding somewhere to put it is often the main obstacle in a retrofit, more than anything about the heat pump itself.

Usually not, even if it looks fine. The coil inside is sized for a boiler's very hot water, and a heat pump supplies water much cooler.

An old cylinder will reheat slowly and drag the heat pump's efficiency down while it does.

Fifteen to twenty years, much like a gas boiler, with the compressor as the part that eventually wears out.

A ground source system's buried pipework lasts far longer — fifty years or more. See what wears out, and when.

An annual service, plus keeping the outdoor unit clear of leaves and debris, which is less attention than a boiler needs.

An unvented hot water cylinder needs its own annual check by somebody qualified for unvented systems.

Some can, but not through ordinary radiators — you need fan coils or underfloor, and careful attention to condensation.

Ground source can do it more elegantly, circulating cool water from the buried loop without running the compressor at all.

That is a defrost cycle, and it is normal on a cold damp morning. Moisture from the air freezes on the outdoor coil, so the machine briefly runs backwards to melt it off.

It lasts a few minutes and then everything carries on as before.

It varies too much for a useful single figure, because it depends on your heat loss, how many radiators need changing and whether there is room for a cylinder.

Roughly half the money is the heat pump and cylinder; the rest is emitters, pipework, electrics and labour. Grants are available in England, Wales and Scotland; Northern Ireland has narrower, income-based support.