What Happens to a Heat Pump in a Power Cut? (UK Guide)

When the power goes off, a heat pump stops completely. The compressor, fan, circulation pump and controller all need mains electricity. Short cuts are rarely a problem. A long cut in freezing weather is different, because the outdoor water pipes on some systems can freeze. When the power returns, give the system a few minutes to restart, then check the controller, the pressure gauge and any fault codes before assuming everything is fine.

The lights go out, the house goes quiet, and one of the first thoughts is often, “What about the heating?”

A heat pump isn’t a boiler. It can’t fire up on gas or oil, and it doesn’t have a pilot light to fall back on. No electricity means no heat from the system. That sounds alarming, but for most homes it’s a short inconvenience, not a crisis. Here’s what is actually going on and what’s worth doing.

What stops when the power goes off

Everything electrical on the heat pump side stops at once:

• The compressor and the outdoor fan

• The circulation pump moving water round the radiators or underfloor heating

• The controller and any motorised valves

• The immersion heater, if you have one

Your radiators will start to cool and the system won’t heat the cylinder. The house doesn’t go cold instantly, though. Heating systems and building fabric hold heat, and a well-insulated home loses it slowly. A cut of an hour or two is usually barely noticeable. A cut lasting many hours on a freezing night is a different matter.

What about hot water?

A well-insulated cylinder holds its heat for many hours. You’ll probably still have usable hot water for a while after the power goes off, but don’t count on it for long. Use it sensibly, and don’t open the tap at every opportunity.

If a storm is forecast, you can use your normal hot water boost beforehand so the cylinder is full of hot water. Stay within your manufacturer’s settings.

The real risk: freezing

For most homeowners, the main worry in a long winter power cut is frost. It depends a lot on how your system is built:

Most of the frost protection on a heat pump depends on power. Controllers can run the circulation pump when temperatures drop, which keeps water moving. In a power cut, that safety net stops. Glycol or freeze-protection valves help, but they aren’t identical in how they work or how well they cope with a very long cut.

If you don’t know what type of system you have, or how it’s protected against freezing, ask your installer. It’s a fair question, and it’s far better answered in October than at midnight in a cold snap.

Frozen condensate pipework is a separate issue. Our guide on heat pump condensate drainage explains why drainage matters in cold weather, and our article on where an outdoor unit should go covers siting and exposure.

What to do during the power cut

• Stay calm and stay warm. Layers, blankets, closed doors, and shutting curtains at dusk all help.

• Don’t run anything dangerous for heat. Camping stoves, barbecues and unvented gas or paraffin heaters must never be used indoors. Carbon monoxide can kill without any warning.

• Don’t open covers on the heat pump. There’s nothing safe to do inside the unit.

• Don’t connect a generator yourself. Plugging a generator into a household socket or fixed wiring is dangerous. A generator needs a proper changeover arrangement fitted by a qualified electrician, and inverter equipment can be sensitive to the quality of the supply, so check with the manufacturer too.

• Don’t assume a home battery will keep it running. Many batteries aren’t set up to supply the house in a cut, and a running heat pump is a heavy load even if they are.

• Check your isolators and breakers aren’t the cause. If only your property has lost power and the neighbours haven’t, look at the consumer unit before assuming it’s the grid.

What to do when the power comes back

Don’t rush. Give the system a few minutes. Heat pumps often wait before starting the compressor again, which is normal.

Then work through this:

1. Check the controller. Is it on, showing the right date and time, and are your schedules intact? Some controllers can reset after a long outage.

2. Check for a fault code. If one is showing, take a photo before you reset anything. Our guide to common heat pump fault codes explains what the main categories mean.

3. Check the system pressure. Compare it with your normal range. A big drop could mean a leak, and it’s worth noting if it keeps falling.

4. Check the breakers. If one has tripped, reset it once. If it trips again, leave it off and call a qualified electrician.

5. Look around the outdoor unit. Check for ice, damaged pipework or water where there shouldn’t be any.

6. Watch how it behaves for the next hour. Radiators should warm up gradually, and the unit should settle into a steady run.

If it won’t restart, shows a fault code that doesn’t clear, or something just doesn’t feel right, our guide on what to check before calling an engineer takes you through the safe checks. If the system keeps stopping and starting after a cut, our article on why a heat pump keeps turning off explains the difference between normal cycling and a real problem.

How to be better prepared for winter

Most of what helps is simple and takes ten minutes:

• Find your isolators, consumer unit and controller before you need them.

• Photograph your controller settings, schedules and normal pressure reading.

• Ask your installer whether your system has glycol or freeze-protection valves, and what happens in a long power cut.

• Keep the area around the outdoor unit clear.

• Run through a pre-winter check while the weather is mild. Our winter maintenance checklist walks you through it.

When to get help

After most short cuts, nothing needs doing. Get help if the heat pump won’t restart, a fault code keeps returning, pressure has dropped, settings have vanished, or you think pipework may have frozen.

Our Fix My Heat Pump service is remote. We go through your fault codes, settings and what the system is doing to work out what’s wrong, and give you or your installer a clear brief for what to do next. We can’t repair frozen pipework or do electrical work over a video call. But we can help you work out whether you’re dealing with a settings issue, a hydraulic problem or something that needs an engineer on site.

Frequently asked questions

Will a power cut damage my heat pump?

A normal cut shouldn’t. The unit just stops. The bigger risks are freezing in a long cold-weather cut and the system not restarting properly afterwards.

How long can a heat pump be without power in winter?

There isn’t one number that fits every home. It depends on the outside temperature, how your system is built and how it’s protected against frost. A few hours in mild weather is rarely an issue. Many hours of hard frost is where you need to take care.

Do I need to do anything when the power comes back?

Give it a few minutes, then check the controller, any fault codes and the pressure gauge. Most systems just restart.

Can I run my heat pump from a generator?

Only if an electrician has set it up properly and the manufacturer says the generator is suitable. Never connect a generator to your wiring yourself.

Is a heat pump less reliable than a boiler in a power cut?

Neither works without electricity. Most gas boilers need power for the controls, fan and pump. The difference is that a heat pump has more equipment outdoors, which matters in a long freeze.

Heat pump not right after a power cut?

Tell us what’s happening, and we’ll help you work out what to check first.

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