When the supply returns after a UK power cut, leave the split alone for several minutes. Compressors are built to wait before they start, and a second blip from the network can reset that wait. Check the rotary isolator and the breaker have not opened. Expect the remote clock to be wrong and Wi-Fi to need the router first. You can restore mode and setpoint once the unit has had its delay. If it trips, smells hot, or never starts the outdoor unit after that wait, stop. Do not bridge anything and do not open the gas circuit.
Silence for a few minutes is the restart delay
Distribution network cuts, planned works, and storms take UK homes off supply without warning. When power returns, indoor units often come back on whatever memory they keep, or they stay off until you press the handset. Outdoor units wait. A few minutes of silence is the restart delay, not a dead compressor. If the power dipped twice, the timer starts again. Give it a single undisturbed wait of at least five minutes after a stable return of power, longer if the manual says so, with one demand for cool or heat. Hammering the on button builds a queue of aborted starts. After that, a silent outdoor unit in cool or heat needs an engineer.
Some units also forget the clock and any on/off timer. A unit that starts at 3am is often a restored schedule, not a ghost fault. Note the rough time of the cut and whether neighbours lost power too. That separates a local supply event from a fault that began earlier and was only noticed when the lights came back. Power loss does not vent gas. A unit that ran well before the cut and runs well after the delay does not need a call that night.
Isolator, clock, router, then a real fault
Look at the rotary isolator and the consumer unit device for the air conditioning circuit. A surge can trip an RCBO. One reset is reasonable if there is no burning smell and the outdoor unit is not damaged or flooded. If it trips again as the compressor tries to start, leave it off. Do not hold the breaker in. HSE electricity guidance is the reason to stop at a second trip. An electrician and an F-gas engineer should sort the cause. GOV.UK certification covers the refrigerant side of that visit. Energy Saving Trust's air-to-air page is a reminder that the outdoor unit is a heat pump doing real work, not a gadget that should be bridged awake.
Restart the router before you blame the air conditioning app. The indoor Wi-Fi module may reconnect on its own once the router is up, or it may need the app open on the same 2.4 GHz network. Often you do not need to re-pair after every cut. Re-pair only if the module does not return. Infrared remotes still work without Wi-Fi. Set cool or heat explicitly. Do not assume auto remembered what you wanted.
Stop if you smell burning, if the outdoor fan buzzes, if lights flash a pattern that returns after one proper restart, or if water leaked because a pump lost power and the pan overflowed. Isolate and book. Mention the trip if there was one. Do not open outdoor panels to look for a blown board. Do not add refrigerant because the unit was off for an hour. A unit that will not complete a start is the fault. A unit that only lost the clock is not.
Frequently asked questions
How long should I wait before deciding it has failed?
At least five minutes after a stable return of power, with one demand for cool or heat. If the manual states a longer delay, follow it. After that, a silent outdoor unit in cool or heat needs an engineer.
Do I need to re-pair Wi-Fi after every cut?
Often no. Get the home router up first. Re-pair only if the module does not return. The infrared remote should still control the unit either way.
The breaker trips as soon as the compressor starts. Can I leave it reset?
No. One trip on the return of power is a stop sign if it happens again. Leave the isolator off. An electrician and an F-gas engineer should sort the cause. Do not hold the breaker in.
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How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.