If the indoor unit lights up and the outdoor unit stays silent, start with mode, setpoint and the rotary isolator beside the condenser. Fan-only mode never starts the compressor. A setpoint the room has already met will not start it either. After power is applied, many UK splits wait a few minutes before the outdoor unit runs. You may check that the isolator handle is on, without opening the enclosure. Do not bridge wires. If the outdoor unit hums and clicks off, the breaker trips, or it stays dead in cool or heat with a real temperature gap, stop and call an F-gas engineer.
Fan mode never starts the condenser
Fan mode circulates room air. The outdoor unit is meant to stay off. Dry mode may run the compressor differently and the outdoor fan can cycle. In fan-only the outdoor fan should not run at all.
A mild day and a setpoint the room has already met
Auto mode on a mild British day can decide the room is close enough to the setpoint and do nothing outside. Cooling with the setpoint above the room temperature, or heating with the setpoint below it, also leaves the condenser idle.
Look at the remote icons, not the marketing name on the fascia. A snowflake, a sun, a fan blade and a water drop are different requests. If you have both an app and a handset, one of them may have left the unit in fan. Set an explicit cool or heat demand, with a setpoint a few degrees the right side of the room temperature, and wait. Note the mode you actually set, and whether this started after a power cut, a garden job, or a storm.
The rotary isolator beside the condenser
Almost every domestic outdoor unit has a rotary isolator on the wall within reach of the condenser. Gardeners, painters and anyone who isolated the unit for a storm can leave it off. The indoor unit can still look alive if its supply is separate, or it can look dead if they share a supply. With dry hands, from the ground, confirm the isolator handle position. Do not remove the isolator cover. HSE electricity guidance is to leave enclosures shut. Do not work inside the consumer unit unless you are competent to do so. Electrical Safety First describes Part P as the building-regulations route for electrical work in the home, which is not a patio experiment on a condenser.
Three to five minutes after power returns
After you turn power back on, wait at least three to five minutes. Compressors are not meant to restart instantly. A clock on the remote that needs resetting is a nuisance. A dead outdoor unit after that wait, while cool or heat is clearly demanded, is a fault. In heating, some units also pause for defrost or because the outdoor temperature is outside the range in that model's data. The manual states the range. Do not guess a lockout code from a forum.
A hum, a twitch, or a breaker that opens
Resetting once after you are sure the isolator and mode are right is enough. Note whether a breaker tripped. Water inside the outdoor electrical section, a burnt smell, or oil on the pipes are stop signs. Do not open the outdoor electrical box to swap a fuse. A blown fuse inside the condenser is a diagnostic result, not a part to replace from a guess. Repeated replacement hides a short.
When mode and a wait are the wrong response
Stop if you hear a hum then a click, if the fan blades twitch but do not turn, or if a breaker or RCBO opens. A second trip means leave it off. Do not bridge the contactor, spin the fan by hand to start it, or open flare nuts. An F-gas engineer, working to the GOV.UK certification rules for refrigerant equipment, should test the outdoor unit. If the problem is the dedicated circuit, they will often want an electrician as well. A still outdoor unit does not, by itself, mean the gas has escaped. No power, wrong mode, or a protection stop look the same from the patio. Refrigerant is not the first question when the outdoor unit has no power.
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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.