The compressor sits in the outdoor unit. If it will not start, the indoor fan can still run and fool you into thinking the system is on. Check heat or cool rather than fan, a setpoint that actually demands work, the rotary isolator, and a wait of several minutes after power-up. One click then a steady run is normal. A click, a hum, then silence is not. Do not bridge the contactor, do not fit a start capacitor from a guess, and do not open the gas circuit. If it will not run after the delay, call an F-gas engineer and an electrician if the breaker is involved.
Fan mode and a satisfied room leave it off
Compressors start only when the controls call for cooling or heating. Fan mode, a satisfied setpoint, and some protection stops leave them off while the indoor fan blows room air. Set a clear mode and a setpoint several degrees the correct side of the room temperature. If the indoor unit blows air but the room does not change, and the outdoor unit is silent, you are very likely looking at a compressor that has not started.
The restart delay you keep wiping out
UK splits enforce a delay so the compressor does not restart against pressure. After a power cut or an isolator cycle, three to five minutes of nothing is normal. Use a clock. If you switch the remote off and on every thirty seconds you keep resetting that timer and the compressor never gets its chance.
The isolator handle, and nothing behind it
Confirm the rotary isolator beside the outdoor unit is on, with the enclosure shut. Confirm the dedicated breaker has not opened. If you are not competent around a consumer unit, leave the breaker where it is and tell the engineer it may have tripped. HSE electricity guidance and Electrical Safety First's Part P notes both point away from opening those enclosures yourself.
A click, a hum, then silence
One click then a steady run is normal. A hum that never becomes a run, or a case that is too hot, means stop and leave it isolated if you can do that safely. Do not remove the outdoor top cover. Do not spin the fan with a stick to encourage a start. Note whether you heard nothing at all after five minutes, or a click and a hum then a stop, and how many times you have tried.
Capacitor, pressure stop, or a seized compressor
An F-gas engineer should decide whether the outdoor unit is getting a start signal, whether the fan and compressor are electrically sound, and whether refrigerant conditions are part of the story. GOV.UK certification is the company rule for that refrigerant work. An electrician should be involved if the circuit trips or the isolator is damaged. Do not order a replacement compressor from a model sticker and a hunch. Low refrigerant can prevent a normal run, but so can a failed capacitor, a high-pressure stop, or a seized compressor. Energy Saving Trust's air-to-air heat pump description is about the heat move, not a way to jump-start the outdoor unit. If ice, oil or a burning smell is present, say so and do not run a second test.
When a bridge, a tap, or a hard-start kit is the wrong fix
Do not press in a contactor with a screwdriver. Do not hit the outdoor unit. A compressor that is stuck or overheated needs a proper test, with the unit isolated. Extra capacitors and hard start devices are electrical work on a refrigerant system. Fitted blind they mask a failing compressor and can create a fire risk. Leave that decision to qualified people. None of those faults are corrected by opening a service port in the garden. Leave the flares tight.
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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.