Air Conditioning Not Blowing Cold

What's happening?

A split system does not make cold. It moves heat out of the room and dumps it outside, using refrigerant that changes state as it goes round a sealed circuit. Air blowing but not cooling means one of the three stages has stopped: the indoor coil is not getting cold, the outdoor unit is not rejecting heat, or the refrigerant charge has fallen far enough that the system can no longer shift much. The first thing to establish is whether the outdoor unit is running at all. Stand next to it: a working condenser hums, the fan turns and the air off it is noticeably warm. If the outdoor unit is silent while the indoor head blows, the fault is in the outdoor half or the wiring between them. If both run and the air is still tepid, suspect airflow or refrigerant.

What usually causes it?

• Filters choked with dust, so almost no air passes over the indoor coil
• Refrigerant charge low after a slow leak, leaving the system unable to absorb much heat
• Outdoor unit not running because of a control, capacitor or supply fault
• Outdoor coil packed with pollen, leaves or grass cuttings, so heat cannot escape
• Unit set to fan-only or dry mode rather than cool, or the setpoint above room temperature
• Indoor coil dirty enough that the fins are bridged and air passes without contact
• System undersized or badly sited for the room, so it never catches up in a heatwave

Is it dangerous?

Losing cooling is not dangerous in itself, but a top-floor flat or a south-facing room in a UK heatwave gets uncomfortable fast, and heat is a genuine risk for elderly people, babies and anyone with a heart or breathing condition. Treat that as a reason to act quickly rather than a reason to panic. Stop using the unit and get it looked at if it is tripping the circuit, smells hot or electrical, or is dripping water onto anything live.

Can I fix it myself?

Two checks are yours and both are common causes. Slide the front panel of the indoor head open and pull the mesh filters out: if you cannot see daylight through them, wash them in warm water, dry them fully and refit. Then go outside and clear anything blocking the condenser: leaves against the coil, a fence panel or a shed built too close, a bin in front of the fan. Confirm the remote is on cool, not fan or dry, and that the setpoint is several degrees below the room temperature. Do not touch the pipework, the valves or anything sealed.

When should you call an air conditioning engineer?

Call an air conditioning engineer if the unit still blows warm after clean filters and a clear condenser, if the outdoor unit never starts, or if you can see ice or heavy frost anywhere. Anyone who needs to check the refrigerant charge must hold an F gas qualification, and their company must be F gas certified: that is the law, not a preference, so it is worth asking before you book.

What will the air conditioning engineer actually do?

• Measure air-on and air-off temperatures at the indoor head to confirm how much cooling is actually being produced
• Check the outdoor unit's supply, contactor and fan capacitor, and confirm the compressor is running
• Take suction and discharge pressures and superheat readings to judge whether the charge is correct
• Clean the indoor and outdoor coils and check the fan speeds against the manufacturer's data
• Leak-test the circuit if pressures point at a loss of refrigerant, and repair before recharging
• Record the refrigerant added or recovered, which is a legal requirement for equipment they service

Who fixes this

• Air conditioning repair

Questions people ask

Sources & References

1. GOV.UK: Qualifications to work with F gas
2. GOV.UK: Checking F gas equipment for leaks
3. Daikin UK: Frequently asked questions