How cold should the air coming out actually be?

On a healthy split system in cooling, the air leaving the indoor head is typically well below room temperature — the engineer's check is the difference between the air going in and the air coming out, not the number on the remote. If that difference has collapsed to a degree or two, something is wrong.

Does air conditioning need topping up like a car?

No. The circuit is sealed and should never lose refrigerant. If it needs gas, it has a leak, and GOV.UK guidance is that a leak found during a check must be repaired as soon as possible and re-tested within a month. Repeat top-ups without a repair are not legitimate servicing.

Can I clean the filters myself?

Yes, and you should — most manufacturers ask for it every few weeks in heavy use. The panel on the indoor head lifts open and the mesh filters slide out. Everything past the filters, including the coil and the fan barrel, is an engineer's job.

Why does it cool one room but not the other on a multi-split?

Each indoor head has its own expansion valve, filters and sensors, so one head can fail while the rest work. It also means the fault is far more likely to be in that head than in the shared outdoor unit.

Low risk

Updated 30 August 2026

Air Conditioning Not Blowing Cold

The unit runs and the fan blows, but the air coming out is room temperature or barely cool.

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Air conditioning work, illustrative image

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

Frequently asked questions

We’re happy to answer all your questions.

Sources & References

Ice On The Pipework Or Indoor UnitCaution System Keeps Needing RegassingCaution Outdoor Unit Not RunningCaution Error Code Or Flashing Light On The UnitCaution

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