Air Conditioning Not Cooling Properly
When an air conditioning system switches on, the fan runs, and cold air seemingly comes out, but the room still doesn't drop to temperature, it's easy to assume the unit is broken. In practice the most frequent culprits are far simpler: a dirty filter, a poorly set thermostat, or an outdoor unit that's been left to bake in direct sun with restricted airflow.
The trick is working through causes in order of likelihood before assuming the worst. Some checks are safe for anyone to do in five minutes. Others involve refrigerant circuits, which by law only an F-Gas certified engineer can open up and work on.
Most likely causes, in order
- Blocked or heavily soiled filters restricting airflow across the indoor coil
- Outdoor condenser unit smothered by leaves, pollen, or positioned too close to a wall or fence
- Undersized system for the room, especially in extensions or conservatories with lots of glass
- Low refrigerant charge from a slow leak, reducing the system's ability to absorb heat
- Thermostat or remote set to fan-only mode by mistake, or a faulty temperature sensor
What you can check yourself
Start with the filters. Most wall-mounted units have a front panel that lifts up to reveal two washable mesh filters. If they're grey with dust, a wash under a tap and a few hours to dry can make a noticeable difference on its own.
Next, look at the outdoor unit. Clear leaves or debris from around it, make sure nothing is stacked against the casing, and check the fan spins freely when the system is running. Confirm the mode on the remote is set to 'cool', not 'fan' or 'dry', and that the target temperature is actually below room temperature.
When it needs an engineer
If filters are clean, the outdoor unit is clear, and the mode is correct but the room still won't cool, the issue is likely inside the refrigerant circuit or the electronics, and that's not something to open up yourself. A refrigerant leak, a failing compressor, or a faulty expansion valve all present in similar ways to the homeowner and need proper diagnostic equipment to tell apart.
Handling refrigerant without F-Gas certification is illegal in the UK, aside from being unsafe, so this is the point to book a visit rather than keep guessing.
What we do on a call-out
We start with a full system check: filters, coil condition, airflow at both units, electrical readings, and refrigerant pressures using gauges connected to the service ports. That tells us fairly quickly whether we're looking at a mechanical fault, an electrical fault, or a leak, and we'll explain what we find before doing any further work.
Keeping it from happening again
- Wash filters every four to six weeks during heavy summer use
- Keep at least 300mm of clear space around the outdoor unit
- Book an annual service so a refrigerant top-up or a struggling fan motor gets caught before it fails outright
FAQs
Common questions
Why is my air con blowing air but it's not cold?
This usually points to a refrigerant issue, a failing compressor, or an outdoor unit that isn't rejecting heat properly. It's rarely a filter problem if air is flowing but simply not cold.
Can dirty filters really stop a unit cooling?
Yes. Restricted airflow across the indoor coil is one of the most common reasons a system underperforms, and it's the cheapest fix available.
Is it safe to top up refrigerant myself?
No. Handling F-Gas refrigerants requires certification in the UK, and cylinders and gauges aren't sold to the public for this reason.
How quickly can you attend if my AC has stopped cooling?
We prioritise loss-of-cooling calls, particularly for commercial sites, and can usually offer a same-week appointment, with emergency cover available outside that.
Keep reading
More from our knowledge base
Fault & symptom guides
Air Conditioning Smells Musty
A sour, damp smell when the AC kicks in is almost always biological growth on the indoor coil, not a fault with the refrigeration circuit itself.
Fault & symptom guides
Air Conditioning Not Turning On
A unit that shows absolutely no signs of life is usually a power or control fault rather than anything wrong with the refrigeration side.
Fault & symptom guides
What Air Conditioning Error Codes Mean
Flashing lights and displayed error codes are the system telling you roughly where the fault sits, well before an engineer even connects test equipment.
Talk it through with an engineer.
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