Air con
Why is my air conditioning not cooling (or blowing warm air)?
3 min read · Reviewed for accuracy before publishing
Nine times out of ten it's one of a few things, and the first ones you can check yourself. Confirm the unit is in cool mode (not heat or fan-only) with the set-point actually below room temperature, then check the filters — a clogged filter alone can stop a unit cooling. If it's still blowing warm after that, it's most often lost refrigerant through a slow leak, or a failing compressor or fan — none of which are a DIY or a top-up-bottle job. Refrigerant work is legally restricted to F-Gas certified engineers, so that's where it goes next.
The checks you can do safely
Set-point below the room temperature, cool mode selected, and — the big one — clean filters. Filters clog with dust and quietly choke airflow until the unit can't cool; sliding them out and cleaning them is a homeowner job and fixes a surprising number of 'not cooling' calls. Also check the outdoor unit isn't blocked by leaves or a bin.
When it needs an engineer
Still warm with clean filters and the right mode? That usually means the refrigerant charge has dropped through a slow leak, or the compressor or fan is failing. Refrigerant is a controlled substance — by law only an F-Gas certified engineer can trace the leak, fix it and recharge. Anyone offering a quick 'gas top-up' without finding the leak is doing it wrong (and illegally).
Why we trace, not just top up
Refrigerant doesn't get 'used up' — if it's low, it's leaked, and simply refilling means it leaks straight back out (and it's an environmental offence to knowingly do that). We find and fix the leak first, then recharge to spec, so it stays fixed rather than becoming an annual bill.
Datum's engineers install and service boilers, air conditioning, heat pumps and plumbing across South Hertfordshire and North London. Guides are written from real jobs, not brochures.