Air con
Why is my air conditioning not cooling (or blowing warm air)?
3 min read · Last updated · Reviewed for accuracy before publishing
Start with the checks you can do safely. Daikin's checklist for a unit that won't cool is: air filters not dirty, air inlets and outlets of both units not blocked, a sensible temperature setting, doors and windows closed and airflow settings appropriate. Confirm the unit is in cool mode, not heat or fan-only. If it's still blowing warm, it needs an engineer: GOV.UK says working on air-conditioning refrigerant needs F-gas qualifications, so it isn't a DIY or top-up-bottle job.
The checks you can do safely
Set-point appropriate and below the room temperature (Daikin's manual suggests operating within 26–28°C for cooling to save energy), cool mode selected, doors and windows closed, and — the big one — clean filters. Daikin's manual says operating with dirty filters gives poor heating and cooling, and it recommends cleaning them every two weeks. Also check the outdoor unit isn't blocked by leaves or a bin; Daikin lists blocked inlets and outlets on both units.
When it needs an engineer
Still warm with clean filters and the right mode? Daikin's FAQ then points you to an installer. A refrigerant leak is one possible cause, and Daikin's manual says the refrigerant normally does not leak, so a low charge is a fault to find.
GOV.UK says you must have qualifications to service and maintain air-conditioning systems and to check them for leaks, and that working with F gas without them is against the law.
Why we trace, not just top up
Daikin's manual says the refrigerant normally does not leak and must not be vented to the atmosphere, so a low charge means something has gone wrong. Simply refilling means the cause is still there. We find and fix the problem first, then recharge to spec, so it stays fixed rather than becoming an annual bill.
“If the charge is low, something has let it out — Daikin's own manual says the refrigerant normally does not leak. Anyone selling you a top-up without a leak trace is selling you the same gas again next summer.”
Sources
- Daikin UK — The air conditioner does not cool or warm the room (check the air filters are not dirty, the indoor and outdoor inlets and outlets are not blocked, the temperature setting is appropriate, doors and windows are closed, airflow settings are appropriate; then contact an installer; accessed 19 Oct 2026)
- Daikin UK — FTXM-N / CTXM-N operation manual (the refrigerant "normally does NOT leak" and must not be vented to the atmosphere; do not repair the unit yourself; operating with dirty filters gives poor heating and cooling; cleaning the air filters every 2 weeks is recommended; maintenance at least once a year is recommended; accessed 19 Oct 2026)
- GOV.UK — Qualifications required to work on equipment containing F gas (for air conditioning and heat pump systems you must have qualifications to install, service and maintain, and check for leaks; "It is against the law to work with F gas if you do not have the correct qualifications"; accessed 19 Oct 2026)
Datum's engineers install and service boilers, air conditioning, heat pumps and plumbing across South Hertfordshire and London. Guides are written from real jobs, not brochures.