The short answer
Most weak-cooling problems come down to airflow, refrigerant or settings. The usual culprits are a dirty filter or blocked coil, a low refrigerant charge (often a leak), a fouled outdoor unit, or simply the wrong mode or temperature setting. You can clean filters and clear obstructions yourself. Anything involving refrigerant must be left to an F-Gas-certified engineer — topping up refrigerant yourself is illegal and rarely fixes the real cause.
When an air-conditioner hums away but the room stays warm, the cause is usually one of a short list of problems — most of them about airflow or refrigerant. This guide walks through the likely causes in order, tells you which you can safely tackle yourself and which need a certified engineer, and explains why a low charge almost always means a leak rather than a top-up.
Weak cooling at a glance
- Most common cause Dirty filter restricting airflow
- You can fix Filters, settings, obstructions
- Needs an engineer Refrigerant, electrics, compressor
- Low refrigerant means Usually a leak, not normal loss
- Re-gas cost £100–£300
- Legal note Only certified engineers touch refrigerant
First, the easy checks
Before assuming the worst, rule out the simple things — they cause most callouts. Confirm the unit is actually set to cooling mode (not fan or heat), that the target temperature is below the room temperature, and that any timer or eco setting is not throttling it back. A surprising number of “not cooling” complaints turn out to be a remote left on fan-only or a setpoint above room temperature. Make sure doors and windows are closed and curtains drawn against direct sun, because an air-con unit cannot win against an open window and a south-facing glare. Check the indoor filter: if it is grey with dust, that alone can halve the cooling. Then check the outdoor unit is not blocked by leaves, furniture or overgrown plants — it has to dump heat outside to cool inside, and a smothered condenser is a common cause of weak performance on hot days.
Airflow problems you can fix
Restricted airflow is the number-one reason a system stops cooling well, and most of it is within your control.
- Dirty filters — remove and clean or replace them; this is the single most effective fix.
- Blocked indoor coil — if the coil behind the filter is furred, it needs a professional clean.
- Obstructed outdoor unit — clear leaves, debris and anything within the airflow path.
- Closed vents or grilles — make sure the air path in and out of the unit is clear.
- Furniture or curtains — keep anything that blocks the indoor unit’s air throw well clear of it.
Of all these, the filter is by far the most common and most rewarding to address. A filter that has not been cleaned for a season can be furred enough to choke the airflow completely, so the unit runs hard, the air it does push out is barely cool, and in bad cases the coil ices over from lack of warm air passing across it. Cleaning it takes minutes and often restores full cooling on its own — always the first thing to try.
| Symptom | Likely cause | Who fixes it |
|---|---|---|
| Weak airflow, warm air | Dirty filter or coil | You (filter) / engineer (coil) |
| Cools a little, never enough | Low refrigerant / undersized unit | Engineer |
| Ice on indoor unit or pipes | Low charge or airflow fault | Engineer |
| Outdoor fan not running | Electrical or fan fault | Engineer |
| Runs constantly, room warm | Heat gain or wrong size | You (shading) / survey |
Low refrigerant almost always means a leak
If the system has plenty of airflow but still won’t cool, and especially if you see ice forming on the indoor unit or pipework, the refrigerant charge may be low. Crucially, a sealed refrigerant circuit does not “use up” gas in normal running — a low charge means there is a leak. Simply topping it up without finding and fixing the leak is a temporary patch that wastes money and vents greenhouse gas. A certified engineer will leak-test, repair, then recharge — see re-gas explained. This work is legally restricted to F-Gas-certified engineers.
When the unit is simply too small
If the air-con cools but never quite catches up on the hottest days, it may be undersized for the room, or the room may have more heat gain than expected from glazing, electronics or occupants. A unit that runs flat-out and still loses ground is a sizing or heat-load problem, not a fault — see what size air con do I need. Improving shading, closing blinds during the hottest part of the day, and reducing heat sources all help, but a genuinely undersized system will always struggle on peak days and needs rethinking rather than repairing. Equally, expecting a single unit to cool an open-plan space it was never sized for is a common mistake. For ongoing prevention, keep up with maintenance and an annual service, which keeps the system performing at the capacity it was designed for. This page is general guidance, not a diagnosis of your specific unit.
Air-con still not cooling after the easy checks?
Get matched with an F-Gas-certified engineer to leak-test, diagnose and put it right safely and legally. A quote costs nothing.
Frequently asked questions
Why is my air-con blowing warm air?
Usually a dirty filter, the wrong mode setting, a blocked outdoor unit, or a low refrigerant charge from a leak. Check the simple causes first, then call an engineer.
Can I top up the refrigerant myself?
No. Refrigerant work is legally restricted to F-Gas-certified engineers. A low charge means a leak, which must be found and fixed, not just topped up.
Why is there ice on my air-con?
Icing usually points to a low refrigerant charge or restricted airflow. Turn the unit off to let it defrost and have a certified engineer investigate the cause.
My air-con runs constantly but the room stays warm — why?
Often heat gain from sun and glazing, or an undersized unit. Improve shading first; if it still cannot keep up, the system may be too small for the room.
Sources & further reading
- HSE — ventilation and indoor environment guidance
- GOV.UK — F-gas: guidance for users, producers and traders
- Energy Saving Trust — keeping a home cool efficiently
- Manufacturer technical data (Daikin, Mitsubishi Electric) — fault guidance
This guide is general information, not a site-specific survey or a substitute for a quote from an F-Gas-certified installer. Installation, servicing and refrigerant handling are legally restricted to F-Gas-certified engineers.