Air conditioner problems: leaking, freezing or not cooling

Updated July 2026 · applies to portable and split systems

Most AC problems trace back to a short list of causes, and the good news is that the most common ones — dirty filters, blocked drains, poor airflow — are things you can fix yourself in ten minutes. The important UK-specific caveat up front: anything involving refrigerant is legally off-limits for DIY. If an American YouTube video is telling you to "top up the freon yourself", ignore it — in the UK, handling refrigerant requires an F-Gas certified engineer, DIY regassing is illegal as well as dangerous, and low refrigerant means a leak that needs finding and fixing, not just topping up. Here's how to tell which camp your problem falls into.

Water leaking from the indoor unit

The most common complaint, and usually the most benign. Air conditioning dehumidifies as it cools; that water should run away through a condensate drain. When it drips into the room instead, the causes in order of likelihood: a blocked or kinked condensate drain (algae and dust build up in the line — clearing it is a routine engineer job, or careful DIY with a wet/dry vacuum on the drain outlet), a filthy filter causing the coil to over-chill and produce more condensate than the drain handles, or — if the leak appeared after ice — see the freezing section below. Portables: an internal tank may simply be full, or the unit isn't level. When it's persistent despite clean filters and a clear drain, book a service.

Ice forming on the unit or pipes

Ice on an air conditioner means the evaporator coil is getting colder than it should, and there are only two real reasons. The one you can fix: restricted airflow — clogged filters, blocked vents, furniture against the unit, or running the system hard with doors open. Switch to fan-only mode to thaw it (never chip ice off), clean the filters, restore airflow, and try again. The one you can't: low refrigerant from a leak, which drops the coil pressure and temperature. If a clean, well-ventilated system re-freezes, stop running it (compressors are expensive casualties of persistent icing) and call an F-Gas engineer to leak-test — not to "regas and go", which just delays the same failure.

Running but not cooling (or blowing warm)

Work through the cheap causes first: mode set to fan or dry rather than cool; thermostat set above room temperature; dirty filters (the single most common cause of weak cooling — clean them and you'll often be amazed); a portable's exhaust hose kinked, over-extended or badly sealed (see the window kit guide — an unsealed window can halve effective cooling); the outdoor unit's coil caked in dirt, leaves or fluff (gently rinse the fins with a hose — power off first); or simply an undersized unit fighting a heatwave (check what your room actually needs). If none of those apply and cooling has faded gradually over weeks, refrigerant loss is back on the table — engineer time.

Smells and noises, decoded quickly

Musty smell: mould or bacteria on a damp coil or in the drain — clean the filters, run fan-only mode for an hour after cooling sessions to dry the coil, and if it persists, a professional coil clean sorts it. Burning smell: switch off and investigate — dust burning off after a long idle period is common and brief; anything electrical-smelling that persists means stop using it. Gurgling: usually the condensate drain or refrigerant flow — harmless if cooling is normal. Grinding, screeching or new rattles: mechanical — bearings, fan or compressor — book a service before it becomes a replacement.

The ten-minute maintenance that prevents nearly all of this

• Clean the filters every 2–4 weeks in heavy use. Slide out, rinse under a tap, dry fully, replace. This one habit prevents weak cooling, icing, leaks and smells — it's the whole game.
• Keep the outdoor unit clear — 30cm+ of breathing space, leaves off the coil, an occasional gentle rinse of the fins.
• Run fan-only for a while after cooling in muggy weather to dry the coil and keep mould away.
• Book an annual service (£70–£120) for a split system — it keeps efficiency at spec, usually keeps the manufacturer warranty valid, and catches refrigerant leaks while they're small. What a service and a proper install look like is covered in our installation guide.
• No, you don't need to cover the outdoor unit in winter — it's built for weather, and covers trap moisture and invite corrosion and nesting. If yours is a reversible unit, it'll be working all winter anyway (the heating side explained).

When repair stops making sense

A well-maintained split lasts 10–15 years, portables 5–10. When a unit past that age needs a compressor or a major refrigerant repair, the maths usually favours replacement — modern inverter units also run meaningfully cheaper (running costs here). For a fitted replacement, benchmark the price and get quotes from F-Gas certified installers.

Quick answers