Cleaning the filters is the single most useful thing you can do for your air conditioning, and it takes about fifteen minutes. A clogged filter makes the unit work harder to move the same air, which means higher bills, weaker cooling and a shorter life for the machine. A fair share of "my AC has stopped cooling" callouts turn out to be nothing more than a filter thick with dust — an expensive way to learn a fifteen-minute lesson. Here's what you can safely do yourself, and the clear line beyond which you shouldn't go.
How often to clean the filters
For most homes, every four to eight weeks while the system is in regular use, plus a proper clean at the start and end of the season. If you have pets, building work going on, or the unit runs most days, lean towards the more frequent end. The mesh filters in a wall-mounted split are designed to be taken out and washed — this isn't off-label tinkering, it's what the manual tells you to do.
How to clean them
- Switch the unit off with the remote, and ideally at the isolator or plug too.
- Lift the front panel of the indoor unit — on most wall-mounted splits it hinges up from the bottom and stays put on its own.
- Slide the mesh filters out. They usually pull down and forward with no tools.
- Vacuum them first with a brush attachment to lift the loose dust.
- Wash in lukewarm water — a soft brush and a drop of washing-up liquid if they're greasy, which is common near kitchens. Avoid hot water, which can warp the mesh.
- Let them air dry completely, in the shade rather than direct sun, and don't be tempted by a radiator or hairdryer. This is the step people skip: a filter refitted damp is an invitation for mould.
- Slide them back in, clip the panel down, switch back on.
Some units also carry small air-purifying inserts behind the main filters. Check your manual — some of these are washable, others are replace-only and washing ruins them.
Keep the outdoor unit breathing
The outdoor unit needs clear air around it to dump heat. Keep roughly half a metre of space free on all sides: clear away leaves, grass clippings, bin bags and anything a climbing plant has ambitions over. After autumn leaf fall it's worth a quick look even if the system is idle. You can gently rinse the metal fins with a watering can or a soft hose to shift dust and cobwebs — never a pressure washer, as the fins bend very easily and bent fins choke the airflow you're trying to protect. And don't stack anything on top of it.
Check the condensate drain
When it's cooling, your system pulls moisture out of the air, and that water leaves through a small drain pipe — usually poking out of an external wall. Dripping from that pipe on a hot day is normal and healthy. What's not normal is water dripping from the indoor unit itself, which usually means the drain is blocked. Have a look at the outside end of the pipe: gunk, algae and the occasional insect nest are the usual suspects, and clearing a visible blockage at the pipe's end is fine to do yourself. If it keeps leaking after that, the blockage is further up and it's a job for a repair visit rather than a coat hanger.
What the smells are telling you
| Smell | Likely cause | What to do |
|---|---|---|
| Musty or damp | Mould or mildew on the filters or the coil | Wash the filters; if it persists, book a professional coil clean |
| Sour or vinegary | Bacteria growing on damp surfaces inside | Same again — filters first, deep clean if it lingers |
| Fishy, or burning plastic | An electrical component overheating | Switch off at the isolator and don't run it until an engineer has looked |
| Faintly sweet or chemical | Possibly a refrigerant leak | Switch off and call an F-Gas certified engineer — this one is never DIY |
Where DIY stops — firmly
Everything above is done from the outside of the machine, and that's the boundary. Never open the sealed casings, indoors or out: air conditioning units contain capacitors that hold a stored charge even after the power is off, and a shock from one can be serious. And never touch the refrigerant pipework. It's a sealed circuit under pressure, and in the UK you must legally hold F-Gas certification to handle refrigerant — that's not our house rule, it's the law. It also covers "topping up the gas", so give any online regas kit a wide berth. If the system still isn't cooling after a filter clean, the likely culprits are low refrigerant or a component fault — our troubleshooting guide walks through how to tell, and none of the fixes at that point are homeowner jobs.
Is an annual service still worth it?
For most people, yes. An engineer will check refrigerant pressure, deep-clean the coil, test the electrical connections and make sure the condensate system is clear — the things you can't safely reach. Many manufacturers also make regular servicing a condition of the warranty, so it's worth checking yours before deciding to skip it. Think of the jobs in this guide as what keeps the system efficient between services: clean filters and a clear outdoor unit are also the cheapest way to keep your running costs down.
When you do need a professional
If something in this guide has pointed you towards an engineer — a stubborn leak, a worrying smell, cooling that won't come back — Coolhouse can match you with a vetted, F-Gas certified local engineer rather than leaving you a pile of quotes to sift. But honestly, most weeks, the best maintenance your system will get is you, a soft brush and a sink of lukewarm water.