The short answer
Air conditioning filters catch dust and fibres before they reach the indoor unit's coil and fan, keeping air flowing freely and the inside of the unit cleaner. Most are washable mesh screens that need cleaning every few weeks in regular use.
On this page
Every indoor unit draws in air from the room, passes it over a cold or warm coil and blows it back out, and that air carries dust with it. The filter's main job is to protect the unit, because a dusty coil moves heat less well and a dusty fan moves less air. Filters are also the one part of the system you're expected to look after yourself.
How air passes through the filter
On a wall-mounted unit, air is drawn in through the top or front, passes through the filters and across the coil, and is then blown back into the room by the fan. The main filters sit just behind the front panel, often as a pair of mesh screens that slide or lift out.
Most split systems recirculate the air already in the room. The pipes between the indoor and outdoor units carry refrigerant, not air, so the system doesn't bring in fresh air from outside, and ventilation is a separate job. What the indoor and outdoor units do explains how the two halves work together.
Types of filter
| Filter | What it's designed to do | Usual care |
|---|---|---|
| Main mesh filter | Catch dust, hair and fluff | Vacuum or rinse every few weeks in use |
| Fine particle filter | Catch smaller particles than the mesh | Clean or replace as the manual says |
| Deodorising filter | Reduce some odours | Clean, refresh or replace as the manual says |
Every wall-mounted indoor unit has main filters. The other two types come with some models, often clipped onto the main filters, and their names and descriptions vary between manufacturers. What they physically do is catch particles as air passes through, and in the case of deodorising filters, hold on to some odours.
The main mesh is fairly coarse. It stops the dust you can see, while much finer particles mostly pass through it.
Ceiling cassettes, ducted systems, floor-standing units and portable air conditioners all have filters too, in different places. Some ducted systems use filters that are replaced rather than washed.
Why a clean filter matters
As a filter clogs, less air gets through it, and that has knock-on effects:
- Less cooling or heating. The unit moves less air, so the room takes longer to reach temperature.
- Harder work. The system runs for longer to do the same job, which uses more electricity.
- Ice in cooling mode. With too little air flowing over it, the coil can get cold enough to freeze. See ice on the pipes or outdoor unit.
- Dirt further in. Dust that gets past a damaged or badly fitted filter builds up on the coil and fan, where it's much harder to remove and can cause smells.
Looking after filters
The manual gives the exact method for your model, but it usually goes like this:
- Switch the unit off, and isolate the power too if the manual says so.
- Lift the front panel and slide or unclip the filters.
- Vacuum off the dust gently, or rinse the filters in lukewarm water with a little mild detergent if they're greasy.
- Let them dry completely, out of direct sun and away from heat, which can warp them.
- Refit them and close the panel.
Every few weeks is a typical interval while the system is in regular use, and more often in a dusty home, a kitchen or a house with pets. Many units show a reminder after a set number of running hours. Don't run the unit without its filters, even briefly, because dust goes straight onto the coil.
Extra filters follow their own schedule. Some can be refreshed and reused and others need replacing, and the manual says which. How to clean the filters goes through the whole job step by step.
What filters don't replace
Clean filters don't make a service unnecessary. Over time some dust still reaches the coil, the fan and the tray that collects condensate, and those need cleaning by an engineer with the right equipment. Manufacturers typically recommend regular servicing, often yearly, and some warranties depend on it.
The outdoor unit has no filter. Its coil stays clear if you keep leaves, plants and clutter away from it and don't cover it while it's in use.
Related guides
- How to clean the filtersSwitch the unit off, lift the front panel and take out the mesh filters, then vacuum or rinse them in lukewarm water and let them dry fully before refitting. In regular use, this usually needs doing every few weeks.
- What a service involvesA service is a clean and check of the whole system, indoors and out, plus tests of how it runs and a look for leaks and wear. Manufacturers typically recommend one regularly, often once a year.
- Musty or unusual smellsA musty smell usually comes from damp dirt on the coil, drain tray or fan, and a dusty smell when heating first starts usually fades. A burning or electrical smell means switching the system off straight away.
- Features worth havingThe features that make the most day-to-day difference are an inverter, heating that copes with cold weather, a genuinely quiet lowest fan speed, good airflow control, useful timers and easy-to-clean filters. Many extras are pleasant rather than essential.