Why does my air conditioner smell?

An air conditioner dehumidifies as a side effect of cooling: room air passes over a coil cold enough to condense water out of it, which is why there is a drain at all. The result is a surface that spends every cooling hour wet, cold and in the dark, with a fan pushing household dust across it continuously.

That is an ideal substrate for mold, mildew and bacteria, and the film they form is what you smell. Three details make it worse:

• Water that sits. A pan that does not drain fully, or a unit installed without the slight tilt to the outside, keeps a reservoir going all season — the drainage design is in condensate and drainage.
• A loaded filter. Dust that gets past a clogged or missing filter lands on the wet coil and feeds the growth.
• Short cycles. An oversized unit that runs in brief bursts leaves the coil wet without ever drying it, which is one more argument against buying more capacity than the room needs.

Unplug the unit first. Each step is cheaper and easier than the one after it, so work down the list and stop when the smell goes.

1. The filter. Wash it, dry it completely, refit it. A filter put back damp is itself a source. Do this monthly in season.
2. The evaporator coil — the indoor one behind the filter. A soft brush and a no-rinse coil cleaner, following the fin direction so you do not bend them. The full method is in how to clean a window air conditioner.
3. The drain pan and drain path. Clear the channel and the weep hole. This is where the water that fed everything collects, and it is the step people skip.
4. The blower wheel, if it is reachable. Dust cakes onto the blades and holds moisture; it is also why a unit gets quieter after a clean.
5. The window seal. Gaps around a unit let humid outdoor air in to condense on cold surfaces, which restarts the cycle.

One habit worth adopting afterwards: run the unit on fan only for ten minutes before switching it off for the day. That dries the coil instead of leaving it wet overnight, and it is the single most effective prevention there is. Several units do it automatically; the modes are explained in timers and eco modes.

Before deep-cleaning anything, run the test in the first pick above: switch the air conditioner off, shut the window, and come back in an hour. If the room still smells musty, the machine is circulating an existing problem rather than creating one.

Common room-side sources:

• Humidity above about 60%, which lets mold and mildew grow on soft furnishings and in closets regardless of the air conditioner.
• A damp basement below feeding the floor above — see basement dehumidifiers.
• A dry plumbing trap in a rarely used bathroom, which smells of sewer rather than mildew and is fixed by running the tap.
• Something in the chassis. Through-wall and window units occasionally admit a mouse or a bird, and that smell is unmistakable and not biological growth.

• Pouring bleach into the pan. It corrodes aluminum fins and the pan itself, and on a window unit the pan is the part you least want to lose.
• Fragrance cartridges and odor bombs. They mask the smell for a few days while the growth continues, and they put additional residue on a wet coil.
• Running it harder. A colder set point means a wetter coil for more hours. The smell gets stronger, not weaker.

1. Monthly in season: wash and fully dry the filter.
2. Once a year: clean both coils and clear the drain path.
3. Every shutdown: ten minutes of fan-only running.
4. Every autumn: drain it, dry it, and store it indoors or cover the outdoor half — storing a window air conditioner is where most chassis corrosion is prevented.