Window and Portable AC Smells: The Water Inside

By Ray Bennett · Updated 2 October 2026 · 6 min read · General guidance, always check your own manual

Window and Portable AC Smells: The Water Inside

Short answer

These smell more than a big system for one reason: a central air conditioner drains its water away down a pipe, and a window or portable unit keeps it in a pan in the bottom on purpose. Warm dark box, standing water, dust blowing through it. The cure is cleaning the pan and the coil, and then a spell on fan only after every use.

A window or portable air conditioner smells worse than a big system, and it is not because it is a worse machine. It is because of where the water goes.

A central air conditioner sends its condensate down a pipe and out of the house. These keep it, deliberately, in a tray in the bottom. So there is standing water inside a warm dark box with dust blowing over it every time you switch it on.

Before you start

What to carry: washing up liquid, white vinegar, a soft brush, an old towel and a screwdriver. Twenty minutes for the filter and the tray, an afternoon to do it properly.

Gas or electric? Electric, and it unplugs, which is the one real advantage these have over a furnace or a split system. Unplug it before anything else and there is nothing live left to worry about.

Smell it while it runs and again a minute after it stops. A smell that only arrives when the fan starts is coming off something inside. One that hangs around the room all day is probably not the machine at all.

One smell to stop for, and one thing never to pour in. Fish, or hot plastic. Those two are the same smell to most noses and it is electrical insulation getting hot. Unplug it and do not use it again until somebody has looked inside.

Never pour bleach into the pan. It attacks the plastic and the aluminium fins, and these machines are built to pick the pan water up and fling it onto the hot coil, so whatever you put in there gets atomised and blown out of the back. White vinegar does the job without any of that.

Why the water is in there at all

The pan is not a fault and it is not overflowing. The machine collects that water on purpose and throws it at the hot coil to help itself cool.

People find the water and assume something is blocked. Nothing is. A ring on the outdoor fan dips into the tray, picks the water up and slings it across the hot coil, where it evaporates and leaves with the exhaust air.

That is why these units have no drain pipe, and it is also the whole reason they smell. The tray never fully dries while the unit is in use.

What each smell is telling you

Check 1

Name it before you take anything apart

Musty, mildew, dirty socks. Growth on the cold coil and in the tray. It is usually worst on the first run of the season after the unit has sat damp for months.

Drains, or sewage. Stagnant water in the tray rather than anything to do with plumbing, because these have no plumbing drain and no trap. On a portable it is often the drain hose sitting in an old bucket of water.

Ammonia, or urine. Two causes and they are very different. One is a particular kind of mould. The other is an animal, because a window unit left in the window all winter is a dry sheltered box with an open grille facing the garden, and mice do move in.

Cigarettes or cooking. Absorbed into the filter and the foam lining, and it comes back every time the fan starts because you are blowing air through it.

Sweet or chemical. Usually warm plastic as things heat up, or the residue of whatever was sprayed in there last time. Refrigerant itself has very little smell.

Cleaning it out, in order

Check 2

Unplug, empty, wash, and dry it completely

Get it somewhere you can work. Unplug it. A window unit is much easier out of the window, and it wants two people because the outside half is the heavy end. A portable only needs the hose off.

The filter. Out and washed in warm soapy water. If it is a foam one that has gone brittle, replace it rather than fight it.

Empty the tray. Tip a window unit gently, outside half down, over a bucket. A portable has a drain plug at the bottom, and there is always more in there than you expect.

Wash the tray. Warm water and washing up liquid first, then a rinse with white vinegar and water. The slime on the bottom is the smell, so it has to come off rather than be rinsed over.

The cold coil. Soft brush and soapy water, or a proper coil cleaner, always working along the fins rather than across them so you do not fold them flat.

The foam lining. This is where an old unit keeps its smell, and it is the part people miss. Wipe it down and be honest with yourself: foam that has been soaked for years may never come fully clean.

Dry it completely. A day standing in moving air, or a fan pointed into it. Putting a damp unit back is starting the clock again.

Keeping it from coming back

This is the whole answer, and almost nobody does it. Cooling leaves the coil soaked, and fan only dries it before anything can grow.

Check 3

Fan only, every time

Fan only, until the coil is dry, after every cooling session. The coil comes out of a cooling cycle dripping wet. Running the fan on its own dries it before anything can take hold. Many units have an auto dry or self clean setting that does exactly this if you switch it on.

Wash the filter monthly. Two minutes, and it keeps the dust off the coil where you cannot reach it.

Take it out at the end of the season. Dry it, and store it inside. A unit left in the window over winter takes in damp, leaves and the occasional mouse.

If it has to stay in the window, cover the outside half. A proper cover, not a bin bag, because a sealed bag traps moisture rather than keeping it out.

Do not put anything scented inside it. A block or a spray covers the smell for a week and leaves the water exactly where it was.

Questions people ask

It only smells for the first minute and then stops.

That is the air pushing whatever has settled out of the coil and the lining. It is the mildest version of the musty one, and drying the coil after each use clears it.

Is a smelly unit actually bad for you?

For most people it is unpleasant rather than dangerous, but if somebody in the house has asthma or an allergy it is worth taking seriously, because you are blowing spores round a room on purpose.

Do the ozone or UV gadgets sold for this work?

They do not remove what is already growing in the tray, and ozone is not something to run in a room people are sitting in. Clean the tray and dry the coil instead.

It smells much worse on humid days.

More water is being pulled out of the air, so more of it is sitting in the tray. It is a sign of how much moisture is passing through rather than of a new fault.

Mine smells and it also cools badly.

Then the coil is probably blocked as well as dirty, which is one job rather than two: what to check on a window unit or on a portable.

I cleaned it and the smell was back within a week.

Then the coil is being left wet after every use. Start the fan only habit above, and if it still returns, the foam lining is holding it and that is a harder job.