Why Your Air Conditioner Smells, and What Each Smell Means

When an air conditioner smells, the type of smell usually tells you the cause. You switch the air conditioner on for the first time in months and the room fills with a smell. Damp, or sour, or something worse. It fades after a few minutes, so most people open a window and forget about it until next time.

That is usually the wrong call. An air conditioner that smells is telling you something specific, and the smell itself narrows it down. A few of them are nothing. One or two mean switch it off now. This page sorts them out.

What each air conditioner smell usually means

Start here, then read the section that matches.

The musty smell, and why it happens to almost everyone

This is the one most people have. It arrives on the first hot day, it is worst in the first few minutes, and it gets a little stronger each season.

The mechanism is simple. When an air conditioner cools, moisture condenses on the indoor coil, the same way a cold glass sweats. That water is meant to run into a drain pan and out through a pipe. In the meantime you have a dark box with wet metal in it, sitting in a warm room, with dust blowing through.

That is an ideal place for growth. Over a season or two a film builds on the coil and in the drain, and every time you switch on, air passes over it and into the room.

Two things make it worse. A dirty filter lets more dust reach the coil, and a partly blocked drain leaves standing water sitting in the pan between uses.

Why does my air conditioner smell like fish?

This one is different from the others on the list and it deserves its own section.

A fishy or urine like smell coming from an electrical appliance is commonly associated with overheating electrical components. Some plastics and wire insulation give off that particular smell as they get too hot, before there is any smoke.

If your air conditioner smells fishy, the sensible response is to switch it off at the remote, then switch it off at the isolator or the breaker, and leave it off until somebody has looked at it.

It may turn out to be nothing. It may also be the only warning you get. The cost of switching it off for a day is nothing next to the alternative, and no technician will think you overreacted.

Why does my air conditioner smell like vinegar?

Sour and vinegary usually belongs to the same family as musty. It tends to point at moisture that has been sitting somewhere for a while rather than moving through.

The usual suspects are the drain pan and the drain line. When the line partly blocks, water backs up and sits. A clean of the drain and the coil normally deals with it.

There is a second possibility worth mentioning. Some filter materials and some adhesives give off an acidic smell when they get damp and stay damp. If the unit was recently serviced and the smell is new, mention that when you call.

Why does my air conditioner smell like urine or ammonia?

Two very different causes share this smell, and they need different responses.

The first is mould growth, which can produce an ammonia like smell as it develops. That sits in the same category as musty and sour, just further along, and it is dealt with by a proper clean of the coil, the drain and the filters.

The second is an animal. Possums, rats and birds get into roof spaces and duct runs, and a nest or a dead animal in or near the return air path produces exactly this smell, far stronger than anything a coil can manage.

The way to tell them apart is usually strength and pattern. Mould smells worse when the unit runs and better when it stops. An animal smells bad whether the unit is running or not, and gets worse over days rather than better.

If you suspect an animal, do not keep running the system. You are distributing it through the house.

Burning and hot dust smells

A brief hot dust smell the first time you use heating for the season is common and usually harmless. Dust settles on warm components over months of disuse and burns off in the first few minutes.

The test is time. If it clears within a few minutes and does not come back the next time you switch on, that was dust. If it persists, gets stronger, or returns every single time, switch the unit off at the breaker and have it looked at.

An acrid or plastic burning smell is never in the harmless category. Treat that the same way as fish.

Rotten eggs, and the one that is not about the air conditioner

Natural gas has no smell of its own. A sulphur or rotten egg odour is added to it deliberately so that leaks can be detected.

If you smell rotten eggs in a property with gas, the air conditioner is not the first thing to investigate. Leave the building, avoid switching anything on or off on your way out, and call the gas emergency line for your provider from outside.

It is on this page only because people notice it when the air conditioner starts moving air around the house, and assume the air conditioner caused it. It did not. It only distributed it.

What you can do yourself

For the musty and sour family, several of these help and none of them cost anything.

1. Clean the filters. This is the single highest value thing on the list and it is designed to be done by the owner. Take them out, wash them, let them dry fully before refitting.
2. Use the dry or fan function after cooling. Running the fan alone for a few minutes after you finish cooling dries the coil instead of leaving it wet.
3. Use the anti mould function if your unit has one. Many manufacturers build this in and almost nobody uses it. See the section below.
4. Check the drain outlet outside. If nothing drips from the drain pipe on a humid day while the unit is cooling hard, the drain may be blocked.
5. Air the room. Open a window for the first few minutes of the season rather than breathing the first burst.
6. Do not spray anything into the unit. Aerosol deodorisers and household cleaners sprayed at a coil make the film worse and can damage the fins.

The button on your remote that prevents this

Most brands build in a function that dries the indoor unit after cooling, and it is one of the least used buttons on any remote. The names differ.

• Gree calls it X FAN, shown as BLOW on some remotes. Pressed in cooling or dry mode, the indoor fan keeps running for 10 minutes after you switch the unit off, to dry the indoor unit.
• Hitachi has FrostWash, a cleaning cycle that freezes and then defrosts the indoor heat exchanger to lift dust and dirt off it.
• Daikin documents a cleaning indicator and, on some models, a dust box that needs emptying when its lamp flashes.

If your remote has any of these, use it every time you finish cooling. A coil that dries out after each use does not build the film that causes the smell in the first place.

Our guides to air conditioner symbols cover what each of these buttons looks like on your particular brand.

Why a clean is not the same as a service

Wiping the front panel and washing the filters is maintenance you do. It is worth doing and it solves a fair share of mild smells.

What it does not reach is the coil itself, the drain pan, the drain line and the fan barrel. Those sit behind the casing, they are where the film actually lives, and reaching them properly means opening the unit up.

A smell that survives a filter clean is a smell that lives somewhere you cannot get to. That is the point at which a service earns its money, and it is worth knowing how often you should service your air conditioner so the film never gets the chance to build.

Why is it always worse at the start of the season?

Because the unit has been sitting still for months with whatever was on the coil when you last switched it off.

Through autumn and winter a split system that is only used for cooling does nothing at all. The coil stays damp for a while after the last use, then dries, and anything growing on it is left undisturbed with no airflow to carry it away. Dust keeps settling through the grille the whole time.

Then on the first hot day you switch it on and months of that arrives in the room in one go. The same unit used steadily through summer smells far less, because it never gets the still, undisturbed months.

The practical version of this is that the best time to have a system cleaned is before you need it, not on the day it first smells.

Ducted systems smell differently

Everything above applies to a ducted system too, with one addition. A ducted system has metres of flexible duct in a roof space, and that duct is part of the air path.

Three things change as a result.

• The smell can come from the ducts rather than the unit. Damaged duct, or duct with a gap at a joint, draws in roof space air, and a roof space smells like a roof space.
• The animal problem is much more likely. Roof spaces are where possums and rats live, and the return air path runs straight through their territory.
• A filter clean helps less. On a split system the filter sits right in front of the coil. On a ducted system the return air filter is often the only thing you can reach, and the smell may be well past it.

If a ducted system smells and the return air filter is clean, the next step is someone in the roof space with a light, not another filter wash.

Which smells mean stop using it now?

Most of this page describes something to book in. These do not wait.

• Fishy. Switch off at the breaker. Commonly associated with overheating electrical components.
• Burning plastic or acrid. Same response.
• Rotten eggs. Leave the building and call the gas emergency line from outside.
• Any smell with visible smoke or scorching anywhere on the indoor or outdoor unit.
• A smell that arrives with the breaker tripping. Do not keep resetting it.

Everything else on this page is a cleaning problem wearing a frightening disguise.

Getting the smell out for good

An air conditioner that smells is almost always an air conditioner with a wet, dirty coil and a drain that is not clearing properly. It is common, it is fixable, and it comes back every season if the only thing that ever gets cleaned is the filter.

Start with the filters and the dry function on your remote. If the smell survives both, it is living behind the casing where you cannot reach it, and that is the call worth making rather than a problem to keep airing out.

We send a licensed technician who works on split, multi head and ducted systems across Melbourne, and the price is confirmed before any work starts.

A smell is only one of the signs an air conditioner needs repair, and the others are worth knowing before the first hot week.

A smell that returns after a clean usually needs the indoor coil opened up properly. Our air conditioner service in Melbourne includes that, and you can request a quote before booking anything in.