Maintenance
Why does my air conditioner smell?
The kind of smell coming out of the vents narrows the cause down quickly. Most of them trace back to moisture sitting where it should have drained away. One of them is not an air conditioning problem at all and needs you out of the house.
6 min read
The short answer
What each smell means
Work out which one you have first. The cause and the urgency differ for each.
| The smell | Usual cause | What to do |
|---|---|---|
| Musty, damp, like a cupboard | Mould or bacteria on the indoor coil or in the drain tray. The most common of the lot. | Clean the filters. If the smell survives that, the coil needs a proper clean. |
| Sour, like a gym bag | Dirty sock syndrome: bacteria growing on a coil that keeps getting wet then warm. Common on reverse cycle systems. | A coil clean. Filters will not touch this one. |
| Vinegar or slightly sharp | Usually the same mould or bacteria as a musty smell. If it smells sharp and electrical instead of sour, treat it like the hot plastic row. | Filters first, then have someone check the coil and the drain. |
| Burning dust on first use of the season | Dust that settled inside the unit over months idle, warming up. Normal. | Run it for an hour. If it has not cleared, get it checked. |
| Hot plastic or electrical | Overheating wiring, motor or capacitor. | Switch it off at the isolator now and call someone. Do not keep running it. |
| Sweet or chemical | Possibly refrigerant. Modern refrigerants only have a faint, slightly sweet smell, so you would rarely notice one. A sealed system should not be losing any. | Turn it off, open a window and call a licensed technician. |
| Rotten eggs or sulphur | Natural gas. This is not your air conditioner. | Leave the house, do not touch switches and ring your gas supplier or emergency services. |
| Sewage or drains | A dry trap on a nearby waste line or a condensate drain tied into plumbing without a proper trap. | A job for a plumber, but mention it to whoever services the system. |
Why damp inside the unit is the usual culprit
Every time an air conditioner cools, water condenses on the indoor coil and runs down into a tray. On a muggy Illawarra afternoon that is a lot of water. It should leave through a drain line and end up outside. When that drain blocks, the unit ends up leaking water into the room.
A smell is often the early stage of the same fault. Before a drain blocks completely it drains slowly, which leaves water standing in the tray between cycles. Warm, dark, wet and fed by a steady supply of dust, that tray suits mould and bacteria well. Once growth takes hold on the coil fins it will not go away by itself.
Smells and leaks often turn up in the same systems for that reason, with the same drain line behind both. Running the fan by itself for a while after cooling helps a lot, because a coil that dries out between uses grows very little.
What to try before calling anyone
These clear a good share of musty smells and cost nothing but an afternoon.
- 1
Wash the filters and dry them completely
A filter that goes back in damp will start smelling all by itself, so let them dry flat and out of the sun for a couple of hours first.
- 2
Check the drain is running
Find the drain line outside and watch it on a muggy afternoon with the system cooling. Water should come out of it. A dry pipe outside and a musty smell inside point straight at the drain.
- 3
Wipe what you can reach
With the power off and the front panel open, wipe the louvres and the visible plastic housing with a damp cloth. Keep the cloth out of the coil fins, which bend easily and block airflow once bent.
- 4
Run the fan by itself for half an hour
That dries out the coil. Open a window so you push the smell outside instead of around the room.
- 5
Make drying it a habit
Leave the fan running by itself for a quarter of an hour at the end of the day. Many systems have a dry or self-clean mode that does it for you, which is worth finding in the manual. Of everything on this list, it does the most to stop the smell coming back.
- 6
Give it a few days
If the smell fades and stays gone, it was the filters. If it comes back as soon as the system runs properly again, it is on the coil and you need someone with the chemicals and the tools to strip the unit down.
Please do not use an air freshener on this
What a proper coil clean involves
Once growth has taken hold on the fins, the unit has to come apart. We take the covers off, treat the coil with a purpose-made cleaner and rinse it, clean the drain tray and the blower wheel separately and flush the drain line so the water leaves. The blower wheel is the part people most often skip and it holds a remarkable amount of grime.
Allow a couple of hours per indoor unit. You notice the difference straight away in the smell and usually in the airflow too, because growth coating a coil also chokes the air moving through it.
A ducted system takes longer because the ductwork and the return can hold growth as well as the unit. Our air conditioning service and repair page sets out what a coil clean includes. For a call-out on the coast, book air conditioning repair in Wollongong.
If the smell arrived along with an air conditioner not blowing cold air, read that guide as well. A coil dirty enough to smell is often dirty enough to cost you capacity.
Common questions
- Why does my air conditioner smell musty when I turn it on?
- Moisture has sat on the indoor coil or in the drain tray since the system last ran and mould or bacteria have grown in it. The smell is strongest in the first few minutes because the fan pushes the stale air across the growth as it starts. Clean the filters, then check the drain is clearing.
- What is dirty sock syndrome in air conditioning?
- It is the trade name for a sour, gym-bag smell that shows up mostly on reverse cycle systems when they switch between heating and cooling. Bacteria build up on a coil that keeps getting wet then warm. The smell releases when the coil temperature changes. It needs a coil clean because the growth sits on the fins.
- Can a smelly air conditioner make you sick?
- A musty system circulates mould spores and bacteria through the room. For anyone with asthma, allergies or a weak immune system that is worth taking seriously. For most healthy people the effect is mild, a stuffy nose or a scratchy throat. Fix it at the source because an air freshener only covers it up.
- Why does my air conditioner smell like burning?
- On the first run of the season it is usually dust that settled inside the unit while it sat idle. That clears within an hour. A burning smell that arrives mid-season, smells like hot plastic or comes with the breaker tripping points at an electrical fault. Cut the power and get it inspected before you run it again.