Why Your Air Conditioner Smells Musty When It Starts Up

That hit of damp gym bag when the unit first kicks on is not the filters, and it is not the drain. It is mould growing on the barrel fan and the indoor coil, and you cannot reach it from the front of the unit.

What is actually growing in there

Behind the filters sits the indoor coil, and behind that is the barrel fan — a long cylinder with dozens of small blades that flings air out into the room. In Darwin, that coil runs cold and wet for most of the year while the air going over it sits at high humidity.

Cold, wet and dark, with a steady supply of dust and skin cells landing on it. That is close to ideal conditions for mould. The black speckling you sometimes see on the vanes is the part you can see. The barrel fan behind it is usually worse.

The smell arrives when the unit starts because the fan begins spinning before the coil has cooled down. It pushes the first lungful of that air straight out into the room.

Why this is worse here than down south

Most of what you will read about aircon maintenance was written for a climate where the unit runs three or four months a year in dry air. Ours run eight months, sometimes closer to twelve, in humidity that sits high for half the year.

A split system in a Darwin bedroom accumulates in one year roughly what a Melbourne unit takes three years to build up. That is why advice like “service it every three to five years” does not transfer. It was written about a different machine in a different climate.

The bit people do not think about

If the smell is mould, then every time that unit runs it is putting mould spores into the air of a room someone sleeps in. In houses where somebody has asthma, allergies or hayfever, this is usually the thing that has been making the difference and nobody connected it.

We get called out to plenty of houses where somebody has spent months assuming it was dust in the carpet.

What actually fixes it

It has to be cleaned past the filters. A catch bag goes around the unit, chemical goes onto the coil and the barrel fan, then the whole thing is pressure washed through so what comes off ends up in the bag and not on your wall. The vanes and the drain tray are cleaned and sanitised, and the drain is flushed through.

Done properly it takes a bit over an hour per unit. Done properly, the smell does not come back for a year or more.

This is the difference between the cleans advertised around town for $80 and what we charge. At $80 nobody is bagging the unit and washing the fan and coil properly. There is not enough in it to do the job that fixes the problem, so what you get is the filters rinsed and a spray of something that smells nice for a fortnight.

Doing more than one at once

Most Darwin houses have three or more units. Because a good chunk of what you pay for a single clean is us turning up, doing them all in one visit costs a lot less per unit than one at a time.

If the bedroom one smells, the others are the same age and have had the same air through them. They are simply further from your nose.

How to tell how bad yours is

Get a torch, take the front cover off — it usually unclips — and look past the filters at the fan behind. If the blades look furry, or grey rather than white, that is what you have been breathing.

Take a photo of it while you are there. It is useful for us, and it usually settles any argument about whether it is due.