September 8, 2026 · Cairns Aircon Installers
You switch it on after a few weeks off and the room fills with a smell like a damp towel that has been left in a bag. Wet cardboard, old socks, a cupboard that has not been opened since Christmas. It is one of the most common calls we get in Cairns, and the cause is nearly always the same.
Why It Happens Here More Than Anywhere Else
Mould needs moisture, warmth, darkness and something to feed on. The inside of an air conditioner in Far North Queensland offers all four, better than almost any other environment in the house.
Every time the system cools, the evaporator coil gets cold and wet. Dust that slips past the filter sticks to that wet surface. When you switch the unit off, the coil stays damp in a warm dark box, with a film of organic dust on it. Do that daily for a couple of years in our humidity and you have grown a colony.
In a southern state the coil dries out for months at a time over winter. Here it rarely gets the chance.
The Smell Is Worst On Startup, Then Fades
That pattern is the giveaway. The first blast of air pushes the accumulated spores and the volatile compounds they produce straight into the room. After a few minutes the air moving across the coil has cleared the worst of it and your nose has adjusted, so it seems to go away.
It has not gone away. You have just stopped noticing it. If the smell returns every time the system has been off for a while, that is mould in the unit, not a one-off.
Where The Growth Actually Is
- The evaporator coil. The finned surface behind the filters. This is the main site, and the fins are packed tightly enough that you cannot clean between them with a cloth.
- The blower wheel. The barrel-shaped fan in the middle of the head unit. It has dozens of small curved blades and it builds up a black felt of dust and mould that is genuinely unpleasant. This is very often the real source of the smell.
- The drain tray and drain line. Standing water with a biofilm in it. This is the same blockage that causes water to leak from the indoor unit, so the two faults often turn up together.
- The filters. Contributors rather than the cause, but they should be clean regardless.
What You Can Do Yourself
- Clean the filters properly. Not a shake outside. Wash them, and let them dry completely before they go back in. Our guide on cleaning aircon filters covers it.
- Look at the blower wheel with a torch. Through the outlet, with the louvres open and the power off. If the blades look furry or black rather than clean plastic, that is your answer.
- Run fan mode for ten to fifteen minutes after you finish cooling, to dry the coil before shutdown. Some systems do this automatically and call it dry mode, self clean or blow. Turn it on if yours has it.
What will not fix it: spray-on cleaners from the hardware shop, deodorising bombs, and air fresheners. They mask the smell for a few days and add another sticky residue for dust to attach to. Bleach on an aluminium coil is worse than doing nothing.
Why A Proper Clean Is Not A DIY Job
Getting the growth off the coil and out of the blower wheel means dismantling the head unit, bagging it, and pressure washing with a purpose-made chemical, then flushing the drain. Done properly the water that comes out is black. It takes a technician an hour or two per head and it needs the right equipment to avoid soaking the electronics or bending the fins.
People do attempt it and we get called to a fair few units afterwards with damaged fins or water in the control board. It is not worth the risk on a machine that cost what yours did.
Is It A Health Problem?
We are installers, not doctors, so we will keep this factual. You are blowing air across a mould colony and into a room you sleep or work in. People with asthma, allergies or a compromised immune system commonly report that it affects them, and a smelly system is worth dealing with promptly rather than living with. If anyone in the house has symptoms you think are connected, talk to your GP as well as to us.
Keeping It From Coming Back
An annual professional clean, timed before the build-up rather than during it, keeps the coil and blower clear. Monthly filter washes in between. Fan or dry mode after cooling so the coil is not left wet. That is genuinely the whole routine, and systems that get it do not smell.
If yours smells now, a filter wash will not fix it. See our aircon cleaning and maintenance service, or get in touch and we will come and look at it.