The aircon itself isn’t making you sick. A poorly maintained one can. Mould in the drain pan, a filter that hasn’t been cleaned in two summers, or a unit blasting cold air straight at your pillow are the real culprits, and all three are fixable in an afternoon.

We’ve been pulling apart split systems and ducted units across Sydney for years, and the homes where someone’s “always getting sick from the aircon” are almost always the homes where the unit hasn’t been touched since installation. The cold air gets blamed. The maintenance log gets ignored.

Here’s what’s actually happening, and what to do about it.

What’s really going on when “the aircon” makes you feel ordinary

Five things tend to be at play, often together.

A filter packed with dust. Every split system has a removable filter behind the front panel. After a couple of seasons it stops being a filter and becomes a felt mat. Every time the unit kicks on, it pushes a fine dust of skin cells, pet dander, pollen and outdoor particulates back into the room. If you wake up with a blocked nose ten minutes after the aircon comes on, this is usually it.

Mould on the coil and in the drain pan. The cooling coil inside your indoor unit is constantly wet during summer. Water condenses on it, runs down into a drain pan, and exits through a small drain line. When that drain slows up, or the coil never gets a proper clean, you grow a slow biofilm of mould. That’s the musty smell that hits you for the first thirty seconds when the unit fires up. On Sydney’s humid late-summer afternoons this is the single most common service-call complaint we get.

Air that’s too dry. A correctly sized unit pulls a fair bit of moisture out of the room. An oversized unit pulls out too much, and you end up with indoor humidity sitting under 30%. That dries your throat and sinuses overnight, and you wake up feeling like you’ve got a cold. You don’t. You’ve got a sizing problem.

Cold air aimed straight at you. Sleeping with the louvres pointed at your face will give you a stiff neck, a dry throat and a headache. That’s a positioning issue, not a health risk in itself, but plenty of people misread it as illness.

Big temperature swings. Walking from 36°C outside into an 18°C room ten times a day is a stress on the body. It won’t give you the flu, but it will leave you feeling flat and headachy. Setting the thermostat to a sensible 23 to 25°C solves it.

Sydney-specific stuff we see on service calls

A few patterns that come up again and again across our service area:

In the inner west and eastern suburbs, older apartment splits often have the condensate drain run with not enough fall, so water sits in the pan and grows mould. You smell it before you see it.

In newer estates around Oran Park, Leppington and Edmondson Park, two-storey ducted systems run their return air through a roof cavity full of insulation dust. If the return filter isn’t sized properly or hasn’t been changed since handover, every hot day pushes that dust through the home.

After bushfire season, every filter in Sydney is loaded with fine smoke residue. We pull filters in October that look like they’ve been run over by a Hilux. Spring pollen does similar damage through September and October.

In older brick veneer homes around Carlingford, Bankstown and the Liverpool corridor, we still find original 90s and early 2000s splits that have never had a professional service. The coils on those units are matted with biofilm. People assume the unit is “just old”. It’s not old, it’s filthy.

The five things to actually do

1. Clean your filters yourself, every 4 to 6 weeks in summer. Pop the front panel of the indoor unit, slide the mesh filters out, vacuum the worst of it, rinse under lukewarm water until the water runs clear, let them dry completely before they go back in. This is not a tradie job. It takes ten minutes. If you do nothing else on this list, do this.

2. Get the indoor coil and drain pan professionally cleaned every 1 to 2 years. This is the bit you can’t reach. A licensed technician pulls the cover, chemically cleans the evaporator coil, flushes the drain line and clears the pan. If your unit smells musty when it starts up, you’re due. AS/NZS 3666 covers air handling hygiene for commercial settings, and the same principles apply at home.

3. Don’t chase 18°C. Set the thermostat between 23 and 25°C during the day, and 22 to 24°C at night. The unit will run more efficiently, your power bill drops, and your body isn’t getting whiplashed every time you walk through the front door.

4. Aim the louvres up, not at people. Cold air sinks. Point the vanes towards the ceiling and let the air fall naturally through the room. Direct flow on the bed or couch is the source of most “the aircon made me sick” complaints we hear.

5. If something feels off, get it looked at. Black flecks blowing from the vents, water staining the wall under the indoor unit, a musty smell that doesn’t go away after the unit’s been running for a few minutes, or symptoms that only happen in one room are all signs that a service is overdue. Any work involving the refrigerant circuit needs an ARC-licensed technician, not a handyman.

Things people worry about that aren’t actually a problem

A few common myths worth clearing up:

Air conditioning doesn’t give you a cold. Colds are caused by rhinoviruses you pick up from other humans, not from cold air. The reason summer colds spread in offices is that everyone’s sharing recirculated air in a closed room, not because the AC itself is producing virus.

Air conditioning doesn’t dehydrate you in the way people think. It doesn’t pull water out of your body. It does dry the air in the room, which can dry your throat and sinuses overnight, and that gets misread as dehydration. Drinking water helps, but the real fix is humidity in the room.

Cold air doesn’t cause pneumonia. Bacteria and viruses do. Sitting under a vent might make you feel rough, but it isn’t infecting you.

When to call someone

Call a technician if any of the following are true: the unit smells musty more than thirty seconds after start-up, you can see water marks on the wall under the indoor head, you’re seeing visible mould around the louvres, your power bill has jumped without a clear reason, or the unit isn’t holding the set temperature on hot days. Most of these are an hour’s work to sort. Left alone they get expensive.

Frequently asked questions

How often should I service my air conditioner? A professional service every 12 to 24 months for a residential split system, annually for a ducted system that runs hard year-round. Filter cleaning at home should be every 4 to 6 weeks during heavy use.

Why does my aircon smell musty when it turns on? Almost always mould or biofilm on the cooling coil and in the drain pan. A chemical coil clean and drain flush will fix it. If the smell comes back within a few months, the drain line probably needs adjusting for better fall.

Can mould in an air conditioner actually make you sick? It can aggravate asthma, allergies and sinus issues, and it’s more of a problem for kids, older adults and anyone immunocompromised. It’s not usually dangerous to a healthy adult, but it’s not something to ignore either.

Should I leave the aircon on overnight? If you set it sensibly (22 to 24°C, fan on auto, louvres aimed up) yes, it’s fine and often better than turning it off mid-night and waking up sweating. The unit uses less power maintaining a temperature than it does fighting back to one.