Here's a scenario that plays out in thousands of Central Coast homes every June: the temperature drops below 10 degrees, you reach for the remote, and fire up the heating for the first time since March. Within minutes, warm air fills the room. But along with it — something else. An earthy, musty, slightly sour smell that you tell yourself will "burn off in a few minutes."
It won't. That smell is mould spores. And you're breathing them in.
What You're Actually Breathing
That "first-start smell" is aerosolised mould spores, bacterial fragments, and mycotoxins — biological particles that have been multiplying inside your unit since the last time it was used. When you switch to heating mode, the warm air dries the internal surfaces rapidly, sloughing dried spores into the airstream at concentrations up to 40x normal indoor levels.
The Seasonal Switch: Why Winter Makes It Worse
Most people assume their air conditioner is clean because it was working fine when they turned it off in autumn. This assumption is dangerous. Here's what actually happens inside your split system across the seasons:
Summer (Cooling Mode)
Condensation forms on the cooling coils — up to 15 litres of water per day for a typical split system. The drain pan catches most of it, but internal surfaces stay perpetually damp. Organic material — skin cells, pet dander, pollen — is drawn in with the return air and settles on every surface. The combination of moisture, organic food source, and darkness creates perfect mould incubation conditions.
Autumn (Off Mode)
The unit sits dormant for 6-12 weeks. The damp internal cavity stays dark and warm as the outside temperature cools. Mould colonies that started in summer continue to grow unchecked. Without airflow, spore concentrations build up inside the enclosed unit cavity. The fan barrel, coils, and drain pan are now coated in a biofilm of mould, bacteria, and organic sludge.
Winter (Heating Mode — The Trigger)
You switch to heating. The coils heat up rapidly (from ambient ~15°C to ~45°C in 2-3 minutes). This thermal shock does two things simultaneously: it dries the mould colonies rapidly, causing them to release spores in self-preservation mode, and it creates thermal uplift that carries those spores into the fan airstream. Within 60 seconds of startup, your air conditioner becomes a biological aerosol dispenser.
Numbers That Matter
A single square centimetre of established mould colony on an AC fan barrel can contain 200,000 to 1,000,000 viable spores. A typical 2.5kW split system fan barrel has a surface area of approximately 800-1,200 cm². The math is not in your favour.
The Health Impact: What Those Spores Do Inside Your Lungs
When you inhale aerosolised mould spores, your respiratory system treats them as foreign invaders. The immediate response varies by individual, but the mechanism is consistent:
- Upper respiratory irritation — The nasal passages and throat inflame in response to the biological particles. This produces the "morning cough" that many people attribute to winter dry air.
- Bronchial inflammation — Spores small enough to pass the upper airway (typically under 10 microns) lodge in the bronchial tubes. The immune system attacks. Inflammation follows. Breathing becomes laboured, especially at night.
- Alveolar penetration — The smallest spores (under 4 microns) reach the alveoli — the microscopic air sacs where gas exchange occurs. Here, the immune response can cause fluid accumulation, reduced oxygen transfer, and permanent scarring if exposure is chronic.
Children are particularly vulnerable. Their lungs are still developing through adolescence, and the epithelial lining of their airways is more permeable than adult tissue. A child sleeping in a bedroom with a contaminated air conditioner is receiving 6-8 hours of continuous mould spore exposure every night.
What To Check Before You Turn On The Heat
Before you hit that heating button this winter, do these three checks:
1. The Smell Test
Turn the unit to "fan only" mode for 5 minutes. Stand directly under the indoor unit and breathe normally. If you detect any musty, earthy, or sour smell — that's mould. No amount of "running it through" will fix it. The colonies are established and need physical removal.
2. The Visual Check
With the unit off, lift the front panel and shine a torch inside. Look at the fan barrel (the cylindrical blower wheel). If you see black, grey, or green patches — that's mould. A clean fan barrel should be uniform grey plastic with no discolouration. Also check the louvers (the air direction vanes) — mould often starts at the edges where condensation pools.
3. The Filter Inspection
Pull the filters out. Hold them up to a light. If you can't see light through them, they're clogged. Blocked filters reduce airflow, forcing the fan to work harder and creating negative pressure zones inside the unit that draw even more moisture into places it shouldn't go. Clean or replace filters immediately — this is the one thing you can do yourself.
Quick Self-Check vs Professional Clean
You can: Clean/replace filters (30 seconds), wipe external surfaces, check for visible mould on louvers.
You cannot: Access the fan barrel (requires full strip-down), clean the coils properly (requires chemical treatment), sanitise the drain pan (sealed inside the unit), or remove established biofilm. That's what the $250 professional deep clean does — and it takes 90 minutes for a reason.
Why DIY Cleaning Makes It Worse
A common mistake: spraying Bunnings "air con cleaner" foam into the unit and calling it done. This is not cleaning. It's cosmetic. Here's what actually happens when you use those spray-can products on a mould-contaminated unit:
The foam penetrates the first 2-3mm of surface buildup and partially dissolves it — but the mould colonies extend 10-15mm into the coil matrix. The foam never reaches the root structure. What's worse, the moisture from the foam provides additional water for the surviving mould, which regrows within 3-5 days — often more aggressively than before because the competing surface layer has been stripped away.
Professional cleaning involves complete disassembly of the unit. The fan barrel is removed and pressure-cleaned. The coils are chemically treated and flushed. The drain pan is cleaned and sanitised. The internal cavity is dried completely before reassembly. This is not a 15-minute spray-and-wipe job. It's 90 minutes of methodical, component-level work.
Protect Your Family This Winter
The Central Coast has one of the highest rates of split system air conditioning per capita in NSW — and one of the highest humidity environments. Coastal air means salt. Salt means faster corrosion. Corrosion means more surface area for mould attachment. It's a compounding problem that makes regular professional cleaning not optional but essential.
If your air conditioner hasn't been cleaned in the last 12 months — or 6 months if you're coastal (Terrigal, Avoca, Wamberal, and surrounds) — it's time. Not next month. Not "when the smell gets bad." Now. Before your family spends another winter night breathing air that's been circulating through a mould colony.
Book a Winter Deep Clean — $250 (Save $30 with Winter Voucher)Same-week bookings available. Central Coast NSW. Call or text 0432 055 804.