Why Does My AC Smell Bad? 6 Causes and Fixes for GTA Homes

You flip the thermostat to cool on the first warm day of June, and instead of crisp, conditioned air, your living room fills with a smell that’s somewhere between a wet basement and old gym socks. It’s a familiar GTA experience — and almost always a sign that something inside your system needs attention, not replacement.

The smell itself is a useful clue. Different odours point to different causes, and most of them are fixable in an afternoon once you know what you’re chasing.

What the smell is telling you

Air conditioners don’t generate odours on their own. They move air across cold, damp surfaces — and whatever lives on those surfaces gets carried into your rooms. That’s why the type of smell matters so much. It points to where the problem is.

Below are the six odours we see most often in Toronto and the wider GTA, what causes each one, and the right fix.

1. Musty or mildew smell — the most common offender

If your AC smells like a damp basement or wet towel, you’re almost certainly dealing with microbial growth on a wet surface. Three usual suspects:

• A dirty evaporator coil with a thin biofilm of mould and bacteria.
• A clogged condensate drain or pan holding standing water.
• A saturated, overdue filter trapping moisture along with the dust.

How to fix it

Start with the easy steps. Swap the filter, then locate the white PVC condensate drain line near the indoor unit and clear it — a wet/dry vacuum on the outdoor end works well. If the smell lingers after a few cooling cycles, the evaporator coil itself likely needs a professional cleaning. It’s not a DIY job: the coil is delicate, sits inside the air handler, and uses specific non-corrosive cleaners.

Musty smells often spike during humid GTA summers because the system runs longer and pulls more moisture from the air. That’s normal — but only if the drain and coil are clean enough to handle it.

2. Dirty-sock smell — biofilm on the coil

If the odour is more sour, sock-like, or cheesy, you’ve likely got what HVAC techs call “dirty sock syndrome” — a specific kind of bacterial film that forms on the evaporator coil when the system cycles between heating and cooling. It’s surprisingly common in GTA homes with central systems that handle both seasons.

A filter swap won’t fix this one. The cure is a proper coil cleaning, and in stubborn cases, a UV light installed near the coil can keep it from coming back. While you’re at it, our summer AC maintenance checklist covers the rest of the annual tune-up that catches issues like this early.

3. Burning, electrical, or hot-plastic smell — stop and call

This one isn’t a nuisance — it’s a warning. Shut the system off at the thermostat and the breaker and don’t run it again until a technician has looked at it.

Possible causes:

• A failing capacitor or scorched wiring at the contactor.
• A seized or overheating blower motor.
• Insulation or debris sitting against a hot component.

There’s one harmless version: a mild dusty smell on the very first cool day of the season, as dust burns off coils and ducts that sat idle through the winter. That should fade within an hour. If a burning smell lasts longer, smells like plastic, or comes with visible smoke, treat it as urgent.

4. Rotten egg or sulphur smell — possible gas leak

A rotten egg or sulphur smell near a vent isn’t actually from the AC — natural gas is odourless on its own, and Enbridge adds a sulphur-like odourant so you can detect leaks. If you smell it:

1. Don’t switch anything electrical on or off.
2. Get everyone outside.
3. Call Enbridge’s 24-hour emergency line at 1-866-763-5427.
4. Once it’s safe, have your HVAC system inspected.

A dead mouse or other small animal in the ductwork can produce a similar smell, but rule out a gas leak first — always. We’d rather you make the cautious call.

5. Sweet or chemical smell — possible refrigerant leak

A sweet, chloroform-like, or chemical smell near the indoor unit or vents can signal a refrigerant leak. Modern refrigerants like R-410A are largely odourless, but additives and lubricants in the system can produce a faint sweet smell when they escape.

Refrigerant leaks are bad for three reasons: efficiency drops, the compressor can be damaged from running low, and the refrigerant itself is regulated and shouldn’t be vented. Shut the system off, ventilate the room, and call a licensed technician — refrigerant handling in Ontario is restricted to certified pros for exactly this reason.

6. Cigarette, cooking, or pet odours — trapped in the system

If your house smells like the last meal you cooked or like the previous owner’s habits, the ductwork, filter, and coil are acting like a sponge. Smoke, frying oil, and pet dander all stick to damp coil surfaces and the inside of ducts, and the blower recirculates the smell every time it runs.

The fix has three layers:

• A fresh, higher-MERV filter (MERV 11 is a good sweet spot for most GTA homes — high enough to capture fine particles, low enough not to choke airflow).
• A professional duct cleaning if the smell is heavy or the ducts haven’t been cleaned in years.
• An air purifier or HEPA add-on for ongoing control. Our guide on indoor air quality digs into the options.

Quick reference: smell to source

Use this as a starting point — but when in doubt, get a professional look.

A simple prevention routine

Most odours are preventable with a small amount of seasonal attention:

• Replace the filter every 1–3 months in cooling season — sooner if you have pets or run a high-MERV filter.
• Clear the condensate drain in spring and again mid-summer.
• Keep humidity in check — roughly 40–50% indoors. A dehumidifier helps when the AC alone isn’t keeping up.
• Book an annual tune-up in late spring, before the first heat wave, so coil cleaning and electrical checks happen before problems develop.
• Clean registers and vents with a damp cloth a couple of times a season.

One small habit that pays off all summer: when you swap the filter, take a quick look at the drain pan and the line where it exits the unit. Catching a clog at the start saves you a smell — and sometimes a water-damaged ceiling — later.

When to call Delson Air

A filter swap and a quick drain flush handle most lingering smells. But coil cleaning, refrigerant work, electrical diagnostics, and duct cleaning need a licensed technician — and a single visit usually solves the problem for the rest of the season.

At Delson Air (“Home Comfort”), we look after homeowners across the GTA — Toronto, Mississauga, Markham, Vaughan, Brampton, Richmond Hill, Oakville and surrounding areas. We’re licensed, insured, TSSA-licensed, and an Enbridge Authorized Contractor, so every diagnosis and repair is held to Ontario’s standards.

If your air conditioner is putting out a smell you can’t track down — or if you’ve caught even a hint of burning, gas, or sweet chemical odour — call (647) 467-9919 or get in touch. Your comfort is our priority, and clean, fresh air through every vent is part of the deal.