First Hot Day of Summer — And Your AC Smells Terrible. Here's Why

It's the first hot day of the Ontario summer. You turn on the air conditioning and instead of cool, fresh air — you get a musty, damp smell flooding through your home. It's one of the most common AC complaints we hear from homeowners across Brampton, Mississauga, and the GTA.

The good news: it's usually fixable without replacing your entire system.

What Causes a Musty Smell from Your AC?

1. Mould or Mildew on the Evaporator Coil

The evaporator coil inside your air handler gets cold and humid during operation — the perfect environment for mould and mildew growth. When the AC sits idle all winter, these organisms can establish themselves. When you turn the unit on in spring, the air blows right through the contaminated coil.

2. Dirty or Clogged Air Filter

A filter that hasn't been changed since last season traps moisture, dust, and organic matter. Over winter, this creates ideal mould conditions. When airflow resumes in summer, the smell circulates through your home.

3. Blocked or Dirty Condensate Drain Line

Your AC removes humidity from the air and drains it away through a condensate line. If this line gets clogged with algae or debris, water backs up and sits stagnant — causing a musty or even sewage-like odour.

4. Ductwork Contamination

Mould can grow inside your ducts, especially if there's been any moisture intrusion. This is less common but produces the strongest and most pervasive smell.

How to Fix a Musty AC Smell

1. Replace your air filter — do this first. A fresh filter costs $15-30 and takes 5 minutes.
2. Check the condensate drain pan — if it has standing water, the drain line is clogged. Pour a cup of diluted bleach into the drain access port to clear algae.
3. Run a fan-only cycle — turn your thermostat to "fan only" for 30 minutes to dry out the evaporator coil.
4. Schedule a professional coil cleaning — if the smell persists, a technician can chemically clean the evaporator coil.

How to Prevent Musty AC Smells in the Future

• Change your air filter every 1-3 months
• Schedule an annual AC tune-up before summer
• Keep the area around your indoor unit dry and unobstructed
• Run the fan for 10-15 minutes after cooling to dry the coil
• Pour a cup of vinegar into the condensate drain every spring

When Is It Time for a New AC?

If your AC is over 12 years old and producing persistent odours despite professional cleaning, it may be time to replace it. Modern high-efficiency AC units are significantly better at managing humidity — which reduces mould risk. GT Home Services installs Lennox central AC systems from $44.95/month with zero upfront on rental.

We Serve All of Ontario

GT Home Services provides AC installation and service across Ontario including: