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Air Conditioning Odours and Smells

Common Causes and Issues

A common issue we see when repairing air conditioners is customers complaining about a foul odour when the AC kicks in. Some customers have tried to “mask” the smell with air fresheners, diffusers or candles. It’s not recommended to cover-up any smells as it could be caused by something serious.

If you encounter unidentified smells with your AC system it’s important to have the problem diagnosed to reduce the risk of any possible health hazards or having major repairs needed later on.

Common Odours

We compiled a list of the most common issues we find when smells are emanating from your air conditioning vents.

Musty, Humid Smell

As you may assume a musty or moist smell is caused by excess moisture or the build-up of humidity. Areas of concern causing this issue may be caused by:

  1. The AC compressor becoming clogged and not draining properly. The moisture can accumulate within the compressor and over time it will begin to smell
  2. This odour can also be present if your AC Filter is past the replacement due date. A clogged filter will allow moisture and condensation to collect within the ductwork. Keep in mind the AC Filter is for the unit itself and not for filtration of air within your home.
  3. Plumbing leaks in your water lines can also cause a musty smell

Whatever the issue causing a musty, humid smell, it should be addressed and corrected to prevent bacteria and mould spores from entering your ducts and vents.

By scheduling an annual HVAC maintenance inspection these issues can be detected and corrected before they create problems.

Burning Smell

Most everyone is familiar with a burning smell. The issue is actually caused, most often, when something is burning. Typically a burning smell is produced by an overheated electrical motor, a short in a circuit board, a problem with wiring or a mechanical issue.

A burning smell from your AC system or found in your utility room can be a serious situation and should be addressed immediately.

Before calling an HVAC expert for assistance – turn off your HVAC system from the breaker box.

Cigarette Smell

This odour, is what it appears to be, caused by someone smoking in your home and the smell becomes fixed within the ductwork. You may want to try and replace any air filters to see if it provides a quick fix. The removal of the smell, may come with time, or more might require duct cleaning to eliminate the odour from its source.


It is prudent to have all unidentified smells inspected and diagnosed by an HVAC professional. Maintenance can not only provide you with peace of mind but it may also eliminate any potentially hazardous situations which may endanger your family.

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