Quick answer
A musty smell when the AC turns on comes from mould or bacteria growing on the wet evaporator coil, in the condensate pan, or on a damp filter, and the blower pushes it through the ducts in the first minute of each cycle. Cleaning the coil and drain and changing the filter usually clears it. If it returns quickly, the house is too humid or the system is oversized.
The reason your AC smells musty when it turns on is that the indoor coil spends the whole summer wet. Air passing over it deposits dust, pollen and skin cells onto fins that are dripping with condensate, and that film is a good place for mould and bacteria to grow. The smell is strongest in the first minute of a cycle because the blower is clearing the air that sat in the coil box between runs.
The smell is called dirty sock syndrome, and it starts on the coil
Technicians call it dirty sock syndrome because that is what it smells like: a locker room, wet cardboard, a basement after rain. It is worst in shoulder seasons when the coil warms and cools repeatedly, and on heat pumps that swing between heating and cooling. On a straight air conditioner in Burlington it typically shows up in late June, once the coil has been wet for a few weeks.
A coil cleaning is the fix. The technician removes the coil access panel, applies a no-rinse foaming coil cleaner that breaks down the biofilm, and flushes the pan and trap below it. A light coil clean and a drain flush are part of a proper AC tune-up at $120–$250; a coil that is heavily fouled takes longer and is quoted as its own job.
Other sources, and how to tell them apart
- A damp filter: a filter that sat in a humid return for four months smells like a wet towel. Replace it, and replace it more often through the summer.
- Standing water in the condensate pan or a slimed trap: a sour, swampy note rather than musty. If there is water on the floor as well, the drain line is the problem.
- Basement return air: in 1950s–60s bungalows in Roseland and Aldershot the returns are often panned joist bays, open to the basement air on their way to the furnace. The system is inhaling a damp basement and delivering it upstairs.
- A burnt-dust smell at the first furnace start of fall is a different thing and normal for a few minutes.
- A rotten-egg smell is not an AC smell at all. That is the odourant added to natural gas; leave the house and call Enbridge's emergency line or 911 from outside.
- A sweet, chemical smell can be refrigerant from a leaking coil. It is faint and rare, but worth a service call.
Why the smell comes back: humidity and an oversized system
If the coil was cleaned in May and smells again by August, the coil is not the root cause. Lakeshore air in Burlington and Oakville sits at high humidity for much of July, and a house that stays above 55 to 60 percent relative humidity indoors keeps the coil, the ducts and the basement damp enough for regrowth. Our answer on why a house stays humid with the AC on explains the mechanics.
An oversized air conditioner makes this worse. It cools the air to the setpoint in a few minutes and shuts off before it has pulled much moisture out, so the coil gets wet without ever drying the house. A whole-home dehumidifier at $1,500–$3,000 installed deals with the humidity directly; right-sizing at replacement time fixes the cause.
Do UV lights or duct cleaning fix it?
A UV lamp mounted at the coil keeps the biofilm from re-establishing, and for a house that has had dirty sock syndrome more than once it is the usual prevention. It does nothing for a smell that originates in the ducts or the basement, and we set out the honest limits in do UV lights in HVAC systems work.
Duct cleaning helps when the ducts themselves are the reservoir: a past flood, a renovation that filled them with drywall dust, or years of pet hair matted in the returns. It does not help a coil problem. We price duct cleaning at $300–$700 for a typical house and will say so if yours does not need it.
Four checks that locate the smell before you call
- Run the fan only, with cooling off, for 15 minutes. If the smell fades, the source is the wet coil box. If it persists, look at the returns and the basement.
- Pull the filter and smell it. Replace it regardless if it is more than three months old.
- Look at the drain line outlet. No drip on a humid day means the line is blocked.
- Put a hygrometer in the main living area. Readings above 60 percent point to a humidity problem the AC alone will not fix.
ZK Mechanical does coil cleaning, drain service and indoor air quality work across Burlington, Oakville and Hamilton from North Service Road. Book a cooling tune-up in Burlington or send us a note with what you are smelling and when it happens.