Quick answer
An AC smells like vinegar when bacteria growing on the wet evaporator coil, in the condensate pan or in the drain trap give off sour, acetic odours. A second cause is ozone from a dirty electronic air cleaner. It is unpleasant, not dangerous: clear the drain, change the filter, and have the coil cleaned if the smell returns each time cooling starts.
If your AC smells like vinegar, the source is almost always the wet side of the system: the indoor coil, the pan under it and the drain. The smell is sharp and sour, strongest in the first minute or two of a cooling cycle, and it fades as the coil gets cold. It is a cleaning problem, not a refrigerant or gas problem. The one exception is a home with an electronic air cleaner, where the sharp smell can be ozone.
Bacteria on the wet coil and standing water in the pan cause the vinegar smell
The evaporator coil pulls litres of water out of the air on a humid day, and dust that gets past the filter sticks to the wet fins. That film feeds bacteria, and some of them produce acetic acid and related sour compounds as they grow. When the system sits idle the film warms up; when the blower starts, the first air across the coil carries the smell to every register.
It shows up most in spring and early summer, in long humid stretches near Lake Ontario, and on oversized systems that shut off before the coil has drained. A filter left in too long, or a gap around the filter that lets dust bypass it, speeds it up.
A sour smell that is there all the time, not only at start-up, points at standing water. The pan under the coil should empty through a 3/4-inch drain and a trap. When algae and slime plug the trap, water sits in the pan for days and turns.
- Watch the drain outlet at the floor drain or condensate pump while the AC runs. A steady drip is healthy; nothing at all means a blockage.
- Open the pump reservoir if you have one. A slimy, sour tank is a source in its own right.
- Look for water marks or rust staining at the base of the coil case above the furnace.
Clearing the line yourself is described in how to unclog an AC drain line. A professional drain cleaning or trap replacement runs $150–$350.
Ozone from an electronic air cleaner: the second cause
Electronic air cleaners charge dust with high voltage and collect it on metal plates, and a small amount of ozone is a by-product. When the cells are dirty or a plate is bent they arc, with a snapping sound, and ozone output climbs. Ozone has a sharp, acrid edge that some people read as vinegar and others as chlorine or the air after a thunderstorm. Switch the air cleaner off for a day: if the smell goes, wash the cells or have the unit repaired.
Coil cleaning versus a UV light on the coil
Coil cleaning removes the film that is there now; a UV light slows it from coming back. They do different jobs, and the order matters. A technician cleans the coil in place with a foaming cleaner, flushes the pan and drain, and checks that the filter seals in its slot. Coil cleaning is part of an AC tune-up at $120–$250.
A UV lamp mounted to shine on the coil keeps the wet surface from being recolonized between cleanings. It is worth considering when the smell returns every season on a clean, draining coil. It will not clean a fouled coil, unblock a drain or remove dust, and the bulb has to be replaced on the manufacturer's schedule to keep working. The price of a UV install depends on the lamp and the duct access, so it is quoted in writing; whether UV lights in HVAC systems work is its own question.
Vinegar, musty or sweet: how to tell AC smells apart
- Sour or vinegar: bacteria on the coil or in the pan, or ozone. Strongest at start-up.
- Musty, like a damp basement: mould in the ductwork, a wet filter or soaked duct insulation. See why an AC smells musty.
- Faintly sweet or chemical: a refrigerant leak, usually with weaker cooling and ice on the copper lines. Turn the system off and book a leak check.
- Burning or hot plastic: an overheating motor or wiring. Shut it off at the breaker.
- Rotten eggs: not the air conditioner. Leave the house and call Enbridge Gas from outside.
What you can do today about a sour-smelling AC
- Replace the filter and make sure it fills the slot with no gaps at the edges.
- Clear the drain and pour a cup of white vinegar into the cleanout. Used this way it is a cleaner, and its own smell is gone in an hour.
- Switch off the electronic air cleaner for 24 hours as a test.
- Run the fan on AUTO, not ON, so the coil can drain between cycles.
Call if the smell is back after a week, if there is water around the furnace, or if the smell is sweet or chemical. ZK Mechanical cleans coils and drains on AC repair and tune-up visits across Burlington, Oakville, Hamilton, Milton and Mississauga. Phone (647) 801-1252 or send a request and say when the smell appears; that detail often identifies the cause before a panel comes off. Other symptoms are covered in the cooling answers.