What the smell coming from your AC actually means

Written by the team at Panda Heating and Cooling · licensed TDLR TACLA128438C · published 5 September 2026

Rotten eggs means leave the house

Start here, because this one is not an HVAC problem.

Natural gas has no smell of its own. Utilities add a sulfur compound specifically so that a leak smells like rotten eggs or sewage, because that is a smell nobody ignores.

If you smell it: do not touch light switches, do not touch the thermostat, do not use your phone indoors. Get everyone out, then call the gas company from outside. Atmos Energy’s emergency line is 866-322-8667.

Call us afterwards, once it is safe. Not before.

Burning smells, and which ones are normal

The first heat of the season. A dusty, hot-metal smell in the first hour or two of the first cold snap is normal. Dust settles on the heat exchanger all summer and burns off. It should fade within a couple of cycles.

Burning plastic, or an electrical smell that does not fade. That is not dust. Overheating wiring, a failing blower motor, or a seized capacitor smell sharp, acrid and chemical rather than dusty. Switch the system off at the breaker and call. This one has a real fire risk behind it and it is not worth waiting out.

The difference matters: dust burning off is a smell that gets better every cycle. An electrical fault gets worse.

Musty and damp — the most common one

A musty, dirty-sock, wet-towel smell that appears when the AC starts is almost always microbial growth on the evaporator coil or standing water in the drain pan.

It is not mysterious. The coil is cold, wet and dark, which is where mould likes to be. In a North Texas summer a system pulls gallons of water a day out of your air, and if the drain is slow or the pan holds water between cycles, something starts growing in it.

What you can do: change the filter first, since a saturated filter holds moisture and smells on its own. Then check whether the smell is strongest at startup and fades — that points at the coil rather than the ductwork.

What we do: clean the coil, clear and treat the condensate drain, and check the pan is actually draining rather than holding an inch of water. It often arrives alongside a drain problem you have not noticed yet.

Chemical, sweet or ether-like

A sweet, faintly chemical smell can indicate a refrigerant leak. Modern refrigerants are largely odorless, so this is less reliable than the internet suggests — but combined with poor cooling, ice on the lines, or a hissing sound, it is worth taking seriously.

Refrigerant is a sealed system. If it is escaping, there is a leak, and topping it up without finding that leak sells you the same repair next summer.

Other smells worth naming

Sewage or drains. Often a dry P-trap on the condensate line letting sewer gas back up, or a plumbing vent issue near the return. Not usually the AC itself, but the AC is what distributes it.

Something died. Occasionally something has. Rodents get into ductwork and attics, and the smell moves through the house whenever the fan runs. Unpleasant, straightforward to deal with.

Cigarette smoke or cooking that will not clear. Odors absorb into the coil and the ducts and get redistributed. Coil cleaning helps.