Your nose is a good early warning system. Some HVAC smells are harmless and fade on their own. Others mean turn the system off now. Here is how to tell them apart.
Musty or mildew smell
The most common complaint, especially in our humid summers. Moisture is part of how an AC works, and where there is standing water and dust, mold and mildew can grow. Common sources:
- A clogged drain line or standing water in the drain pan
- A dirty indoor coil
- Ductwork sweating in a humid attic or crawlspace (see ductwork problems)
- An old, damp air filter
Start with a new filter and check the drain (our guide to an AC leaking water shows how). If the smell stays, the coil and drain pan need cleaning.
Dirty socks
A dirty sock smell, often noticed when a heat pump switches between heating and cooling, usually comes from bacteria growing on the indoor coil. A coil cleaning usually fixes it.
Burning dust when the heat first comes on
Normal. Dust settles on the heat exchanger or heat strips over the summer and burns off the first few times the heat runs. It should fade within an hour or so. If it does not, change the filter and call.
Burning plastic or electrical smell
Not normal. An overheating motor, wiring or electrical part can smell like burning plastic or hot metal. Turn the system off at the thermostat and the breaker, and call.
Rotten eggs or sulfur
Natural gas has a rotten egg smell added to it so leaks can be detected. If you smell it near the furnace or gas line, do not flip switches or use your phone inside. Leave the house and call your gas utility or 911 from outside.
Your air conditioner does not use natural gas. If you smell gas when the AC runs, the source is likely a nearby gas appliance or line, and you should treat it the same way.
Exhaust or smoky smell
Exhaust fumes from a gas furnace can mean the vent pipe is blocked or disconnected, or the heat exchanger is cracked. Turn the furnace off and call. Make sure your carbon monoxide alarms work. Carbon monoxide itself has no smell, which is why alarms matter. Read our guide to carbon monoxide and furnace safety.
Vinegar or sour smell
A vinegar-like smell can come from mold or bacteria on the coil or in the drain pan. It can also be an overheating electrical part. If it is strong or comes with a burning smell, turn the system off and call.
Dead animal
Rodents and other animals sometimes get into ductwork, especially in attics and crawlspaces. The animal has to be removed, and the duct may need repair. This is one of the few situations where the EPA says duct cleaning makes sense (see is duct cleaning worth it).
Sewage smell
Some condensate drains connect to a plumbing drain through a trap. If that trap dries out, sewer gas can come back up. Other times a plumbing vent is near an air intake. Either way, it needs to be found and fixed.
When to turn it off and call
- Rotten egg smell (leave the house first)
- Burning plastic, electrical or hot metal smell
- Exhaust fumes
- Any smell along with a CO alarm, headaches or dizziness
Call North Alabama Heating & A/C at (256) 534-2941. We have served Huntsville, Madison and North Alabama since 1977, Monday to Saturday, 7 AM to 7 PM. See our HVAC repair service.
Quick answers
Why does my AC smell musty?
Usually mold or mildew from standing water: a clogged drain line, a dirty drain pan or coil, or ducts sweating in a humid attic. Replace the filter and clear the drain. If the smell stays, have the coil and pan cleaned.
Is a burning smell from the heater normal?
A light burning dust smell for the first few minutes of the season is normal and should fade within an hour or so. A burning plastic or electrical smell is not. Turn the system off and call.
What should I do if I smell gas near my furnace?
Do not touch switches or use your phone inside. Leave the house and call your gas utility or 911 from outside. Do not go back in until it is safe.
