Most common causes
- Musty or moldy smell. Moisture on the indoor coil and in the drain pan lets mold grow, especially if the drain is slow or the system sat unused for a while.
- Dirty sock smell. Bacteria build up on the indoor coil, often on heat pumps and systems that cycle on and off in mild weather. The smell is usually strongest at startup.
- Burning or electrical smell. An overheating blower or fan motor, a failing capacitor, or damaged wiring can give off a sharp burning or plastic smell. This needs attention right away.
- Dusty burning smell at first use. The first run of the season can burn off dust from the heat strips or furnace parts. It should fade within an hour. If it does not, turn it off.
- Rotten egg smell. Natural gas has an added rotten egg odor. If your furnace shares the air handler with the AC, a gas leak can be carried through the vents.
- Dead animal smell. A rodent or bird in the ductwork or near the indoor unit can cause a strong, sour odor that does not go away.
What you can safely check
- If you smell burning, turn the system off at the thermostat and switch off its breaker. Do not turn it back on until it has been checked.
- If you smell gas or rotten eggs, do not touch any switches. Leave the house and call SoCalGas or 911 from outside.
- For a musty smell, replace the air filter and check that the drain pan is not holding standing water.
- Find where the condensate drain ends outside and make sure water is coming out while the AC runs.
- If there is a cleanout cap on the drain line near the unit, pour a cup of plain white vinegar into it to help clear algae.
- Make sure supply vents and return grilles are not dusty or blocked. Wipe the grilles you can reach.
Call a technician if
- You smell burning or hot plastic (turn the system off first)
- You smell gas (leave first, then call SoCalGas or 911)
- A musty smell comes back after a new filter and a clear drain
- You see mold on the grilles or around the indoor unit
- You suspect an animal in the ducts
What affects the repair cost
A filter and drain cleaning cost little. Cleaning the indoor coil and drain pan is a maintenance level service. Burning smells usually lead to a motor, capacitor, or wiring repair that depends on the part. Duct cleaning or animal removal depends on how much ductwork is affected.
