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Ask A Tech: “Help! My air conditioner smells like…”

May 23, 2018

Smell can be a key indicator of major HVAC system malfunctions. Irregular smells are great ways for home and business owners to self-diagnose certain issues. If you aren’t confident in your ability to properly identify an odd scent coming from your vent or unit, you’ll want to call an HVAC expert right away to ensure that you or your system are not at risk. If you can identify one of these odors, adhere to the advice given to prevent any health or system concerns.

Rotten Garbage

There can be many reasons why a foul smell is coming from your ductwork. As the longest standing Carrier dealer in LA, we have seen it all. From old food left from a builder in new construction homes, to dead animals, we have found any number of things in ductwork that can fester and create an unpleasant smell. While it’s not inherently dangerous, you’ll still want to give an HVAC tech a call to have whatever is causing the odor out of your ducts right away.


Your air conditioner acts as a natural dehumidifier, so it is fairly common for it to collect more water than it can hold over time. To prevent this unpleasant mildew smell, regularly replace your filter and have your system maintained. This will ensure that any signs of mold or mildew are cleared away and that your system is clean and dry.

Cigarette Smoke

If a current or former resident was a smoker indoors, the smoke would have been sucked up into the evaporator coil. Once this potent smell has been absorbed by the coil, it can blast the smell back into the home. All that is required is a thorough cleaning of the evaporator coil to prevent this problem further.

Stinky Feet

Similar to mildew collection, stagnant water caught within your HVAC system can create a foot odor smell. Simple maintenance can eliminate excess water and clean your system to rid your ducts of this unpleasant smell.


Raw sewage is not only an unpleasant smell, but it can also be an indication of a serious threat. If you are smelling sewage from your vents, there is a good chance that one of your sewage pipes has burst near your ductwork. This will leak toxic and combustible methane gas into your home. Do not expose yourself or others to the gas if at all possible and call both a plumber and HVAC contractor to ensure both systems are in working order.

Rotten Eggs

Inherently natural gas is odorless, this can make it a silent killer because it is very dangerous to our health to inhale. Gas companies have added a harmless chemical, mercaptan, to natural gas to help homeowners identify lethal leaks. If you notice a sulfur, rotten egg, smell, exit your home or building and call your gas company immediately.


If it smells like something is burning inside your home, don’t assume one of your neighbors are barbecuing. Rather, shut your system off immediately and check to see if the smell gets worse near the unit. If so, it could be a sign that your AC fan or compressor is malfunctioning. Have your system serviced before flipping it back on.

Exhaust Fumes

This smell is of course going to be more associated with gas powered equipment, such as a vehicle. That’s what makes it easier to identify for homeowners when this smell arises. If you start to smell exhaust fumes through your vents or around your unit, it could be an indication that your engine is leaking fluids. If this is the case, call an HVAC tech immediately.


If you’ve ever been to an old war reenactment or are a old gun enthusiast, you’ll know that gunpowder has a distinct smell. For those unfamiliar with gunpowder, it is a sulfuric, metallic smell that is not dissimilar to firework smoke. If this scent is coming from your vents, it could mean major problems for your system. It is a sign that your circuit board or fan motor has shorted out. To avoid further system damage or risks, call an HVAC technician immediately for servicing.

If you have any other questions that you want answered, or to address any of these mild or serious HVAC system issues, give Brody Pennell a call @ (310) 896-4911.

About The Author

Brody Pennell

Since 1945, Brody Pennell Heating & Air Conditioning is the committed to helping homeowners in the greater Los Angeles area experience total home comfort. In 2021, the readers of the Los Angeles Times voted Brody Pennell the Best HVAC Company in the area.

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