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Why does my AC smell?


It’s the middle of a heat wave, and you’re drinking an iced tea on your couch. Your drink is condensating a lot more than usual. “That’s weird… I’ll go check the AC”.

As you get up, you notice your skin sticks to the couch more than it usually does. There are beads of sweat forming on your forehead. You start getting nervous.

Once you get up and walk toward the vent, the smell of burning plastic sears your nostrils. Your AC is broken, and you realize you have a few days left of the heat wave with no air conditioning. Heartbreaking.

Unlike carbon monoxide leaks, bad roofing, or an electrical problem, air conditioners are usually pretty obvious about their breakdowns. You’ll almost always see something weird, smell something awful, or feel the oppressive outdoor heat begin to crush your innards and immediately begin draining your body of any retained water.

Today’s topic is smells. There are a few smells that a bad AC system can create, and we’re going to talk about a few of them. We’ll also talk about the causes and potential fixes for these smells.

As always, we highly recommend hiring a professional to do your HVAC work. They’ve had years of training and education about all things HVAC. This means they know how to pull a disconnect switch without liquifying their internal organs. They know how to install a capacitor without sending your roof into the stratosphere. They know how to add refrigerant without causing long-term brain death. Seriously, just let them do their job.

Alright! Let’s get to sniffing!

Why does my AC smell like burning plastic?

If you have an older system, this is a common one. When you have an old system, chances are it’s had a lot of repairs. Most of the time, these repairs are done under a home warranty. Home warranty companies are… how do I put this politely?... rather frugal when it comes to their budget for repairs.

This means that the repairs done on your HVAC system are just quick fixes, rather than actual repairs that solve the root problem. Over time, these repairs add up to a system that is bound for frequent failures.


There’s a good chance that burning plastic smell is coming from a burning wire in your AC system itself. If you check your thermostat and it’s giving an error message, or if it’s turned off completely, there’s a good chance that’s what it is. Turn off your electricity and investigate your AC wiring (usually in your attic) to make sure it’s not going to cause a fire.

A burning plastic smell, in a worst-case scenario, can mean an active fire somewhere in your electrical system. Your AC picks up the smoke and then ejects it into your air. Do a quick look around and ensure that your walls don’t have any hotspots, and that you don’t have any outside fires. Be sure to check your attic, basement, or wherever your AC fan is. If you find a fire, get away from the house and call 911 immediately. No, you don’t have time to save your 1999 First Edition Charizard Pokémon card. Yes, even if it’s the holographic print.

If you smell burning plastic after you’ve had your AC off for a long time, it could be your AC system burning off some dust. Check for fires or burning wires. If you can’t find any, let it run for a couple hours. Chances are, the smell will go away.

Why does my AC smell like a car exhaust?

Remember the long-term brain death I mentioned in the intro? That’s where this comes into play.

Your AC should not smell like your aunt’s 2003 Honda Civic after a cold start. If it does, it’s probably a broken refrigerant line.


Your air conditioning system uses a refrigerant (usually freon) to cool down the air that cycles through the system. Here’s how it works (WARNING: BORING SCIENCE LESSON AHEAD!)

First, your system uses a compressor to squeeze freon down and give it a very high pressure. The act of pressurizing the freon makes it very hot. I know, heat doesn’t make things cold. Just give me a second.

The hot, pressurized freon is sent into a series of coils, which allow it to condensate into a liquid. Then, the liquid freon is passed through a specialized valve that quickly and drastically lowers its pressure, forcing it into a gaseous state. This massive drop in pressure has the opposite effect as the first step, and the freon becomes extremely cold. It’s then circulated around the air hoses, drawing out the heat and making the air cold before it’s pushed out into your home.

The freon is then funneled back into the compressor, and the whole thing starts over.

When that freon starts to leak into parts of the system where it isn’t supposed to be, it can create that car exhaust smell that’s plaguing your nostrils.

If you smell that, you need to call an HVAC pro right away. As mentioned, inhaling loose freon can make your brain go bye-bye. Open your windows and turn on all your fans to get ventilation. This will send the freon harmlessly into the outside air, rather than into your innocent little lungs. If you can’t get proper ventilation in your house, find somewhere else to be while you wait for it to get fixed.

Why does my AC smell like moldy socks?

If your AC system smells like the inside of a high school boy’s gym bag, you have a problem. Luckily, this is the least dangerous smell on our list. You’ll definitely want to get it fixed, but it won’t cause immediate brain damage or a fire. There are two main causes for this smell.

If you have drainage problems in your AC system, you likely have some stagnant water somewhere in the pipes or condensation pan. As this water sits for a while, bacteria and fungus can fester in it and start to stink up your house. Call an HVAC pro to check out your system and see why water is accumulating.


The second cause is a clogged filter. This often goes hand in hand with the first reason, and can mean your filter has some mold or fungus growing on it. Once your condensation problem is fixed, you’ll need to replace your filters. A clogged air filter not only smells bad and reduces AC power, but it can even break your system entirely. That’s where we can help.

Filter King’s online filter store has any size of filter you could possibly need. Seriously, if you need a new filter, use our list of sizes to find yours and then just have it sent directly to your house. Super easy.

Bad AC smells are a nuisance. But once you confirm that there’s no fire or that you can ventilate your house in case of a freon leak, they’re usually no big deal. If you have any air conditioner/filter related questions or comments, don’t hesitate to drop us a line and we’ll be happy to help!