Air Conditioner Smells When It’s Turned On? Here’s Why

Air Conditioner Smells When It's Turned On

What died in here!?

That’s not a question you want to pop into your head as you walk into your house on a hot summer day. Especially when you realize the smell is coming from the very thing you need to keep your home cool.

But sometimes air conditioners can smell to the point you need to turn them off.

There are several reasons for a smelly air conditioner. However, the most common is a musty smell due to long-standing water in the drain pan or drip lines where bacteria has begun to grow. Dark, humid locations such as the evaporator coil are also prone to mold and mildew.

I’ll detail all the reasons below, so keep reading. Hopefully, you’ll soon have your cool, fresh-smelling home back again.

Technician fixing smelly air conditioner
How repulsive is it that your air conditioner emits odors? Find out its source and address the problem at its very root with the next tips!

Why Your Air Conditioner Smells

As mentioned above, there are multiple reasons for a smelly air conditioner. Some of these could indicate a serious problem, so if you do think the odor you smell is similar to something below, shut off your system and call an HVAC technician .

To be clear, I’ll indicate in what circumstances you should do so.

Here’s a list of several odors and what they may mean.

Exhaust Fumes

If you’ve ever started your car in advance on a cold winter day and forgot to crack open the garage door before doing so, you know exactly what exhaust fumes smell like. But since your air conditioner doesn’t run on the same type of engine your car does, it doesn’t create exhaust.

However, if you smell something that seems similar, you may have a leak in your refrigerant line. A leak will negatively impact the performance of your air conditioner but more importantly, it’s hazardous.

If you think you smell car exhaust fumes, turn off your system and call an HVAC professional. As a safety measure, the law states that only certified technicians can deal with refrigerant issues.

Cigarette Smoke

Don’t smoke, but your home smells like to do? Or you do occasionally smoke but your home smells like you’re a chain smoker?

Over time your air filter and evaporator coil will absorb the odor of cigarette smoke. When your air conditioner is running and the air is getting pushed through the filter, that old, embedded odor is being released into the air again.

To completely overcome this, stop smoking. Alternatively, you’ll need to be religious about changing your filter and cleaning your evaporator coil. Heavy smokers should probably change their air filters more frequently than the manufacturer recommends.

One excellent way to freshen up your air conditioner is to use charcoal filters. These are excellent for eliminating odors – especially smoke.

Is Something Burning?

Something other than a cigarette, that is.

If the air pushing out of your vents smells like something’s on fire, there could be an issue in one of the many electrical components of your system. This could include circuit boards, wiring, your blower motor, compressor, and more.

This is a potentially dangerous situation, so turn your air conditioner off immediately and contact an HVAC professional.

Dirty Socks

Call any HVAC professional and tell them your air conditioner smells like dirty socks, and they’ll know exactly what you mean. The phrase “dirty sock syndrome” is common in the industry.

More than likely, anyone who doesn’t need to run their air conditioning year-round has been hit with the smell of cheesy feet a time or two.

The problem here occurs when you turn your air conditioning back on for the first time at the beginning of the summer season . That’s because mold and mildew have had time to grow, thanks to standing water in your air conditioner. It’s not harmful or dangerous, it just stinks.

This issue is especially problematic with heat pumps. It can even be smelled in the winter time because the system needs to go through a defrost cycle, which is essentially running in the air condition mode. This AC mode runs long enough to wet the coil, and to stimulate the smell.

To stop this from happening in the future, clean your evaporator coil and drain pan/line before starting up your system in the summer. Ideally, make this part of your annual maintenance.

Skunk or Rotten Eggs

Does it smell like Pepé Le Pewis roaming free in your home? Then yes, you probably should be alarmed, just not for the reason you might think.

Natural gas is odorless but deadly.

Finger pointing out air conditioner filter
It is very important to turn off the gas supply if you notice this peculiar smell coming from inside your air conditioner.

A chemical called methyl mercaptan—also known as methanethiol—is an odorant that’s added to natural gas by many utilities as a safety feature. Some claim that smells like a skunk and some say it smells like rotten eggs.

So assuming you have neither a skunk nor rotting eggs sitting somewhere in your home, there’s a good chance you have a gas leak.

As soon as you catch one of the smells and there isn’t an obvious reason for it, turn off your gas supply and contact your utility company.

Rotting Garbage

The smell of something rotting may indicate a dead animal, such as a rodent, that has found its way into your ductwork and died there. Running the air conditioner is just pushing the odor through your home.

If you’re anything like me, you have no desire to deal with this on your own. And depending on where you live, there may be some sort of guidelines you need to follow with regards to dead wildlife.

Get the animal removed, and then thoroughly clean the spot where it was found. After running your air conditioner for a while, the smell should dissipate.


So what does your air conditioner smell like?

To recap we’ve discussed the following:

  • Exhaust fumes. This could be due to a problem in your refrigerant line and should be dealt with by an HVAC technician immediately.
  • Cigarette smoke. Cigarette smoke will eventually infiltrate and remain in your system. In order to get rid of the smell, you’ll need to keep your filters and evaporator coil clean.
  • Something burning. If something smells like it’s burning, maybe something is burning. There are many electrical components throughout your system, so if you smell something burning, turn it off immediately and call a technician.
  • Dirty socks. This could be due to mold or mildew growing in standing water in your drain pan.
  • Skunk or rotten eggs. Assuming you don’t really have a skunk or rotting eggs in your home, there’s a good chance you’re smelling a chemical that has been added to natural gas, which is odorless. Turn off your main gas line and call your utility immediately.
  • Rotting garbage. This may indicate a dead animal somewhere in your HVAC system, perhaps your ductwork.

Some of the above can be dealt with yourself but a few of them will require professional help.

Hopefully, we’ve identified whatever smell you’ve noticed in your home, and you’re able to deal with it. While you are here, why not check out some of our related posts below? Perhaps we can help you with something else.

I've been helping homeowners with appliance repair since 2016. Starting out as an enthusiastic amateur, I've since worked with many Appliance, HVAC, and DIY experts over the last 7+ years. My mission is to help fix your appliances and prevent future issues - saving you stress, time, and money. Visit my author page to learn more! Read more