Foul Air Conditioner Smells & Their Causes

Firing up your AC and flopping on the couch after a long, hard day at work is a truly satisfying feeling. That is, until the satisfaction is completely ruined by a pungent smell that begins to escape your AC unit and quickly wafting around the room.

Unfortunately, the reality of a stinky air conditioning unit is all but too real. A fully functional unit should be odorless, so if you’re smelling anything odd at all when you turn on your air conditioner, it’s more than likely a signal that you have a problem with it.

However, pinpointing exactly what’s wrong with your air conditioning system can be tricky. Different sorts of smells can signify different problems within the unit. Below we’ve broken down some of the most common air conditioning smells and what they could mean.

Rotten Eggs

The mere mention of the smell of rotten eggs can be enough to turn most people’s stomachs, so having that particular smell blasting out of your air conditioning is a truly dire situation.

If you’ve found yourself in this spot, there are two likely reasons that your air conditioning unit is emitting an unpleasant odor.

Firstly, it could be a gas leak. A leak is a serious health threat and should be addressed immediately. A gas leak can lead to carbon monoxide poisoning, which can cause brain damage, heart problems and, in extreme cases, can be fatal.

If you’ve noticed your air conditioning giving off a smell akin to rotten eggs, firstly open up all the windows and doors in the room for ventilation and then turn off the unit. It would be a good idea to turn off the home’s gas connection at the mains for a short period of time to be safe, too. Next, call a professional air con technician to identify and rectify the problem.

A less dangerous cause of a rotten egg smell coming from your air conditioner could be a dead insect or small animal inside the unit. Insects and other small animals can crawl into your unit and get trapped, before eventually dying.

If the cause of the smell is a dead animal, a technician can diagnose this and easily fish out the critter. You can take steps to prevent this from happening by pest proofing your air conditioner.


The smell of mould coming from an air conditioning unit is a common complaint, especially in humid areas. The cause of the smell is pretty self explanatory, it’s likely that your unit has accumulated mould from stagnant water.

All air-conditioning units have a drainage system that collects drops of condensed water and filters them out. However, when a drain is clogged this water won’t filter out and can accumulate and become a breeding ground for bacteria.

If you have an HVAC (heating, ventilation, and air conditioning) system installed in your home, mould can become an even bigger problem as it can spread through the air conditioning ducts onto everything else. If this is the case, then your best bet is to promptly call an HVAC technician to examine your system before things escalate.

Breathing in mould can cause major health issues like pneumonia, bronchitis and asthma attacks, amongst others. It also negatively affects indoor air quality. So, if you find yourself getting a whiff of a mouldy smell when switching on your AC, make sure you get it checked out.


The smell of burning when switching on your air conditioner can be a dangerous sign. It could indicate that there are wiring issues, a mechanical problem with your air conditioning fan motor or compressor, or an electrical component failure.

However, the burning smell could also be something as simple as dust burning off. To figure out whether you have a big problem at hand, switch your aircon on for twenty minutes. If the smell remains after that time then it’s likely you have a mechanical failure which could short the system or, worse, start a fire.


Smoking cigarettes inside is a likely culprit for the reason your unit may be smelling like nicotine. The bad smell can pass through the air filter and make its way up into the evaporator coil, so when the AC unit is next used it will push out the smell for a long time.

Cigarette residue tends to build up and while this smell isn’t a dangerous risk, it’s also not very pleasant. You can rectify the problem by switching out the dirty filters or calling a technician for a professional clean.


No-one wants the foul smell of feet wafting into their home and affecting the air quality when switching on their air conditioner. However, this type of bad smell is so common that workers in the AC industry have even made a slang term for it, “dirty socks syndrome”.

This smell is usually associated with stagnant water that’s been trapped inside the unit by a fault in the drainage system. A professional technician will be able to diagnose the drainage issue and ensure that water flows through smoothly again.


It may be obvious, but a chemical smell emanating from your air con is a dangerous sign. Air conditioners use a range of different fluids and chemicals, and the presence of a smell could mean that any of these are leaking.

A chemical odor could also mean that the refrigerant gas is leaking. Refrigerant gas is a coolant that's used in reverse cycle air conditioners and can be harmful to both your health and the environment.

When refrigerant gas is deeply inhaled it can cut off vital oxygen to your cells and lung. Even a small amount of refrigerant gas can cause mild health problems like a headache, nausea, dizziness and irritation of your eyes, ears and throat.


The smell of sewerage wafting through your bedroom or living area probably sits pretty high on a list of things that are unenjoyable. This certain smell usually means a backed up sewer line or ruptured sewer vent pipe.

Methane gases from the sewerage can leak into the duct work of your air conditioner. If this happens, it can cause a serious health hazard. Methane gases can be dangerous and high concentrations can displace oxygen in the air.

Alternatively, the putrid smell may also be an indication that there is a dead animal in your air conditioner. Small animals or insects may climb inside the vents when they are sick or injured and eventually die in there. The decomposition of their bodies can create a rotten smell, similar to the smell of sewerage. A trained professional can easily remove the animal or insect and improve your indoor air quality.


Is a smelly air conditioner dangerous?

While a foul smell being emitted from your air conditioner doesn't necessarily mean a health hazard, there's a good chance that there's something wrong with your unit. On the other hand, many smells coming from an air conditioner can be seriously dangerous, so it's a good idea to have air conditioner smells checked out as soon as they begin.

Can I spray Lysol in my air conditioner?

Many people think that spraying a disinfectant like Lysol into an air conditioner is a sure fire way to kill germs and neutralise any nasty smells. However, we can not stress enough how dangerous doing this can be. Lysol is highly flammable due to its high ethanol alcohol concentration. If the disinfectant comes into contact with the electric motor it can create a flash explosion that can start an electrical fire.

Time to call the experts?

If you are still having issues - it's time to call in the experts. Our Brisbane-based air conditioning repairs team are on hand to give you the assistance you need. Call us on 07 3283 5566 or fill out a quick quote to get your air con back up and running!

Experiencing other air conditioner issues?

Check out our other guides on common problems below:

My air conditioner heat mode is not working

My air conditioner will not turn off

My air conditioner is making a loud buzzing noise

My air conditioner is not turning on

My air conditioner is icing up

My air conditioner is blowing hot air

My air conditioner is leaking water

My air conditioner is sweating inside

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