Air Conditioner Smells Bad When Turned On

Air conditioners are intended to provide cool air during the warm weather months, but there are various potential problems that can occur when using this type of mechanical device. For instance, when an air conditioner emits a foul odor, it could be an indication of a more serious issue.

There are several different reasons why an air conditioner might emit a foul odor when turned on. One of the most common is that the unit needs to be cleaned or serviced, as this can help prevent bacteria from building up inside and cause bad smells while running.

Another reason could be due to a problem with the compressor itself; if it has been damaged by moisture or other fluid leaks then this will cause an unpleasant stench during operation.

As the weather gets warmer, more people are turning on their air conditioners to cool down their homes. Making sure your air conditioner is working well is important, but if you turn it on for the first time and notice strange smells coming from it, you should have it checked out to determine the problem.

Common A/C Unit Smells

a white ac on wall

Mildew Smell

If your air conditioner smells like a wet towel, it may be because mildew, mold, or fungus is growing inside or on the outside of the machine. The stench of mildew coming from your air conditioner may be the result of improper drainage.

This keeps moisture in the system, which gives off the smell. Besides the compressor not draining well, your air conditioner may need its filters to be cleaned or changed.

Decay Smell

The decomposition of an animal within your air conditioner or HVAC system will cause it to smell like rotten eggs or decay. This probably happened in the winter when your air conditioner wasn't running. If you smell this smell coming from your unit, don't try to clean out the dead body yourself.

Many dead animals carry diseases or other health problems, so getting rid of them could be hazardous for your health. Instead, you should call a professional HVAC technician for help.

Sewage Smell

A sewage smell coming from your air conditioner can be dangerous, just like a smell of rotten eggs. If you smell sewage when you turn on your air conditioner, your sewer system may have backed up into or near your unit, or a pipe may have burst and let sewer water into or near your unit.

Gunpowder or Electrical Smell

Many older homes have electrical problems, so it's also possible that an older air conditioner might make a smell that seems like something is burning. One way to get rid of a burnt smell coming from an air conditioner is to replace a burned-out blower fan.

A short circuit board may also cause an air conditioner that hasn't been used for a long time to smell like fire when it's turned on after being off for a long time.

Gas Smell

If you switch on your air conditioner for the first time and smell gas burning, it could be a minor issue or a serious one. Dust settling in the heat exchanger HVAC system, for example, can give off an odor similar to that of combusting gas. However, knowing where the odor is coming from is essential for figuring out whether or not you have a serious condition that might require assistance.

Dust is probably to blame for the vents' gas-like odor. However, if the odor is not coming from the vents, a leaky pipe fitting or a leak in the A/C unit's equipment may be to blame. In the event of a gas leak, it may be necessary to immediately involve other parties, such as your gas company, therefore it is best to call in a professional specialist to help you locate the leak and fix it.

Oil Smell

An oil leak at the unit could be the cause of the greasy odor emanating from the air conditioner. A technician can ensure the leak stops by tightening fittings on the unit, such as those located at the tank, the oil line, and the filter.

A broken oil burner is another possible source of the oily odor emanating from an air conditioner. An oily odor coming from your appliance may indicate a clogged burner or a failing fuel pump, both of which require immediate attention.

Unusual noises coming from the device could potentially indicate other issues. An air conditioner that gives off an oily odor should be checked by a professional HVAC specialist.

Old Cigarette Smell

If the air conditioner has been turned off for some time in a property where smokers currently dwell or previously did so, the new occupants may notice a stale cigarette odor. A specialist with knowledge in A/C maintenance should inspect your unit and clean the evaporator coil if you notice a musty odor coming from the system.

Reasons Your Air Conditioner Gives Off a Bad Odor When Turned On

Dirty Air Filters

Air filters naturally gather dust and other particles from the air, but if they are not changed at regular intervals, mold and mildew can grow on them, spreading unpleasant scents throughout the house.

The efficiency of your air conditioner may suffer and a host of other issues may arise, if the air filter is dirty. As part of your regular maintenance, make sure to inspect and replace your air filters.

Dirty Evaporator Coil

A dirty condenser coil is another thing that can make an AC smell bad. The liquid refrigerant is turned into gas in the condenser coil so that cool, clean air can flow through your home. In some cases, though, these coils can get too wet, which lets mold and mildew grow on them.

You can sometimes clean these coils on your own, but it's always best to call a pro. Find the coils, which are usually hidden behind a panel on the inside or near your air handler (the inside unit), and clean them with a soft brush, warm water, and a small amount of mild detergent.

Problems in Your Ductwork

If your air pipes are broken or have leaks, pathogens from outside can get in and grow mold and mildew, which will make your home smell bad. Leaky air pipes show themselves by not cooling the same way from room to room and not cooling well even after the AC has been on for a long time.

Even though you can fix small tears on your own, it's usually best to call an HVAC expert. They can clean the ducts with special, safe tools and fix them so they don't leak or get damaged again. They will also be able to look at the system to see if there are any other problems.

Clogged Drain Lines

Microorganisms like algae, mold, mildew, bacteria, and more can grow in your water drain line if it gets clogged. During the heat exchange process, these drain lines are supposed to take any extra water from your unit.

However, problems can happen if they don't work right, get damaged, or just wear out over time. Most clogs can be fixed at home with a few simple household things and a little bit of work. But if the problem is more serious, you may need the help of a professional.

Stagnant Water in Drain Pan

If the drain lines get clogged, water can collect in the drain pan of the AC itself. Water that doesn't move is a good place for mold, fungus, and other unpleasant things to grow. If you see water in your air conditioner's drain pan, you can get rid of it with a towel or a wet/dry vacuum and clean the pan with vinegar, bleach, or something similar. You'll need to clear your drain line or call a professional to figure out what's wrong.


Air conditioner problems can come in all shapes and sizes. The air conditioner smell, however, is a common problem shared by many homeowners. We hope that this article helped answer questions that you may have had, and we hope that you now have a better understanding of what is happening in your AC unit.

GQ Heating and Air is a full service HVAC company that provides maintenance for all of your air conditioning equipment. Our experienced technicians have been trained in the most advanced techniques for maintaining your system, which means they can ensure it will continue to run efficiently and effectively. We are dedicated to providing you with the best Air Conditioning services such as:

  • Air Conditioning Installation
  • Air Conditioning Repair
  • Ductless Air Conditioning
  • Heat Pump Systems
  • Thermostat Services
  • Zone Control Systems

Our team of experts will carefully inspect your system to identify any potential problems before they occur, so you can rest assured knowing that your system is being taken care of!

Schedule your consultation today at [email protected]!

We offer affordable rates for all types of repairs and installations

We go above and beyond for every project to exceed your expectations while providing superior service

✆ 714-389-3925

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