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My Air Conditioner Smells: What’s Wrong?

an air conditioning vent that is blowing smelly air through the home

If your air conditioner smells when it runs, there is a good chance that something within the system is broken and needs to be fixed. In some cases, it’s actually a sign of a problem that could be making your health worse. There’s also a chance that it’s something simple you can resolve on your own. Either way, it’s important to do some investigation and troubleshooting. The best option is always to have your AC system checked out by a heating and cooling specialist, to make sure everything is working as it should.

Musty or Moldy Smells Coming From the AC

Humidity within the AC system is one of the most common causes of bad smells coming from the AC when it runs. Many people don’t realize that the AC doesn’t just cool the air in your home on a hot day. It also removes humidity. When there’s too much moisture in the system, it means the AC isn’t working properly. Excess moisture can lead to mold and mildew growing inside the ductwork—the network of ducts that carry cooled air from the AC unit to the rooms of your home.

Excess moisture in the system causes musty, moldy smells to come out of the vents when you turn on the AC. Some people describe these smells as the smell of dirty socks or stinky feet. This moisture buildup can be caused by several things, including the following:

  • A clog in the AC drain line causing water to build up in the drain pan
  • Dirty evaporator coils
  • Dirty AC filters—especially if the bad smell comes out the first time you’ve turned on the AC in warmer weather, but then goes away after it runs for a while
  • A water leak somewhere in the AC system

A professional should evaluate and fix any leaks. A pro can also clean out your AC ducts, clear drain clogs, clean your AC coils and resolve any mold or mildew problems in the system.

Note that dirty AC filters can cause a lot of other problems besides just a musty smell in your home. Cleaning or replacing your AC filters is a simple job that most homeowners can easily handle on their own. This should be done at least once a month. It will need replacing more often if you use the AC a lot, if you live in an especially dusty area or one with lots of pollen or if you have pets that shed lots of hair indoors. Not changing out the filters can significantly shorten the life of your AC over time.

Burning Smells Coming From the AC

If the bad smell coming from your AC is more like a burning smell, it could mean that one of the AC system’s electrical components is burning out or going bad. This is potentially a very serious problem, so it’s best to turn off the AC and call a professional as soon as possible to take a look. Remember that if something within the AC is burning, it could start an actual fire in your home.

When there’s a burning smell coming from the AC, it could mean there is a wiring problem, or a motor somewhere within the system is overheating. There could also be a short in the circuit board or within the fan motor. Another possibility is plastic parts within the AC that are overheating and melting.

Any of these could be a very dangerous situation. That’s why it is important to turn off the power to the AC. Then, contact a reputable AC professional as soon as possible to come to inspect the system and find out where the problem lies.

Cigarette Smoke Smells Coming From the AC

If anyone in your home smokes cigarettes—or someone who used to live there did—the smell of smoke could be trapped in the evaporator coil and the filter. The smells from this could persist, even if there’s no longer anyone in the home who smokes. Changing the AC filters will help. The best thing to do if you’re smelling cigarette smoke is to have your AC system checked and thoroughly cleaned by an HVAC professional.

Sulfur or Rotten Egg Smells Coming From the AC

If you smell rotten eggs or sulfur when you turn on the AC, this could be a sign of another serious situation. A sulfur smell is often caused by a gas leak. If you smell this scent when you turn on the AC, turn it off immediately and reach out to a heating and cooling specialist for help.

A Sweet or Chemical Smell Coming From the AC

When there’s a sweet or chemical smell coming from the AC, this could be caused by a refrigerant leak. A refrigerant leak will also cause other symptoms, like your AC failing to cool your house down properly, or running for too long as it struggles to cool the home. You might also notice ice on the evaporator coils in the outside unit or high electricity bills. A trained HVAC professional should always fix a refrigerant leak.

Other Smells Coming From the AC—Sewage or Dirty Trash

People report all kinds of funny smells coming from the AC. Each smell indicates something, and it’s not always easy to figure out what’s going on. In most cases, it’s best to call in a professional to solve any AC issues you may be having.

If you smell a stinky sewage smell while your AC is running, it could be from sewer gasses that are backing up into your home. This could be caused by dry P-traps, a ruptured sewer vent pipe or a backed-up sewage line. A reputable plumber or HVAC professional will be able to identify the cause and figure out the right solution.

If you smell something rotten coming from the vents when you turn the AC on, it’s possible that an animal crawled into the AC ductwork or one of the units and died. Even a small animal, like a mouse, can give off a powerful stench when it is decomposing. A heating and cooling professional can take a look at your entire AC system and figure out where the bad smell is coming from.

How Long Do AC Capacitors Last?

If you think your AC capacitor may have gone bad, it’s time to reach out to a heating and cooling specialist for help. Pros have the knowledge to diagnose and repair any part of your AC system that needs fixing, including a burned-out capacitor. But how long do AC capacitors last?

The capacitor is the part within the AC that gives a surge of power to the unit when it first turns on. After that initial power surge, the capacitor keeps running throughout the AC’s cycle, just at a lower level. If the capacitor is going bad, the AC will struggle to start up, or won’t start up at all.

The right capacitor should last for 10 to 20 years without any problems. Sometimes, capacitors fail sooner than that because of issues like heat exposure or the wrong capacitor for the particular AC system. These factors can shorten the capacitor’s lifespan.

If the capacitor is going bad, the AC won’t start up right away, or at all. The capacitor itself will probably look swollen, and you may see fluid leaking out of it. Even if you can locate the capacitor yourself and see that it needs replacing, doing it yourself is typically not a good idea, for several reasons. For one thing, there are several different types and sizes of capacitors. Choosing the wrong one for your AC system is an easy mistake to make, and will lead to further problems for your AC.

Additionally, since the capacitor stores electricity, it is a dangerous part to try to replace yourself. Since the capacitor stores energy, even after you shut off the power to the AC, the capacitor still holds a charge. Handling it carries a real risk of electric shock. If you believe your AC capacitor has gone bad, it’s best to reach out to an HVAC professional for help. They can provide the necessary AC repairs or part replacements.

an ac unit with a bad ac compressor

What Does a Bad AC Compressor Sound Like?

When your AC compressor starts going bad, you may notice certain sounds and other indications that can point to this particular problem. The compressor is the outdoor AC unit. So what does a bad AC compressor sound like? When the compressor is going bad, you may hear a whistling or squealing sound coming from the unit when the AC is running. A high-pitched sound like this could indicate a worn bearing somewhere within the compressor.

Other AC noises you might hear are growling or grinding noises. This would indicate not just a worn bearing, but a bearing that has seized up inside the unit. Another possibility is rattling or knocking noises, which could mean that there’s a loose or unbalanced part within the unit—possibly a crankshaft or connecting rod or a piston pin.

If you start to hear any of these common types of noises coming from the outdoor unit when your AC is on, it’s a sign that the compressor isn’t working. To get your AC back up and running, it’s best to rely on a heating and cooling specialist. If you can have the unit serviced early enough, the AC specialist may be able to fix the part that has broken. If the problem goes on for too long, it can cause the whole unit to fail, in which case you would need to replace the compressor itself.

Your AC system is one of the most important—and most expensive—systems in your home. Keeping it in good condition will extend the life of the entire system. Hiring an HVAC specialist for regular AC maintenance is a savvy way to protect your AC and your home. Staying on top of problems as they occur will help you head off bigger, more costly AC repairs down the line.

ABC Can Keep Your AC Working Efficiently

Dealing with AC issues requires lots of technical know-how. Instead of trying to figure it out yourself, contact ABC Home & Commercial Services. Our professionals can efficiently diagnose and repair your AC problem.

Stephen Richardson

Stephen Richardson is the HVAC Director for ABC Dallas and Fort Worth. Stephen has over 37 years of experience in the HVAC industry and has been with ABC for 21 of those years. Before joining ABC, Stephen held positions as a Building Engineer, Refrigeration Service Technician, and AC/Refrigeration Mechanic. In his off-time, he enjoys cooking, restoring vintage tractors, and SLR photography.

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