Why My Air Conditioning Smells Bad – And How to Fix It

Summer is in full swing, which means one thing for many homeowners – air conditioning! While AC can make your home nice and cool, it’s not uncommon for it to produce an unpleasant smell.

If you’ve been noticing a bad smell from your air conditioning, don’t worry – we’re here to help. Keep reading our detailed guide to learn why your air conditioning smells bad, how to fix it, and when to call an HVAC professional.

a person covering his nose because the air conditioning smells bad

Air Conditioner Smells Musty

We’ve all been there. You come home from a long day at work, ready to relax in your cool, comfortable home, only to be greeted by a musty smell emanating from your air conditioner. Or maybe you notice bad smells right when you turn on your AC for the first time of the season. Either way, it’s far from a pleasant experience.

So why does this happen? Well, there’s actually a handful of things that can cause your AC unit to smell musty when it runs. Here are the top causes and how to fix each of them.

Clogged Drain Line

A clogged drain line is one of the most common causes of musty smells coming from AC units. The drain line carries away condensation that collects inside the AC unit. If it becomes clogged, water can start to back up and cause all sorts of problems, including mold and mildew growth, leading to that musty smell.

How to Fix

  1. Start by locating the AC unit’s indoor unit (usually located in your basement, attic, utility closet, or garage).
  2. Turn off its power, then remove the air handler’s access door.
  3. Underneath the AC coils, you will find the condensate drain pan. There should be a small cap or plug near the unit’s base. Remove this and check to see if there is standing water in the pan. If so, go outdoors and find where the drain line empties.
  4. Then, use a wet/dry vacuum to suction out the water.
  5. Next, flush the drain line with a garden hose to remove any debris that may be causing the blockage. If you can’t get it to drain, you will have to call an HVAC contractor.

High Humidity Levels

Another common cause of musty AC smells is high humidity levels. AC units work by removing moisture from the air while cooling the air in your home. But if the air outside is already very humid, it can be difficult for the AC unit to keep up. This can lead to condensation building up inside the unit and, eventually, that musty smell.

How to Fix

To fix this problem, you’ll need to lower the humidity levels in your home. One way to do this is to use a dehumidifier in conjunction with your AC unit. You can also try opening windows and doors to let in drier air from outside or running fans to help circulate the air in your home.

Dirty Evaporator Coils

a person using a soft brush to clean the dirty coils

The evaporator coils are responsible for removing heat and humidity from the air in your home. But over time, these coils can become dirty and clogged with dust and dirt. And with condensation collecting on the coils, this can lead to a breeding ground for mildew and mold.

Not only do dirty evaporator coils make it difficult for the AC unit to remove heat, but it also generates that familiar musty smell that we all know and love (and by love, I mean hate).

How to Fix

  1. Turn off the power to the AC unit.
  2. Next, remove the access panel to the coils.
  3. Use a soft brush to gently remove any dust or dirt built up.

Note: Don’t use a lot of force or abrasive material on the coils. If you damage them, you can cause a refrigerant leak which is very expensive to repair. Once the coils are clean, replace the access panel and turn on the power to the AC unit.

Standing Water in Drain Pan

As mentioned before, AC units have a drain pan that collects water that condenses inside the unit. If this pan starts to fill up with water, it can cause all sorts of problems.

How to Fix

To fix this problem, you’ll need to empty the water from the drain pan.

  1. Remove the access door of your indoor unit (remember to power it off first).
  2. Then, take off the small cap or plug near the unit’s base and check if there is standing water in the pan.
  3. Suction the water with a wet/dry vacuum.
  4. If the drain pan keeps refilling with water, read how to clear a clogged drain line above.

Frozen Coils

The coils in your AC unit can also become frozen, preventing the AC unit from removing heat from the air. This can lead to that musty smell and a whole host of other problems.

Note: Usually, frozen coils are a symptom of another problem. You might have a refrigerant leak, dirty coils, a clogged filter, a blocked condenser, a failing compressor, or humidity levels that are too high.

How to Fix

  1. Turn off the power to the AC unit.
  2. Next, locate the air conditioner’s air handling unit and remove the access panel.
  3. Place a fan blowing into the panel opening to help melt the ice. You can also try a hair dryer to speed things up, but we don’t recommend it. Electronics plus water and likely no GFCI outlet nearby is a bad accident waiting to happen.
  4. Once the ice on the coils melts completely, clean them with a soft brush and replace your filter if it’s dirty too. Turn your AC back on, cross your fingers, and hope your AC returns to its normal cooling with no bad smells.

If the coils refreeze, you should call a local HVAC technician to inspect and repair them.

Clogged Filter

man washing a reusable ac filter

A dirty air filter is another common cause of musty smells coming from AC units. The air filter’s job is to trap airborne particles like dust, pollen, and pet dander, so they don’t circulate through your home.

However, if the filter gets too full, it can start to restrict airflow, collect moisture, and cause many issues. Keeping your filter clean is one of the best preventative maintenance tasks to avoid other problems with your air conditioning system.

How to Fix

  1. Start by locating the AC unit’s indoor blower unit. There should be an access panel that you can remove to get to the filter.
  2. Once you have the filter in hand, check if it’s visibly dirty. If so, simply replace it. But, if you have a cleanable filter, wash it with warm, soapy water and let it air dry. Portable air conditioners usually have washable filters, while central AC units typically use replaceable air filters.

Air Conditioning Smells Like Eggs

If your air conditioning is smelling like eggs, but no one in your house is cooking up some omelets, your AC unit can be the cause. Here are the top reasons why your AC might smell like rotten eggs.

Gas Leak

If your AC unit smells like eggs, it could be a sign of a natural gas leak somewhere in your home. Air conditioners don’t use natural gas, but the leak could be occurring elsewhere in your home and getting sucked into the air vents and then recirculated by the AC system.

Note: Natural gas is odorless, but utility companies add a sulfur-based compound (which smells like eggs) to it so people can smell it if there’s a leak.

How to Fix

If you have a natural gas leak, you should evacuate the premises immediately and call 911 and your local utility company. Do not turn on any lights, appliances, or AC units as this could create a spark and cause an explosion.

Refrigerant Leak

a technician checking for a refrigerant leak

Another reason your AC might smell like eggs is a refrigerant leak. The refrigerant in your AC unit helps remove heat from the air. It is also what makes your AC unit cold. If there’s a leak, the AC unit won’t be able to cool properly, and the musty smell could be coming from the refrigerant gas itself.

How to Fix

If you think you have a refrigerant leak, it’s best to call a local HVAC technician. They will be able to repair the leak and recharge your AC unit with refrigerant.

Overheating AC Unit

AC units have a lot of components. If they overheat, they produce an egg or burnt sulfur smell.

How to Fix

Start by checking the air filter and ensuring it’s clean. A dirty air filter can restrict airflow and cause the AC unit to overheat. If your AC has an outdoor unit (the condensing unit), check for blockages. This component expels the heat from your home outdoors. If it is blocked, your AC unit could overheat.

If the AC unit is still overheating, it’s best to call a local HVAC technician. They can diagnose the problem and make any necessary repairs.

Critters and Insects

a technician checking for critters and insects inside the AC

Another reason your AC might smell like eggs is that critters or insects are living (or dying) in your AC unit. Mice, rats, and other small animals can crawl into the AC unit and die, causing a foul odor.

Insects can also get into the AC unit, ductwork, and vents and build nests, causing a musty or eggy smell and poor airflow.

How to Fix

If you think there are critters or insects in your AC unit, start by checking the air filter and making sure it’s clean. A dirty air filter can provide a hidden place for critters to build their nests.

Next, check the AC unit’s outdoor unit to ensure its coils are clear of dirt, leaves, branches, etc. If there is anything blocking it, remove the obstruction and turn on the AC unit.

Note: If you still think there are critters or insects in your AC unit, it’s best to call a local pest control company. They have scopes that they can use to view the inside of your ductwork, walls, etc., and can diagnose the problem and make any necessary repairs.

Sweating Ducts

Another potential source of that egg smell coming from your AC unit is sweating ducts. Ductwork is made of metal, and when the AC unit is running, the air inside the ductwork will be cold. If the air outside the ductwork is much warmer, the outside of the ductwork can sweat (just like a glass of ice water sweats).

This formation of condensation can drip inside your walls and lead to a musty smell and mold and mildew growth. And, if the problem is bad enough, it can also lead to water damage.

How to Fix

  1. Check the AC unit’s outdoor condenser unit and make sure nothing is blocking the airflow. If there is, remove the obstruction and turn on the AC unit.
  2. Next, check the AC unit’s evaporator coils and make sure they’re clean. Dirty coils can restrict airflow and cause the AC unit to work harder and sweat more. You can also try using a dehumidifier to reduce condensation on your ductwork too.

If your ductwork continues to sweat, it’s best to call a local HVAC technician. They can investigate the problem and make any necessary repairs.

A Fresh Smelling AC

If your AC unit smells bad, it could be due to various problems. The most common cause of a bad smell is a dirty air filter. But, other causes can include a natural gas leak, a refrigerant leak, overheating, a clogged drain line, or critters and insects. You can use our tips above to eliminate these foul odors and get your home smelling fresh and clean again.

If you can’t figure out why your AC unit smells bad, it’s best to call a local HVAC technician. They can diagnose the problem and make any necessary repairs.

About The Author

Jonathon is a mechanical engineer with over ten years of experience in the HVAC industry. He has hands-on experience with all types of HVAC systems.

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