What is That Odor Coming from the AC? Bad AC Smells Explained

Living in Houston, Texas, it is not uncommon for air conditioning units and heat pumps to give off  certain odors during the first few uses each season. While a musty smell from your AC or heater is normal when you first turn it on after not using it for a while, there are other odors that may indicate a more serious problem.

Let’s take a closer look at the different air conditioning smells, their possible causes, and when you should reach out to a local HVAC expert for assistance.

Common Types of Bad AC Smells

1. Dirty Socks or Smelly Feet

One of the most common and unpleasant odors is the “dirty sock smell” that reminds homeowners of, well, dirty socks, dirty gym clothes, vinegar, and smelly feet. This musty smell is common and is usually a result of the buildup of moisture combined with mold, mildew and dust.

It occurs when mold and bacteria build up on the evaporator coil or other areas of your air conditioning system due to long periods of inactivity or when your condensate drain line is clogged. Fortunately, this is usually not a dangerous issue. However, it can be annoying, embarrassing, and disruptive to your comfort throughout the cooling season.

What You Can Do

  • Start by cleaning or replacing your air filters, as they act as the first line of defense against mold, dust and bacteria. They should be changed once every three months or more often.
  • Make sure you are maintaining your condensate drain by pouring a cup of plain white vinegar (not bleach) down the drain each season.
  • Scheduling regular maintenance in the fall for the heater and spring for the AC can prevent these types of odors.

How Your AC Technician Can Help

  • If the smell persists, it’s best to contact your local HVAC contractor to clean your evaporator coil, inspect your ductwork, and look at your condensate drain.
  • If you notice that musty smells tend to return seasonally, ask your favorite HVAC contractor about options for UV lights and whole home air purification systems. These strategies can be particularly helpful for those with asthma or allergies and can improve your indoor air quality.
  • Regular maintenance on your air conditioner or heat pump can deter the development of bad smells coming from your AC.

2. Burning Smells Coming From Your AC

If you notice a burning smell coming from your air conditioner, it is crucial to turn off the system immediately and contact an HVAC technician for assistance. A burning smell may indicate frayed wiring that has caught fire, an overheated motor, a circuit board malfunction, or another hazardous electrical issue. If the smell persists or gets worse, you may need to call the fire department.

  • Burning Dust – A burning smell after an air conditioner or heater has not been used in a while is common. This odor may be concerning when you smell it but should go away quickly.  It is usually no cause for alarm. (This smell is most prevalent when heaters start-up in the winter.)
  • Burning Plastic – An air conditioner or heat pump has many electrical components and wires covered with plastic. If you can smell plastic burning and your AC is starting to act up, turn your system off to avoid any further damage or at worst, an electrical fire.
  • Burning Rubber – This may indicate a broken fan belt if you have a particularly old machine or a commercial system. A malfunctioning fan will cause your AC system to shut down and not cool properly.

What You Can Do When You Smell Burning

  • Document what you are smelling and call a local AC repair company.
  • Turn off your air conditioner or heat pump and notice if the smell goes away or gets worse.
  • Schedule regular pest control to shore up areas where rodents can burrow into your AC and ductwork and cause problems.
  • If the smell gets worse and/or you also see smoke, call the fire department.

How Your AC Technician Can Help

  • Licensed AC technicians will give your system a once-over to determine the cause of the burning smell. Sometimes burning smells are a result of an electrical problem from a part that has worn out and sometimes these issues can be caused by rodents who find themselves in the wrong place at the wrong time. Because electricity can be dangerous to handle, call an expert to get to the bottom of what is going on for your own peace of mind.
  • Regular AC maintenance can often spot parts that may be starting to fail before they actually fail. In this way, we can avoid AC breakdowns before they become an emergency.
  • Also, it bears repeating, but always make sure you have enough working smoke alarms in your home. They should be replaced every five years.

3. Trash, Rotten Eggs, Sulfur or Skunk Smell

Another unfortunate but common odor associated with air conditioners is a putrid, trash smell. Homeowners describe it to us as smelling rotten eggs, sulfur or skunk spray. There are a few different causes for these terrible smells.

  • If a rodent or other critter was seeking shelter or looking for water around air conditioning units or ductwork, a foul smell could indicate that they have passed away inside or nearby. We have seen and removed lizards, snakes, and rodents of every variety. Electrocution is often the cause of death.
  • If you’re still smelling a putrid smell, it could indicate a gas leak in your furnace or other gas appliance. Natural gas and propane providers add Methyl mercaptan (methanethiol), a strong odorant that smells like rotting trash, cabbage or garlic, to make gas leaks easier to identify. Rob says it smells like cut bell peppers and onions. In such cases, turn the gas off immediately. There is a cutoff for each gas appliance and a cutoff at the home’s gas meter. The Fire Department will respond if you have a suspected gas leak if you dial 911. Turn off your air conditioner, so it will stop circulating air around the home and make gas leaks harder to find.  If you’re unsure which appliance is the culprit, call a plumber. If you suspect, the leak is related to your furnace, call an HVAC professional to resolve the issue.
  • Keep in mind that heat pump systems are all electric, so they will not be the source of a gas leak. Other household appliances such as your gas oven or stovetop, clothes dryer or water heater may also be the culprit.

What You Can Do When You Smell Trash Coming from Your AC Vent

  • If you suspect a gas leak, leave your home to call for help. Gas buildup in a home can be dangerous and explosive. Call 911, Fire Department and your favorite plumber or HVAC company right away.
  • If you suspect you have a dead critter filling your indoor air with really putrid smells, you can get in touch with a pest control service and your HVAC company. Either should be able to help you get to the bottom of the smell.

How Your AC Technician Can Help

  • A licensed AC technician can safely inspect your system and determine the cause of the malfunction and the source of the foul odor.
  • AC companies recommend heaters and gas appliances such as furnaces receive a yearly inspection in the fall. This can prevent catastrophic failures from faulty furnaces, gas leaks and other safety issues.

4. Exhaust Smell from Your AC

Although exhaust fumes are typically associated with cars, they can also be detected by some from an air conditioning unit or heat pump. If you notice an exhaust or chemical smell, it could indicate refrigerant fluid leakage into the motor or another warm part of the system. This is extremely rare for most people to smell.

Some describe the smell of refrigerant as a sweet, chemical smell. That type of smell often means your AC or heat pump system has sprung a big leak and is losing its refrigerant charge.

What To Do If You Smell Exhaust or a Chemical Smell Coming from the AC Outside

  • To prevent further damage to your unit, turn it off immediately and call an HVAC professional for assistance.
  • Refrigerant and other by-product gasses from exhaust can be dangerous to breathe in high concentrations, so move to a place where the air is clean such as outdoors.

How Your AC Technician Can Help

  • A licensed AC technician can inspect your system and determine the source of the exhaust or chemical smell if it is coming from your heating and cooling system. They can also gauge your refrigerant charge to see if that could be part of the problem.
  • We recommend smoke and carbon monoxide detectors in every home as an extra safeguard. We carry these on our trucks at all times. They add an extra layer of protection for homes using gas-powered appliances, like furnaces. Carbon monoxide is a by-product of exhaust, and it can cause poisoning making you feel sick to your stomach or woozy. It can cause death at concentrated levels. Unfortunately, it does not have a characteristic smell.
  • Call 911 if you have a carbon monoxide alarm going off in your home. The Fire Department will respond.

5. Cigarette Smell Coming From Your AC

We sometimes hear complaints about cigarette smoke coming from air conditioning vents.  Your air conditioner cannot generate these odors by itself. What usually is happening is that someone is smoking near an air intake and the air conditioner is distributing the air around the home.

Even if you have filters, it may not filter out all of the smoky smell. Another cause for cigarette smells is the build-up of cigarette smoke residue (tar and byproducts) within your AC system and ductwork. It is very difficult to remove nicotine and cigarette smells from homes that have been smoked in regularly.

What You Can Do

  • Changing your air filters more frequently can help, but it may not completely solve the problem.
  • Ask guests who smoke to smoke outdoors, so cigarette smoke odor does not get into your home.

What Your AC Technician Can Suggest for Reducing Smoky Odors

  • We often suggest a comprehensive duct cleaning from a duct cleaning specialist to help remove cigarette smoke odors, but it can be difficult to completely remove them all.
  • Another solution we suggest is installing a whole home air purification system. Active air scrubbers are far superior to passive disposable air filters. Power Vac of America, Inc., is who we recommend for duct cleaning.

Common Fixes for Most Bad AC Smells

When your air conditioning unit emits unpleasant odors, it’s best to have a professional determine the cause. However, there are some troubleshooting steps you can take to mitigate the smell while you wait for an AC technician to respond.

1. Call for Professional Help

Be sure to document what you are smelling, when it started, and where it seems to be coming from if you can determine the source. This type of information can be invaluable when you call an AC technician to help you get to the bottom of the bad smell problem.

2. Check the Condensate Drain

The condensate drain pan located underneath the evaporator coils can develop mold due to normal wear and tear and the drain line can become clogged up. An AC technician can check to see if this is the problem. They can blow out your drain if it is clogged. Regular maintenance on your condensate drain is an easy DIY task.

3. Address Excess Moisture or Humidity Problems

Excessive moisture in the ducts, water leaks and uncontrolled humidity can result in a musty, mildewy or dirty sock smell. This can arise also because you have the wrong size air conditioner or heat pump. Contact an HVAC professional to help you get to the bottom of this type of problem and to eliminate the odor. Excess humidity is a common problem on the Texas Gulf Coast.

4. Inspect the evaporator coils

An AC technician may also ensure that the evaporator coils are not frozen because this situation can lead to excess moisture and bad odors. Dirty air filters can prevent proper airflow and cause water droplets to accumulate and freeze on the coils, leading to the evaporator coil freezing up. Clean or replace the air filters regularly to prevent this issue. A refrigerant leak can also be responsible for frozen evaporator coils.

Schedule Seasonal Maintenance for Odor Prevention

To maximize the performance of your air conditioner and prevent unwanted odors and breakdowns, AC technicians and manufacturers recommend a regular inspection and service for your heating and cooling system.

Recommended AC Maintenance Schedule

  • Air Conditioner or Heat Pump: Schedule service in the spring before the hot summer weather arrives.
  • Furnace or Heater: Schedule service in the fall before the cooler weather sets in. In the Houston area, that timing generally starts in November.

By following these maintenance guidelines and promptly addressing any unusual odors from your air conditioning unit, you can ensure a comfortable and odor-free environment in your Houston home. Remember to rely on the expertise of local HVAC professionals, such as Ruud Pro Partner™ Terry’s A/C & Heating, for comprehensive inspections and solutions to maintain the efficiency and performance of your HVAC system.

Weird Smells Coming from Your AC? We Can Help You With That.

Terry’s A/C & Heating has been helping customers in Houston, Texas from the West End in Memorial to Fort Bend County’s Katy, Richmond, Fulshear, Sugar Land and points nearby since 1981. Our family-0wned business is a Ruud Pro Partner™ and we provide air conditioning service and replacement for all major brands.  Call us at 281-495-7830 to get on our schedule and let us help you get to the bottom of a pesky AC odor quickly!

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