A clean air conditioner with no mechanical faults shouldn’t give off a foul odor. If you smell dirty socks, rotten eggs, or another bad aroma coming from the unit, it’s a sign that something is wrong.
There are many possible explanations for a smelly AC, from mold and mildew growth to electrical issues and pests. In this guide, we’ll explain what each type of odor means and how to deal with it.
What Are Common Air Conditioner Smells and Their Causes?
An experienced HVAC technician can often diagnose air conditioning malfunctions just by how a home smells when they enter. Here are the most common odors homeowners report smelling, why they happen, and how to fix them.
Musty or Dirty Sock Smells
Believe it or not, “dirty sock syndrome” is a real AC problem. It refers to the AC emitting a foul odor when it’s first turned on, typically in the spring. The odor is caused by mold and bacteria buildup in your HVAC system. If you don’t change your filters frequently, contaminants can enter the unit and build up on the evaporator coil.
Rotten Egg Smells
If you own a gas furnace or other gas-powered appliances, be careful if you smell rotten eggs. Utility companies use an additive that makes natural gas smell like sulfur or rotten eggs to alert people of a gas leak. Without that additive, natural gas is odorless and colorless, and you would have no way of knowing a leak is happening.
When your home is filled with gas, even a tiny spark from static could cause a massive gas explosion. If you smell rotten eggs, leave your home immediately and contact your gas company for an inspection.
Burning or Fishy Smells
If you smell a mild burnt fish odor, that could be a sign an electrical component in your air conditioner is burning or melting. Electrical insulation gives off a fishy scent when it heats. When you smell this odor, turn off the air conditioner and contact an electrician or HVAC contractor. Running the unit with melting wires could lead to damage or a housefire.
Decomposition Smells
It’s common for small animals to dig through your air ducts and get stuck inside, resulting in a horrible rotting stench. If you think a dead animal is in your ductwork, you’ll probably need professional help to remove it. An HVAC contractor can locate the animal, remove it, clean the ducts, and repair any damage.
Mildew and Moldy Smells
Mold growth can build on dirty air filters and spread to other parts of your HVAC system. Smelling mold can also mean you have an overflowing condensate drain pipe or that condensation on your ducts is causing water damage. If your condensate drain line has a clog, you may be able to ease it out yourself.
Dirty drain pans can also emit odors, so clean and sanitize them regularly.
Sweet Smells
If you smell something sweet near your air conditioner, it could mean your AC unit is leaking coolant. The United States Environmental Protection Agency (EPA) regulates who can perform repairs involving refrigerant, so you’ll need to have a Section 608-certified technician patch the leak.
Dusty and Musty Smells
Smelling dust could mean your ducts are due for a cleaning. Dirty and improperly installed air filters can also cause your home’s air to smell musty. Try changing your air filters to see if that makes a difference. If it doesn’t, schedule an appointment with an HVAC contractor.
Plastic and Chemical Smells
When you first run a new air conditioner, plastic and chemical smells are common. Give the unit some time to breathe and cycle through the air in your home. If the smell remains after a few days, contact your HVAC technician and ask them to check for rubbing or burning components.
How To Prevent Bad Air Conditioner Smells
Buildup forming on your evaporator coil, in your ductwork, or on other components can make your AC stink. To keep your AC system smelling fresh, follow these tips:
- Change your air filters every 30–60 days.
- Use the correct filter sizes and install them properly.
- Have a licensed HVAC technician perform biannual tune-ups on your HVAC.
- Have your air ducts professionally cleaned every 3–5 years.
- Clean your condenser and evaporator coils once a year.
- Repair damaged air ducts.
- Keep air vents clear and free of dust and debris.
- Open your windows to let fresh air into your home.
- Seal holes in your attic to keep critters out.
Even if you do everything right, an older air conditioner can start to stink over time and worsen your indoor air quality. Using an air purifier in your home helps keep your air from smelling stale.
DIY Solutions for Minor AC Odors
While some AC issues require professional help, homeowners can often address minor odor problems themselves. Here are some DIY solutions for common AC smells:
- Clean the condensate drain line: A clogged condensate drain can lead to water buildup and odors. Typically, this line is a PVC pipe near the outdoor unit. Pour a mixture of equal parts water and white vinegar into the line, let it sit for 30 minutes, and flush it with clean water.
- Sanitize the evaporator coil: Dirty evaporator coils can breed mold and bacteria. To clean them, turn off your AC system, remove the access panel, and gently brush away any visible debris. Spray the coils with a commercial coil cleaner, then rinse them thoroughly with clean water.
- Change the filters: Fresh filters can significantly improve your home’s air quality. Once you’ve found the right filter for your AC model, install it according to the instructions on the frame.
Contact a professional technician for the following issues:
- The AC unit is making unusual noises.
- There are signs of significant mold growth within the system.
- The smell persists after you’ve tried DIY solutions.
- There’s a suspected gas or refrigerant leak.
- You smell a burning odor.
