Reasons Your Air Conditioning Unit May Smell Strange

March 6, 2024

Have you started noticing unpleasant odors whenever your AC is running? Bad smells coming from an AC are a common problem and can be due to something minor or potentially indicate a more serious issue. The good news is that determining what is causing the odors is often fairly easy, and most of the causes are also fairly easy to prevent and overcome.

1. Dirty Evaporator Coil

If the cool air flowing out of your air conditioning smells moldy or musty, it most commonly indicates that the AC evaporator coil is dirty. The evaporator coil is the main interior component of a central AC system and is located within the air handler compartment that also houses the blower or fan. When the system is running, cold refrigerant continuously flows through the evaporator coil and absorbs heat from the air moving over the coil. Excess moisture in the air also constantly condenses on the coil and then drips down into the drain pan situated underneath it.

The combination of moisture on the coil and the warm air moving over it produces the ideal conditions for mold, mildew, and bacteria growth, all of which can contribute to moldy and musty odors. Dirt and dust also continually build up on the evaporator coil and can also result in your AC giving off musty odors.

Scheduling an annual AC tune-up is an easy way to both prevent and overcome these odors since part of this service includes completely cleaning the evaporator coil. Having the evaporator coil cleaned regularly is also important for ensuring your AC cools as effectively as it should and will also help to prevent the coil from freezing up.

If you frequently have issues with mold and mildew forming on the evaporator coil, you may want to also install a coil-sanitizing UV light unit. This type of unit is mounted inside the air handler so that the ultraviolet light it produces shines directly down onto the coil. UV light has extremely powerful germicidal and fungicidal properties and will eliminate any mold, mildew, and bacteria on the coil. The light shines on the evaporator coil 24 hours a day throughout the entire air conditioning season, which means it will also prevent any new growth from forming. The process works because when microorganisms like these are exposed to UV rays, their DNA gets altered in a way where they can no longer reproduce and are effectively destroyed.

2. Clogged Condensate Drain Line

The warm air and moisture can also often lead to mold, mildew, and algae forming inside the drain pan and condensate drain pipes, which can also cause your AC to smell moldy and musty. When the condensate drain system is working correctly and drains moisture away as it should, you usually won’t have any issues. The problem is when mildew, algae, and slime start to clog up the drain pan or drain lines so that the system no longer drains properly. This then leads to standing water in the system that allows much more bacteria, algae, mildew, etc. to grow and start giving off nasty odors. These smells will be most noticeable when you’re standing near the air handler and condensate lines, but you may also notice the smells wafting up through your vents as your AC runs.

If you’re facing this situation, you’ll want to have a technician clean and inspect the entire condensate drain system since leaks in any of the drain lines can also contribute to mold growth and nasty odors. This is another issue that you can typically prevent with annual AC maintenance since the technician will usually clean the drain pan and drain lines. There are also drain pan tablets you can use to prevent odors, as they are specially formulated to prevent mold and anything else from growing inside the pan.

3. Dirty Ductwork

Dirty ductwork is another issue that can contribute to bad odors and cause the air coming out of your AC system to smell musty. Dirt, dust, and debris always collect in air ducts over time and will start making the air smell unpleasant. All of the debris also makes an HVAC less effective since it creates air resistance and makes it more difficult for the system to circulate air properly. These issues are why it is recommended to have your ducts professionally cleaned every three to five years or so.

Pests like cockroaches, dust mites, mice, and other rodents are also notorious for nesting inside air ducts, especially over the winter. If pests are present, the bacteria contained in all of their nesting materials, urine, and feces will give off an unpleasant smell. Even worse is that some of the pests are likely to die inside the ducts and will give off an even worse smell as they start rotting or decaying. These issues are why experts always recommend scheduling a professional duct cleaning service after dealing with any type of pest infestation.

Another issue is that mold can sometimes start growing inside air ducts. This usually happens when the ducts in areas like basements, crawlspaces, or attics aren’t properly insulated. If these ducts aren’t insulated, the temperature difference between the air surrounding them and the air moving through them can lead to condensation forming and create a high risk of mold growth.

Insulating the ducts and having them cleaned to remove any mold growth will usually be sufficient to overcome this issue. However, if you frequently have issues with mold growth in one part of your duct system, you may want to install an air-sanitizing UV light unit. This type of unit will kill any mold spores in the air and also shine throughout that section of ductwork to kill any mold on the walls of the duct and prevent it from returning.

4. Overheating Blower Motor/Electrical Issues

A burning smell from your AC is generally a sign of a more serious issue and not something you should ever ignore. This type of smell typically indicates that the blower motor is having to work too hard for some reason and starting to overheat. The motor can overheat because it’s starting to wear out or is coated in lots of dust, and it can also happen simply because the air filter is too dirty.

An acrid smell similar to burning plastic often instead means there is an electrical issue such as a short circuit that caused one of the wires to start melting. In either case, you should immediately shut your AC off to prevent damage and then have a technician inspect everything.

5. Refrigerant Leak

The different refrigerants used in newer AC systems are completely odorless. However, if you have an older system that still uses Freon, you may smell a sweet chemical odor if the evaporator coil or one of the refrigerant lines inside the home is leaking. This is also not an issue to ignore since a refrigerant leak will limit the effectiveness of your AC and can lead to other major issues. Another reason is that Freon is extremely harmful to the environment, which is why you should get the issue taken care of as soon as possible.

With 35+ years of experience, Brennan Heating & Air Conditioning is one of the most trusted HVAC companies in Seattle and the Puget Sound area. We specialize in air conditioning and heating maintenance as well as repairs and installation, and we also offer professional air duct cleaning and a full range of indoor air quality, plumbing, and electrical services. For help overcoming the bad odors coming from your AC or to schedule any other home service, contact us today.

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