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Jul 30, 2021 2:49:29 PM by Lauren Mangano

7 Reasons Why Your AC Smells (& What to Do About It)

The last thing you want when trying to cool off on a hot summer day is to notice a foul odor coming from your AC system. Unfortunately, this problem is all too common. But with all the different smells that can be coming from your AC, how do you know what the cause is and how to fix it? We'll go over the seven most common AC smells and what to do about them.

common AC smells

1. Smells Moldy or Musty


A stale, musty smell is the most common smells that homeowners notice from their AC system. If there is excess humidity or moisture in the HVAC system, you could end up with mold and mildew flowing through the ductwork. The main cause is the accumulation of water in the drain pipe or drip lines (or the ductwork), which can cause fungus or mildew to grow.
In the event of a leak, you might even see water dripping from your indoor unit in addition to noticing the stench. 
Another common cause for this smell is dirty air filters. If you live in a hot and humid climate and turn your AC on after a long period of time, moisture can accumulate in your filters and produce this moldy smell.Changing AC filter


If this problem occurs you should have a professional improve the drainage and have the AC lines and ducts thoroughly cleaned to remove the smell. In case of a leakage, a professional will also need to fix it to prevent further issues.
But the best ways to protect your belongings and prevent this problem from happening in the first place are to replace your air filters, schedule annual or bi-annual AC tune-ups, and install a whole-home dehumidifier. Connected directly to your existing HVAC system, the dehumidifier automatically removes the correct amount of moisture from the air and deposits the condensate outdoors.
You can also look into systems that are better at balancing moisture and removing humidity, like the Unico High Velocity system. Less humidity in the home also means you can set your thermostat to a warmer temperature, saving you money as a result.
Unico High Velocity Makes Humidity a Problem of the Past
Annual tune-ups will ensure everything is working properly and running efficiently, and will help detect problems before they occur.

2. Smells Like Sulfur


If you're noticing a rotten egg smell from your HVAC system, the causes can differ greatly. You may have dead rodents or other small animals stuck in the ductwork. If you have a ductless system, small creatures like lizards can even get stuck in the indoor unit. 
On the other hand, you may have a more serious problem: a gas leak. While natural gas doesn't emit a smell, utility companies put an additive called mercaptan that gives off an identifiable scent. Another sign of a gas leak is a patch of dead plants from the gas line underneath the soil. Learn about more gas leak warning signs here.
Finally, a sulfurous smell may be due to a ruptured sewage pipe near your air duct system. This is another issue that can be dangerous if not handled properly.


If you suspect a dead animal is in the ductwork or system, you will need to find the exact location and remove the cover to get the animal out. It might be best to have a professional do this so that they can also make sure there is no other damage.

If it seems like the smell is coming from a gas leak, you will need to call your gas company to shut off your gas and hire a licensed repair professional. If this is the case, don't use matches or lighters, open as many windows and doors as possible, and leave the home/building.
In the event of a ruptured sewage pipe, have a licensed plumber come out to inspect and repair the pipes if needed.

3. Smells Like Dirty Socks/Stinky Feet


This unpleasant smell can be caused by dirty air filters and evaporator coils, clogged condensate drain pans, and stagnant water.


To stop this smell, simply change your air filters every few months or schedule a professional to come out and clean the system or unclog the drain. Once it's unclogged, you can pour a little bit of distilled white vinegar in to help kill algae and fungi and eliminate bad odor.

4. Smells Like Something Is On Fire


If your system smells like something is burning, it probably means that something is, in fact, on fire. There could be an overheating motor or wiring issue, for example. Alternatively, the smell may be due to dust settling in the unit after a period of inactivity. Once the AC is turned on after a long time, the dust can burn off and produce an unpleasant smell.AC issue, condenser fan issue


If it smells like burning plastic, gunpowder, or something else on fire, you should immediately turn off the system and contact a professional to come out and diagnose the issue. If you are using the AC for the first time in a while and suspect an accumulation of dust, you can wait a little to see if the smell fades away. Again, if the odor remains and the system starts to malfunction, turn it off and call your local HVAC company.

5. Smells Like a Skunk/Gas


The smell of gas or a skunk-like stench is another tell-tale sign of a gas leak within the system. As stated earlier, natural gas itself does not have a smell; but the methyl mercaptan in it will be noticeable if there is a leak.


Again, if there's a gas leak, you will need to call your gas company to shut off your gas, hire a professional to make repairs, and follow safety precautions. 

6. Smells Like a Car's Exhaust


The most common cause of this smell is a leak in the refrigerant line. Refrigerant's role is to absorb heat from inside the home and allow the rooms to be cooled, so a leak will negatively impact the system's performance. As the refrigerant does its job, it also releases chemicals into the air. R-22 refrigerant was commonly used for decades, but is being phased out due to the harmful emissions from the Chlorofluorocarbons (CFCs) it contained. If your system still uses R-22 and there is a refrigerant leak, have it checked by a professional as soon as possible.


To combat refrigerant leaks and stop this smell, have your local HVAC company top off the system's refrigerant and repair the leak. Checking the refrigerant charge is another aspect of an annual tune-up -- a maintenance that's critical to a well-functioning HVAC system.

7. Smells Like Cigarette Smoke


If you or someone has smoked in the home, the odors could accumulate in the filter and evaporator coil. As a result, the tobacco particles are knocked loose and sent back through your vents when you turn on the air conditioner. 


The best solution here is to smoke outdoors, but you can also change your air filter often and schedule an AC tune-up to alleviate the smell if it's already there.ECI comfort service call
Whatever the odd odor coming from your system is -- you can never go wrong with calling a professional to help figure out what's wrong and make the necessary repairs.
Taking preventative measures like changing the air filters, scheduling annual maintenance, and installing indoor air quality products will also be a big help.

If you live in the Delaware Valley/Greater Philadelphia area and would like to find comfort within your home, visit our website or give us a call at 215 - 245 - 3200 to learn more.

This entry was posted in Safety, Air Filters, Air Conditioners, Air Conditioning

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