When your HVAC system starts emitting unpleasant odors, it could be a surefire sign that it needs servicing.

Different smells signal different concerns. A musty smell can mean that condensation has led to mold growth in the area. The smell of burning plastic may indicate that the unit is overheating. Many times, a technician can quickly get to the root of these smells, if they are detected early.

Here are some of the most common HVAC smells to look out for and what they could mean:

Musty or Moldy

A smell like mildew or mold is the most common smell in this list. Condensation happens inside the AC while it runs, and when the moisture fails to drain properly, it tends to escape into the ducts and cause mold development. Spotting the mold buildup and cleaning it up will solve the problem.

Burning Plastic

If your HVAC system smells like it’s overheating, it might be because it really is. Before attempting to troubleshoot the problem, turn off the unit. Check if the air filter is dirty, as debris can restrict the airflow, which overheats the electric resistance heaters. If it’s not an air filter issue, it could be a problem with the motor or the wiring. If this is the case, it’s best to leave it to the pros; attempting to fix it may cause injuries.

The first few times your heater kicks in again in the fall or winter, it may emit a burning odor. But this could smell be just the dust burning off, and it will eventually go away on its own. Wait 20 to 30 minutes to see if the smell dissipates.

Rotten Egg

The smell of rotten eggs or sulfur signals a natural gas leak. Although natural gas is odorless on its own, most utility providers add this smell to alert inhabitants or technicians of a hole. Don’t take leaks lightly. Open the windows, get out immediately, and call the gas company.


If you see oil coming from a leak, tighten a fitting in the tank, filter, or oil line. If you do not see oil leaks, it’s likely that the oil burner is not working correctly due to a bad fuel pump or a clogged burner. In this case, zero in on the source of the smell by using your other senses.

For example, listen for knocking, banging, or any unusual sound. Look out for soot and smoke, or look for excess water or any other liquid in the area. If the smell is from burning plastic or leaking gas, turn off the unit and call an HVAC technician immediately.

Cowboy Services AC & Heating is the HVAC company of choice for families in Dallas and surrounding areas. Serving the HVAC needs of the community for almost three decades has armed us with the knowledge to take on any HVAC repair or installation job. We know how to best approach an HVAC problem just by the smell the system produces, and we know exactly where to check for worn, loose belts, or faulty wiring.

Above all, we make sure to replace the faulty parts according to the manufacturer’s specifications and use only top-grade materials. Call us at 1-214-775-9971 today or fill out our form, and we’ll contact you as soon as we can.

Smell That? The Different HVAC Odors and What they Mean
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