What causes HVAC smell and what do they mean

When you turn your air conditioner back on in spring, you may notice some funny smells coming from the HVAC system. Knowing what these HVAC smells mean is important, as they might singal a more significant issue that needs addressing. So here are some of the most common HVAC smells and the problems causing each one.

Rotten Eggs

One of the leading causes of rotten egg smells is natural gas. Because natural gas is odorless, a sulfur-like smell is added to make it easier for us to detect leaks.

If there’s a leak next to your HVAC system, the gas can seep into your ductwork and escape through the vents. That’s most likely why you would smell rotten eggs coming from your HVAC system.

If you smell rotten eggs in your home, immediately evacuate all the occupants from the building and call 911 from outside. Don’t attempt to locate the source of the leak yourself. A professional technician has the knowledge and tools to find and fix the leak safely.

run for your life meme


If you have a sewage smell in your home, it could be coming from your HVAC unit. This can happen for a couple of reasons, and it’s not always the homeowner’s fault.

swer line system in a house

The first reason is that you may have a blockage in your sewer line from leaves or other debris that are caught in the line. If your HVAC system is close to a sewer line, a fractured pipe could be the source of the smell. The smell from a leaking sewerage line can seep into your vents, causing it to spread throughout your home. Always contact a professional immediately to identify any leaky sewer pipes. A plumber will clear out any blockages and recommend solutions to prevent future blockages from happening again.

Or, you could have a dirty air filter inside your HVAC system. Air filters have the job of collecting pollutants, so when they become clogged, they start to emit a foul odor. Regular HVAC tune-ups should prevent this from happening.

Dirty Socks

You’re probably familiar with the musty smell that occasionally occurs when your air conditioner has been left on for a long period of time. It’s not exactly pleasant, but it can be easily fixed by turning off the unit and allowing it to cool down for a few hours.

woman with a dirty sock

Unfortunately, there are times when this odor is accompanied by a much worse smell — like gross dirty socks. That’s because the odor is typically caused by mold growing on the evaporator coil.

Mold growth inside your HVAC system can lead to various health issues, including respiratory problems and allergies. In some cases, your entire HVAC system may need to be replaced if significant damage has been done due to mold growth.

If you smell dirty socks from your HVAC system, contact a technician for a HVAC tune-up and cleaning. And make sure you schedule regular tune-ups to keep your coils clean throughout the year.


Mold and mildew can quickly grow in the evaporator coil, ductwork, and blower housing when the unit is not in use. There may be a few reasons causing the smelling musty odor when you use your air conditioner or heater.

The best way to get rid of this smell and to keep it away, is to have your HVAC system serviced by an air conditioning technician. They’ll clean and sanitize every internal component of your unit to get rid of dangerous mold spores.

mold meme

Electrical Odors

This smell is often described as burning plastic or hot metal. The odor can be caused by dirty air filters or another issue in your HVAC system or in your home. If you notice an electrical smell coming from your HVAC system, check the following areas:

    • Condenser unit – The condenser is a fan-like device that cools the air before it enters your house. If the unit has been operating for long periods or has low airflow, it may overheat and produce an electrical odor.
    • Compressor – The compressor is located inside the outdoor unit. It pumps refrigerant through the system to remove heat from your home. If the compressor malfunctions and does not function properly, it could produce an electrical odor when it comes on for cooling purposes.


There are a few reasons your HVAC system may smell like oil. One is because the oil filter is dirty and needs to be replaced. If you don’t change your filters regularly, they can become clogged and cause excess oil to leak into the system.

inside ac unit

Another reason could be that you have a leaky pump belt or compressor seal. If this happens, you will need to have the unit repaired by a professional technician before it worsens and damages other components – or even causes a fire in your home.

Struggling to find the source of your HVAC smells? Contact the HVAC pros at ASI to get rid of those annoying odors.