Unusual HVAC Smells and What They Mean

HVAC header
When you turn your air conditioner back on after a break in use, you may notice some funny smells coming from the HVAC system. What in the heck is that?

There are several common causes to be aware of – and some may require immediate attention or for a service technician maintenance check. Here are some of the main reasons you may experience different HVAC smells emanating through your home.

Rotten Eggs

A rotten egg smell coming from the HVAC system could be a natural gas leak.

Natural gas is odorless but has a distinctive rotten egg smell when an additive called mercaptan is added to natural gas to make it easier for us to detect leaks. It is added as a safety measure to increase detection and prevent major incidents.

Contact a technician immediately if you notice a strong rotten egg smell near or around your HVAC system. Don’t try to locate or fix the leak yourself; leaving your home is best until the leak has been fixed.

run from smell meme


If you notice a strong, unpleasant sewage smell coming from your HVAC system, sewage is likely the culprit.

sewer line photo
The smell is caused by a buildup of sewage in your system, which may cause the odor to travel through your duct system. If you believe you may have this problem, you’ll want to take action as soon as possible by booking a service check with Bell Bros.

Here are some things that may cause smells in your system:

Clogged drains

If any drains near your home are clogged or backed up, then sewage can build up in the drainpipes. And the sewage may travel through your HVAC duck system and into your home. The sewage will not only cause an unpleasant smell, but the sewage can actually leak into your furnace and air conditioning equipment, causing extensive damage. You should immediately call a plumber and HVAC technician if you notice this problem.

Water or sewer leaks

If your HVAC system is next to a fractured sewer line, this could lead to sewerage getting into your HVAC system and vents. This will cause damage to the equipment if it isn’t addressed quickly enough. And you’ll want to promptly get rid of the foul smell passing through your home.

Dirty Socks

There are several reasons you might get a dirty sock smell from your HVAC system.

woman holding a dirty sock
The first thing to check is the evaporator coil. This is the part of your HVAC system where the air passes over as it cools. If it has mold or other debris, it can cause odors to come from your vents. You also want to ensure there isn’t any standing water in front of or behind the unit.

The best thing to do if you have a smelly sock odor is to call a HVAC technician to come out and inspect your system. If it is moldy, you don’t want to breathe in the spores as they come through the vents.

Sometimes dirty sock smells aren’t caused by something serious like mildew or mold growth. Sometimes, they’re caused by something simple like dead bugs on a vent grill or dust bunnies that get stuck inside them. These things may seem silly, but they can cause some serious odors!

pest meme


The musty smell that comes from the HVAC system is not only a sign of poor air quality but also an indication of high humidity that may have been caused by a leak in your home a duct leak.

The air ducts in your home can become clogged with dust, dirt, and other debris over time. This can result in poor air quality and even cause serious health problems. Especially if the musty smell is caused by mold growth on coils or ductwork.

mold everywhere meme

Hire a professional to carry out an inspection and clean. In addition to removing the musty odor, cleaning out your HVAC system will ensure that your heating and cooling system runs efficiently.

Electrical Odors

Electrical odors can be caused by several different issues. The most common reason for the smell is an electrical short circuit when two wires touch each other. This can happen at any point in the system and is usually caused by damaged wiring or water exposure. However, it can also be caused by loose connections or worn-out components like wire insulation or fuses.

If you smell an electrical odor from your HVAC system, immediately contact a technician for help.



You can smell oil in your HVAC system because the oil used in compressors and motors has leaked out of its sealed containers. This can happen if the seals have failed or there isn’t enough oil in the system.

oil inside a HVAC system

You may also notice oily stains on your floor or carpet, which are another sign of a leak in your unit.Struggling to find the source of your HVAC smells? Contact the HVAC experts at Bell Bros and eliminate those annoying odors.