5 Common HVAC Odors (And What Causes Them)

The HVAC unit in your home is one of the most-used appliances, and it’s inconvenient and uncomfortable when it isn’t working. Fortunately, it’s also pretty good about giving signs when it needs servicing.

One of the key warnings many homeowners notice is bad smells coming from the unit. An odor problem doesn’t necessarily mean a repair will be expensive. It could simply be caused by a dirty air filter or condensation that has created mold in the unit.

Discover how to troubleshoot five common HVAC odors so your HVAC system can keep you comfortable year-round.

Identify and Eliminate Five Common HVAC Odors

When troubleshooting the HVAC smell, use your other senses as well. Listen for banging or knocking, and look for excess water on the ground or other fluids that don’t belong.

Note: Usually, the smells are not dangerous and just indicate that your system needs service. But some smells, like rotten eggs or gas, are more serious. If you experience an electrical, burning, or gas smell that doesn’t go away, turn off the unit and call your HVAC technician immediately.

1. Electrical Odors

If your HVAC unit smells like it might be overheating, it very well could be. Turn your unit off and check the air filter to see if it is dirty, as this can restrict the airflow and overheat the electric resistance heaters.

If it’s not an air filter issue, turn off the electricity at the master switch. An electrical odor typically is a problem within the motor or the wiring. Attempting to fix either of these items can result in severe injuries. A professional HVAC technician will need to check out the problem and make repairs.

Note that the first few times the heater kicks on in the fall, it may emit a burning odor that can smell electrical and dusty. If you are just turning on your heater, this could be the issue and it will eventually go away on its own. Give it 20 to 30 minutes and see if the smell dissipates. If it does, it probably was just the dust burning off. If it continues, contact your HVAC technician.

2. Rotten Egg Smell

Provided there aren’t any rotten eggs in your home, then the smell of rotten eggs or sulfur most likely indicates a natural gas leak. On its own, natural gas is odorless, so most utility providers add the aroma to alert people of a leak. Don’t take this lightly. Turn off the unit, open the windows to your home, get out of the house immediately, and then call the gas company.

3. Gas Smell

Sometimes, when you first turn on the heater, it can smell like gas is burning. During the summer, dust settles within the HVAC on the heat exchanger. When the furnace is turned on, the dust gets burned off. If the smell isn’t coming from the vents, it’s most likely a leak at a pipe fitting or within the equipment. You should not attempt to fix this problem. Call your HVAC technician to find the leak and make repairs.

4. Musty or Moldy Smell

A smell like mildew or mold is probably the most common ailment of HVAC systems. When the AC is running, there is a lot of condensation within the unit. If the moisture doesn’t drain properly, it can escape into the ducts and cause mold. Finding where the mold buildup is and getting it cleaned up will solve the problem. An HVAC technician has the knowledge to handle the problem and ensure that it doesn’t recur.

Note that mold and mildew smells are not serious problems with the unit, but poor air quality increases the risk of respiratory infections in both adults and children. Although you can still use the unit while you’re waiting to make repairs, you shouldn’t put off repairs for the sake of your family’s health.

5. Smell of Oil

The smell of oil comes from one of two things. First, look for an oil leak. If you see oil, you may just need to tighten a fitting in the tank, oil line, or filter.

While this isn’t a dangerous issue, it does need to be taken care of by a qualified repair technician. It should be noted that a new furnace may emit the burning smell for the first 24 hours of operation, but if the smell continues beyond then you need to call the professionals who installed the unit.

If you do not see oil, then the most likely problem is that the oil burner is not working properly. However, there are many things that could be causing the oil burner to malfunction, for example, a clogged burner or a bad fuel pump. Watch for soot and smoke, and listen for unusual sounds. These can indicate a more serious problem that should be repaired by a service technician.

It’s not only odors that indicate a problem. Puddles of water near the unit can be just as problematic. If you notice water leaking from your indoor unit, see if you can trace the water to its source. It could be a leak within the unit, excess condensation, or a tear in the insulation foam. If you don’t know what’s causing the pooling water, call your technician.

Dealing With HVAC Odor

A common reason that many HVAC units create more odors is that homes are built very tightly today, often without the necessary ventilation. This causes the unit to run out of air for combustion. This can be the leading cause for foul HVAC odors for many homeowners, which raises concerns about indoor air quality.

Another indicator of an indoor air quality problem is the smell of dirty socks. Although this smell may be coming from the air conditioning unit, it is not actually the unit that is the problem. Most likely, organic material has not been heated to a high enough temperature to prevent its growth.

In most units, the heat exchanger kills the organic material, but those with heat pumps do not reach those temperatures. When the spores and mold are released into the air through the heat ducts, it produces poor quality air. An HVAC technician can diagnose the problem and recommend solutions.

Make sure to replace the air filter according to the manufacturer’s instructions. Learn how to find the best air filter for your home. Have your HVAC system inspected annually by a professional technician to look for faulty wiring, loose or worn belts, and general wear on the unit. By having an annual service, you have peace of mind that you’ve done what you can to make sure your HVAC unit is ready for the seasons ahead.