The HVAC unit in your home is one of the most-used appliances, even though it is easy to forget about unless it isn’t working. Fortunately, it’s pretty good about giving signs when it needs servicing. One of the key things that many homeowners notice is a bad aroma coming from the unit, and this is a sign that something needs attention.

When troubleshooting the smell, use your other senses as well. Listen for banging or knocking. Look for excess water on the ground or other liquids which don’t belong. Many times, the smells are not dangerous, just an indicator that your system needs service. However, some smells, like rotten eggs or gas are more serious.

Here are some common HVAC odors and what they may indicate:

Rotten Egg Smell or Gas 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. Although on its own, natural gas is odorless, most utility providers add the aroma to alert people of a leak. Don’t take this lightly. Open the windows to your home, get out of the house immediately, then call the gas company.

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.

Electrical Odors

If your HVAC unit smells like it might be overheating, it very well could be. Turn your unit off before attempting to diagnose the problem. First, check the air filter to see if it is very dirty as this can restrict the air flow and overheat the electric resistance heaters. If it’s not an air filter issue, make sure the electricity is turned off at the master switch. A professional HVAC technician will need to check out the problem and make repairs. 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.

In the fall, the first few times the heater kicks on, it may emit a burning odor that can smell electrical and dusty. If you are just turning on your heater, this could be a problem which 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.

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. When 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.

Mold and mildew smells are not problems with the unit that are serious, but the poor air quality does increase 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.

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 a burning smell for the first 24 hours of operation, but if the smell continues beyond this, then you need to call the professionals who installed the unit.

If you do not see oil, then the problem most likely 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 indicate a more serious problem that should be repaired by a service technician.