Why There’s a Bad Smell Coming From Your Furnace

If you catch a whiff of a bad odor every time your furnace is running, it’s important to identify the cause. Here are the most common smells you may encounter, and what you can do about them.

Rotten egg smell

A sulfuric rotten egg smell is usually a sign of a natural gas leak. The foul aroma is added by most utility companies in order to help alert homeowners when there’s a leak. A natural gas leak is an emergency situation. If you smell rotten eggs coming from your air vents, there are a few steps you need to take. If the smell isn’t very pronounced, and you suspect the leak is small, open all doors and windows in your home to encourage airflow before contacting your utility company. If the smell is very strong, and you suspect you’re dealing with a larger leak, evacuate yourself and your family from the home. Once you’re somewhere safe, contact your local utility company and report the leak.

Burning or electrical smells

A burning smell could indicate a mechanical problem, issues with wiring, or component failure. But, it can also be fairly harmless. If you notice a burning smell the first few times you turn your furnace on for the year, it may just be months of dust burning off. After not being used for most of the year, your furnace may collect its fair share of dust. Wait and see if the smell goes away after about 20-30 minutes. If the smell doesn’t go away, contact an HVAC technician for an inspection. Depending on the cause, you may need to get an electrician involved if the problem is with wiring. You may also need to repair or replace your heating system if a crucial component is failing.

During an annual air conditioning maintenance visit, our technicians ensure that both the indoor and outdoor unit are free from dust and debris that can cause burning odors and other problems in your home. If it’s been some time since you’ve last turned on your air conditioner, consider having it examined and maintained before you encounter any unwelcome odors or surprises.

Dusty smell

Even in a dry, desert climate, dusty smells shouldn’t make their way indoors. Dusty smells can result from dirty air filters. Along with an unpleasant smell, dirty filters can decrease the overall efficiency of your air conditioner. That’s why we recommend that you replace your filters at least once every three months. If you have pets, or if someone in your home has allergies, it’s best to replace your filters more often—about once a month.

  • Vacation or seasonal home: every 6 months
  • Home without pets: every 90 days
  • Home with pets: every 60 days
  • Home where one or more people have allergies: every 30 days
  • Home in an area with bad air pollution: every 30 days

One quick way to judge if your filters are dirty is to shine a flashlight at them. If no light shines through, it’s time to change the filter.


To keep dusty smells at bay, consider upgrading to filters with a higher MERV rating. MERV ratings reflect how effective an air filter is at removing certain categories of airborne particles. A Filter with a MERV rating of 16 or higher is considered a HEPA filter, which provide the highest protection from dust, dander, and more. You can learn more about the ins and outs of MERV ratings, HEPA filters, and all things clean air in our guide to Everything You Need to Know About Air Filters.

Dusty or dirty smells can also result from leaky ducts allowing dust to flow throughout the system. While unpleasant, these dusty smells can cause respiratory irritation as well. To avoid this, have your ducts cleaned annually by a qualified company. Because air ducts are located behind walls, in the attic, or beneath the floors in your home, it can be hard to physically access them to locate any problems. It is important to leave this job to the professionals, as attempting to clean your ducts on your own can be dangerous.

Musty or moldy smells

Mold and mildew in your ductwork can pose health problems to people with respiratory conditions, along with producing less than pleasant smells when your furnace is running. The most common causes of musty or moldy smelling air in a home include:

Leaking air ducts

Since your air ducts are generally located in an attic, crawlspace, or otherwise out of the way, you might not give much thought to them. That is, until they spring a leak. If your ducts have leaks, mold can grow in the spaces where dust and humid air enter the ductwork. You’ll need a duct sealing and cleaning from a professional in order to address this, since the location of air ducts makes them often difficult, if not impossible, for the average homeowner to access.

Fixing bad smells and more: Annual heater maintenance

From bad smells to uneven heating, annual heater maintenance is the best way to keep all sorts of furnace issues at bay. At Chas Roberts, we inspect your system inside and out to make sure no component is overlooked. Our experienced technicians work hard to ensure that your furnace is ready for chilly winter nights. Here’s a preview of what you can expect from the Chas Roberts 26-point maintenance plan.

Thermostat calibration

The standard deviation between actual indoor temperature and what your thermostat displays is two degrees. If the difference is higher, it’s best to either have the thermostat re-calibrated or replaced altogether if re-calibration doesn’t solve the problem.  Manual thermostats always have to be replaced if they become uncalibrated. However, programmable thermostats can be re-calibrated by an HVAC professional.

Burner flames inspection

If you have a gas heater, the technician will take some time to inspect the heater’s burner flames. Burner flames should be bright, even, and blue. Blue flames are optimal because they’re hotter than yellow flames, indicating that your heater is working efficiently and is not wasting gas. Yellow flames should be considered a warning sign that could point to dirty burners, combustion problems, or an excess of oxygen.

Heat exchanger inspection

Heat exchangers can develop cracks that could allow carbon monoxide to enter your home and endanger you and your family. During an inspection, the technician will thoroughly check for obvious issues like cracks and other damage. If cracks are present, replacing your heat exchanger should immediately jump to the top of your “to-do” list. Since exposure to carbon monoxide is life-threatening, it’s important to have your heat exchanger replaced as soon as possible to minimize the risk.

Combustion air check

The air that a gas furnace needs in order to burn safely is called combustion air. Without an adequate supply of combustion air, the heater won’t be able to function properly. During a maintenance inspection, the technician will ensure that your furnace is getting enough combustion air, and that the air pressure is appropriate.

Trust Chas Roberts to keep your furnace running smoothly

Even in a mild climate like Phoenix, a reliable heater is still a valuable asset. After all, winter is the time for cozy family gatherings and fun holiday memories—not the time for costly heater breakdowns. Contact us for help with all your furnace questions.