What is That Strange Smell Coming From My Furnace?

It’s not uncommon for your furnace to give off different odors when it’s running - some are completely normal and will dissipate over time as the unit runs, and some smells can be an indication of a problem.

Common causes of furnace odors

Below are some of the most common causes of strange furnace smells. If you read through the list and discover that the cause of the unpleasant odor in your home is something more serious, contact a heating professional immediately.

For starters, if you haven’t turned on your furnace yet this season, it’s likely that when you first run the system, you will notice a “burning” smell. This smell is a result of heating up the dirt, dust, pet hair, etc. that has settled inside your unit during its off-season. If the burning smell doesn’t disappear after a few heating cycles, contact a professional.

Dirty air filter

If you haven’t changed your furnace’s filter in a while, it may have a build-up of dirt, dust, pet hair, and other contaminants. Not only can this obstruct air flow and hinder the overall efficiency of your heating system, but it can also lead to unpleasant odors in your home. After all, conditioned air from your furnace passes through this dirty filter, and in doing so, can make the dirt, dust, etc. become airborne.

Cracked heat exchanger

A cracked heat exchanger can be the source of a potentially deadly carbon monoxide leak and needs to be replaced immediately. The heat exchanger in your furnace is responsible for heating the air. As time passes, normal wear and tear could cause this device to crack. When this happens, you may (or may not) notice a chemical smell in your home when the heater is running. This is a component that should be replaced as soon as possible to prevent inefficiencies and potentially far more serious problems.

Natural gas leak

If your heating system is powered by natural gas and you notice a sulphur or “skunky” smell – sometimes referred to as a rotten egg odor, leave immediately. Do not operate anything that could cause a spark, including lights, cell phones, flashlights, and appliances. From a safe location, call 911 and your local utility, which will dispatch a crew to check on the source of the odor and make the area safe.

Dampness in the area

If your furnace is in the basement, you may notice that your home smells musty when the system is running. This area of a home is often prone to being damp and humid if your basement doesn’t have any waterproofing mechanisms in place, especially after a rainfall. Moisture can settle inside your furnace, and when the furnace runs, you may notice a humid, damp smell throughout your home.

Mold growth

A moisture-filled basement can also lead to the development of mold. Mold may be growing in or around your furnace area. When your heating system runs, it picks up the mold spores and pushes them out into your home. Mold has a musty, damp smell. Not only does it have an unpleasant odor, but it can also cause serious health problems and is an issue that should be dealt with immediately.

Overheating of the unit

If you notice a “burning” smell in your home when the furnace is running, don’t ignore it. This odor could indicate an electrical issue. Turn your furnace off and contact a heating professional immediately for assistance.

Vent problems

Have pets in your home and notice an odd or unpleasant odor when the furnace is running? Check to be sure there isn’t pet hair or urine around the vents in each room. As air is pushed through your vents, it can pick up odors from your pet and disperse them out through your living space.


If you notice a “decaying” smell when your heater is running, you may need to have your system checked for a deceased rodent. Additionally, a “smoking” smell could indicate that your chimney is blocked and in need of cleaning. This could potentially become a fire hazard, so be sure to address this problem as soon as you notice it.

Need help maintaining your furnace?

At Oncourse Home Solutions, we offer affordable Heating Maintenance and Heating Repair plans for homeowners. When you are enrolled in our plans, just call us and we will take care of the rest when it comes to an annual tune-up, or scheduling repair work.