What's That Musty Smell Coming From Your Furnace In The Winter

When preparing your furnace for the chilly winter season, you want to take steps to make sure the system works perfectly all winter. You might focus on things like testing the system, changing the filter, and inspecting the furnace for damage. While operating the furnace in winter, you might pay particular attention to any odd noises it makes that could indicate serious problems, like squealing, banging, or rattling. You want to catch issues early, so repairs can happen in a timely manner.

Don't limit yourself to listening for strange sounds, though. Pay attention to other oddities coming from the furnace, such as unusual odors. Some smells from the furnace are part of normal operation, while others indicate you should call for repairs right away. One of the smells coming from your furnace that you should never ignore is a musty odor. Certainly, this type of smell could mean something that is not worrisome, such as having a system that hasn't run for a few months. However, under the worst-case scenario, the musty odor could relate to a buildup of mold or moisture where it shouldn't be. Having mold in your furnace could lead to health problems for people exposed to the air inside your home, such as with sneezing, irritated eyes, and breathing issues. Learn more about when musty smells are worrisome and when you can ignore them.

The most worrisome reason for a musty odor coming from your furnace is having mold in your home's ducts. When moisture enters the ducts, it could lead to mold or mildew growth inside the system. Areas of the furnace or ductwork that aren't sealed properly may allow humid air to enter the system, causing moisture retention. To remove the mold or find the source of moisture that's penetrating the system, call a furnace repair professional for a duct cleaning and inspection. Ductwork repairs or sealing may be necessary.

If you fail to change the furnace filter on the proper schedule, it could lead to musty odors and potential mold exposure. How often should you change your furnace filter? Swap out the filter every two to three months. As it operates, an air filter naturally collects dust, mold spores, and bacteria floating through the air, eventually becoming clogged. Changing it means fresh filtering material can successfully grab these contaminants again. If a clogged air filter becomes damp, it might allow any mold spores to begin growing, enhancing the moldy, musty smell that you're experiencing when running the furnace. Replacing the clogged filter can eliminate the musty smell.

If you find mold on the filter while replacing it, you should stop moving it and put on a mask and gloves. Then, carefully place the contaminated filter directly in a plastic bag. Call a professional to inspect the system to see if the mold growth has spread to any other areas.

When you haven't run your furnace for a long time because of warm weather, having a musty smell the first few times you operate it is normal. Dust accumulating in the system during idle months may resemble musty odors as the furnace burns off the dust. During times of high humidity in summer, humid air might linger in the furnace. Once air and heat begin regularly moving through the furnace again during cold weather, the stray moisture evaporates or the dust completely burns off and the musty smell disappears. If this is the problem, it might take running the furnace for a few days to completely eliminate the musty odor. If the smell disappears after a few days, it's not worrisome.

Sometimes, the musty smell relates to the evaporator coils in the HVAC system. The evaporator coils are often located near the furnace, but they're only used to operate the air conditioner. The refrigerant inside the coils absorbs heat from the air in summer during AC operation. Condensation forms on the evaporator coils as they operate in summer, causing dust to stick. If the air conditioner and furnace use the same air handler, which is normal in residential HVAC system designs, the evaporator coils are at the top of the furnace. During winter while the furnace is running, the heated air's proximity to the dirty coils can generate a mildewy and musty odor. Fix this issue by calling a pro to clean the coils.