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Why Does My Gas Fireplace Smell Musty?

Musty smells that emanate from your fireplace are not only unpleasant, but they often indicate that hazardous mold and mildew have accumulated in your chimney. Left untreated, chimney mold and mildew will continue to spread throughout your household and into your lungs.  

If your fireplace smells musty, the cause is likely moisture buildup, mildew, or mold. Air humidity, rainfall, chimney damage, and poor air ventilation can impact the dampness of your chimney, causing fungal growth and moisture buildup. 

So, what can you do about the musty smell coming from your fireplace? There are several ways to combat a damp chimney, including repairing your fireplace vent, chimney sweeping, installing or repairing your chimney cap, or having your chimney inspected or waterproofed. 

What Is Causing the Moisture Buildup in Your Chimney?

Did you know that the average gas furnace puts 1.5 gallons into your chimney every hour? This moisture can buildup, allowing mold and mildew to grow in your fireplace.

  • Poor ventilation. Poor ventilation causes moisture to condense within the chimney, especially in cold weather. If your fireplace vent needs repairing, call a professional chimney servicer immediately. 
  • Humid air conditions. The moisture that has built up in your chimney may just be air humidity. If you live in a humid environment, you should regularly sweep your chimney and place a dehumidifier near your fireplace.
  • Broken chimney cap. Over time, chimney caps can corrode and rust, causing the cap to disintegrate. Broken chimney caps allow rain and snow water to enter your fireplace.
  • Rain or Snow. Rain or snow can fall down into your chimney, causing a moisture buildup in your fireplace. 
  • Cracks in the chimney or vent area. Sometimes, there is a crack in your chimney or vent area that allows excess moisture to build up in your fireplace. If this is the case, you should have your chimney repaired and waterproofed to prevent further buildup. 

The Hazards of Chimney Moisture, Mold, and Mildew

The accumulation of moisture in your chimney or fireplace invites mold and mildew to grow in your home. If your chimney smells musty, you likely smell mold spores that are spreading from your fireplace into your house. Since mold and mildew spread via airborne spores, the presence of mold and mildew in your home will cause respiratory problems such as asthma, bronchitis, shortness of breath, and respiratory infections. 

If you detect mold in or near your fireplace, it is very important that you eradicate the mold quickly and completely so that it does not spread throughout your home. 

How To Eradicate Excessive Moisture From Your Fireplace

Replace or Repair Your Fireplace Vent

The accumulation of moisture within your chimney indicates that there is a problem with the air circulation. Instead of pushing the moisture out of your chimney, your vent has allowed moisture to accumulate in a damp, dark environment, hosting mold and mildew. 

You should always turn on your fireplace vent when you use your fireplace and allow it to run 5-10 minutes after your turn your fireplace off. This will allow your fireplace to fully dry out after use, and it will increase airflow, preventing mold and mildew growth.

If your vent is not working, or if it does not push much air out, have it checked and serviced by a professional fireplace servicer immediately. There may be some kind of blockage in your vents, such as dirt, dust, animal nests, or pet dander, and a professional may need to clean out the service panel beneath or above your fireplace. 

Regular Chimney Sweeping

Mold and mildew buildup can be extremely harmful to your health, and beyond that, any fungus that grows in your chimney is liable to spread to other areas in your house. Chimneys that have mold or mildew growing in them require many sweeps, preferably on a monthly basis. Frequent sweeping is necessary to completely eradicate the mold or mildew growth. 

If you do not have the money to hire a chimney sweep on a monthly basis, you may want to invest in a chimney sweeping kit. I recommend the Gardus RCH205-B SootEater, which is a flexible, expandable sweeping brush that attaches to a drill. Learning to sweep your own chimney will allow you to spend less money and regularly sweep your chimney.

Gardus RCH205-B SootEater Rotary Chimney Cleaning System, Cleans Open Chimneys up to 18′ with 6 Flexible 3′ Rods, Includes Trim-to-Fit Spinning Chimney Whip

When you are removing the mold and buildup from your chimney, you will want to cover the floor and all furniture in the room with plastic tarps. After sweeping, clean the entire area of your chimney with an antifungal cleaner and a disposable sponge or towel. Vacuum the area thoroughly after you sweep, and clean any surfaces, curtains, and window blinds near the fireplace.

Replace Your Chimney Cap

The moisture buildup in your chimney may be remedied by a chimney cap. If you do not have a chimney cap or if your chimney cap is damaged, rainwater will enter your fireplace, evaporating and condensing within your chimney. These are the perfect conditions for mold growth. 

In addition, moisture buildup and acidic chimney emissions can cause the chimney cap to rust and disintegrate over time. If you suspect that the cause of the moisture buildup is a defective chimney cap, have it replaced immediately. 

Have Your Chimney Inspected and Waterproofed

Sometimes, moisture buildup is caused by leaks or cracks in your chimney and fireplace area. Cracks and holes allow excess moisture to accumulate in your chimney and fireplace, inviting mold and mildew to grow in your home. 

These cracks are often caused by the buildup of acidic components such as hydrochloric or sulfuric acid that your fireplace produces when it burns. Chimney damage can also be caused by the excess moisture itself. In freezing temperatures, chimney moisture can crack and shatter brick or stone chimneys, leaving cracks for moisture to seep into your fireplace. 

If there are any signs of leakage such as dripping, visible mold near your mantle, water damage on the ceiling or wall near your fireplace, or puddles of accumulated water in your fireplace, call a professional fireplace servicer. You will need to have your chimney or vent inspected for acid buildup, damage, and cracks. If there are no leaks or cracks to repair, then you will need to have your chimney or fireplace waterproofed. 

How To Prevent Moisture Buildup in Your Fireplace

Whether you have experienced mold and mildew growth in your fireplace or just want to prevent it, there are some easy but necessary steps to take in order to ensure that your fireplace does not accumulate excess moisture. 

  • Always use your fireplace vent when you light your fireplace and leave the vent running for 50-10 minutes after you have turned off your fireplace.
  • Place a dehumidifier near your fireplace, especially when the weather is cold or humid. 
  • Have your fireplace professionally serviced annually, at a minimum.
  • Regularly clean your fireplace area with antifungal cleaners.
  • Regularly smell your fireplace in order to detect fungal growth.
  • Watch for water damage or fungal growth around your fireplace. 


If your fireplace smells musty, it is very important to thoroughly inspect your fireplace and chimney in order to make sure that there is no mold and mildew growth. If there is excess moisture trapped in your chimney or fireplace, call a professional fireplace servicer as soon as possible to keep the spread of the mold and mildew under control. 

Left untreated, the mold and mildew will spread, costing you thousands of dollars in-home repairs and likely harming your respiratory health.


  • Alanna Greene

    Alanna is an avid traveler who lives in Michigan. In addition to writing for Temperature Master, he also sells crafts on Etsy and takes long walks through the forests near her home.

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