After coming in from the snowy outdoors, the sound of your furnace kicking on is a welcome one. When the warm air is accompanied by a musty smell from your furnace humidifier, it isn't so welcome. Regular maintenance and cleaning will get rid of the smell and stop it from coming back.
When a musty smell comes from a furnace humidifier, most people usually don't notice it until it gets into the air ducts that bring warm air into your home whenever the furnace runs. The air blowing in will be warm, but it will also smell like a damp basement. If you investigate the furnace, the smell will be strongest around the whole-house humidifier attached to the unit. The musty smell usually occurs with drum-style humidifiers.
The odor is caused by mold growing inside your furnace humidifier. The interior of the unit is dark, wet and poorly ventilated, which makes it an ideal environment for fungal growth. Mold can grow on the interior walls; it can also live on the absorbent pad inside the humidifier. Every time your furnace runs, it blows mold spores and their musty smell into your home, which can not only lead to health issues for your family but can also lead to mold growth in other areas of your home.
Getting rid of the mold will get rid of the musty smell. Create a solution of equal parts white vinegar and water. Using a sponge, wipe away all traces of the mold. If you see mold growing on the humidifier's pad, remove it and soak it in the vinegar and water solution for 20 minutes. Rinse it clean to get rid of the dead mold. If the mold won't rinse away, the pad needs to be replaced with a new one.
To avoid having the musty smell return, check your furnace humidifier for mold growth on a regular basis. Before winter temperatures arrive, check the interior of the unit. Even if you don't see any mold, wiping down the walls with the vinegar solution won't hurt. It kills any mold spores that might be present before they can grow. Check the humidifier again halfway through the season and again when spring arrives.