An unusual or musty odor is often the first sign of mold in air ducts or in air vents.
In many cases, you will smell it before you can see it.
If you notice a musty or moldy smell that goes away when you are not using your air conditioning or heating, that can narrow down the cause of the smell to mold in your HVAC system. If you notice a recurring problem, usually in areas with poor ventilation or a lot of moisture, get it checked out right away. Mold spreads through spores, so you won’t necessarily see actual patches of mold growth right away. By the time you do see a noticeable amount, infestation can be quite serious. If people or pets in your home start experiencing symptoms whenever the AC is on, mold in air ducts might be the cause. If there is enough mold in or around the vents, it can spread in the air, causing allergy-like symptoms such as headaches, nausea, irritated nose and throat, and itchy eyes. Mold grows in ductwork when two things are present: moisture and warm temperatures. If your AC unit is oversized for the space you have, it can cause mold in the air ducts from creating too much moisture by cooling too quickly. Setting your AC on a very low temperature can sometimes lead to mold around the vents. When the cool air from the vents meets the warm air in the room, the temperature difference can cause moisture in the air to condense on surfaces in and around the vents. If that moisture builds up and doesn’t dry out, it can lead to mold growth. If your ducts have leaks that let in warm air, this temperature differential can cause the same issue. The combination of cold air in the ducts and warm air in the walls can cause moisture to condense on the ducts and provide the perfect environment for mold to grow.