What Causes Mold on the Central Heat & AC Unit?

When you think about problems with your central heat and air conditioning unit, you might think about sweltering in the summer or freezing in the winter when it does not run.


Mold can cause problems with the unit as well, including bad odors and health problems.

Mold can grow inside an air conditioning unit if the condenser pan does not drain properly. This leads to increased moisture levels inside the unit, which promotes mold growth. Heating and air conditioning units can be located in crawlspaces; these wet environments increase the humidity within both units. If you have a humidifier attached to your central heating unit, failing to regularly clean and maintain it can allow mold to gain a foothold in the system.


The first sign most people notice that indicates mold is growing on or in the central heat and air conditioning unit is the smell. Mold has an unpleasant musty, earthy smell. When it grows on the heating or AC unit, mold spores blow into your home along with the heated or cooled air. When you view your heating and air conditioning unit, you might see mold growing on the outside. When you look at the condenser coil or inside of the humidifier, you will see green, wet mold growing.


Mold in your central heat or air conditioning unit can make it unpleasant to run the device. Your family can either go without heated or cooled air, or can have to endure the musty mold smell blowing into the house. People who are sensitive to mold can experience adverse health effects, like headaches, nausea and respiratory problems when exposed to the spores. Mold spores blowing through your air ducts can take root there, growing a new patch of mold inside the ducts that might spread throughout your home.


If mold grows on the condenser coil of your air conditioning unit, wash it away with a solution of one-half cup bleach and one gallon of water. Use the same solution to wash away mold growing on the exterior of your unit. If the mold shows up in the furnace humidifier, use a mixture of one part white vinegar to one part water to clean out the mold. Prevent mold growth by maintaining your unit, making sure that the condenser pans drain properly and that the area where the unit is located does not become too humid.