1. Precipitation Fouling - This results from sediment of dust, rust, fine solids, and other entrained solids
2. Chemical Fouling - This type of fouling occurs when the depositions are formed as a result of a chemical reaction
3. Biological Fouling - This occurs when organisms grow on heat exchanger surfaces. Problems arise from algae and other microbes such as barnacles and zebra mussels. At certain times of the year when microbes typically bloom, colonies several inches thick may grow across the heat exchanger, affecting thermal performance.