There are described apparatus and methods wherein ultraviolet light kills and/or degrades and vaporizes microorganisms and organic material which naturally form over time on a heat exchanger. As this matter is eliminated, the pressure drop is decreased (i.e., airflow is increased) and the heat exchange efficiency (capacity) is increased. Less energy per Btu removed is used by the cooling system, and less energy is used by the HVAC system to move air.