Abstract: A heat controlled ultraviolet light apparatus includes a source of ultraviolet light, a cover, and a heating or cooling element that heats/cools the space or gap between the ultraviolet light source and the cover. Accordingly, the ultraviolet light source may be maintained at an optimal temperature thereby maximizing the efficiency of the ultraviolet light source in producing ultraviolet radiation. The apparatus may further include a temperature sensor and a control circuit to automatically control production of heat/cooling by the element based upon the ambient temperatures experienced by the ultraviolet light source during use. Methods are also provided for sanitizing heating and cooling coils of various devices such as an HVAC system, and cooling systems such as a refrigeration unit and an evaporative cooler.