There are disclosed methods of controlling the presence of surface and airborne microorganisms in an air handler. The method may comprise: energizing a germicidal lamp to emit distributed UVC radiation; positioning the germicidal lamp in proximity to the heat transfer coil, wherein the intensity of the UVC radiation striking a heat transfer coil of the air handler is to a degree independent of the distance of the germicidal lamp to the heat transfer coil; directing the UVC radiation across the coil face to the coil's tube and fins; reflecting and directing the ultraviolet by the coil's tube and the fins-throughout the coil, thereby increasing the flux density of the ultraviolet radiation and the dosage of the radiation applied to airborne microorganisms and microorganisms carried on the surface; continuing to irradiate the surface with the UVC radiation from the germicidal lamp at least intermittently until the surface is organically clean.