UV disinfection light is in a circulator loop that draws from a tee into one of the hot tub jet supply hoses and returns to the hot tub drain line. A second circulator injects heated water into the disinfection loop via closely spaced tee arrangement. Heat injection loop circulates through a tankless coil in the top of a buffer tank. Scalding protection is unnecessary because when the hot tub is in operation the flow in the disinfection loop is increased so much by the hot tub mondo pump that the heat injection flow is overwhelmed by the disinfection loop flow. Alternative design would replace closely spaced tees with plate heat exchanger and zone control. Control consists of a cycle timer that turns the disinfection loop on for a while and off for a while. A delay timer activates the heat injection control after the disinfection loop has been running for a few minutes so that the temperature sensor in the disinfection loop has had a chance to stabilize. Both the cycle timer and the heat injection controller are set up to activate larger relays to save wear and tear on the controllers, but the controllers are so inexpensive and the current draw of the pumps and the UV light is so low that this was probably unnecessary. Subsystem is in former cellar stairway now that hot tub is in patio where cellar stairs used to was.