Abstract: A system for heating a hot tub or spa from an existing potable hot water supply is disclosed. The system uses a heat exchanger element which isolates the potable hot water supply from the hot tub water. A failure in the heat exchanger does not cause intermixing of the potable and hot tub water and thus the system meets local plumbing codes. The heat exchanger consists of a center pipe surrounded by two concentric tubular sleeves. The center pipe carries the potable hot water from the hot water system. Surrounding the center pipe is an annular sleeve which has a plurality of longitudinal slots located on the inner surface adjacent to the center pipe. The sleeve is, in turn, surrounded by an outer jacket through which the hot tub water flows. A failure in the center pipe or the sleeve causes water to flow out through the slots, alerting the owner of the heating system that a failure has occurred.