Thermal water circulation requires that the hot water line to the subject fixture must be on a continual uphill pitch. The thermal flow will stop immediately at the first down turn of any magnitude. The return water to the heater should also slope downhill to the water heater, but that is not quite as important, since that water will usually be at ambient temperature and will be displaced by the rising hot water. If your hot and cold water lines meet these criteria, or can be repitched to conform to the requirements, then the NIBCO valve can be installed between the hot and cold water lines under the sink or bathtub.