4" Universal Appliance Adaptor with Back-Flow Preventer For: Takagi, Paloma, Rheem, Noritz tankless water heaters. This Universal Adaptor has seals on both ends and has a back-flow preventer built in. Useful in colder climates to prevent cold air from flowing back into your heat exchanger. Universal adaptor connects tankless water heater to the 4 Z-Flex Z-Vent Stainless Steel Venting