4" Z-Vent Universal Appliance Adapter w/Backflow Preventer 4" Universal Appliance Adaptor with Back-Flow Preventer For some (not all) 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