Z-Flex 6" back flow preventer is 3" in diamter.  This is a must for vertical tankless water heater installations.  The damper (back flow) prevents freezing air from coming back down through the vent stack and potentially causing freeze damage to tankless water heater heat exchangers which is not covered under warranty from any tankless water heater manufacturer.  Single walled venting, this stainless steel category II/III venting accessory has gasketed connections for a seamless installation.