The Lochinvar & A.O. Smith 100322375 21" Sidewall Termination Concentric Vent Kit is an OEM venting component designed for compatible direct-vent gas water heaters and boilers. Its concentric pipe design combines combustion-air intake and exhaust-gas discharge through a single sidewall opening, simplifying installation while maintaining proper ventilation. The kit is ideal for residen... https://www.partshnc.com/100322375-lochinvar-ao-smith-21-sidewall-term-conc-vnt