In a preferred embodiment, there is provided a water heater venting assembly for directing a combustion air and a flue gas between an outdoor atmosphere and a water heater, and which includes a generally hollow housing defining a combustion air aperture, a flue gas exhaust conduit disposed in the housing, and an adjustable inlet duct coupling assembly having a retention member for placement in the housing proximal to the aperture, a gasket for placement around an outer periphery of the aperture and a combustion air supply member. The retention member is for retaining the supply member in fluid sealing engagement with the gasket to effect fluid communication between the aperture and the supply member, and is sized to permit slidable movement of the supply member relative to the gasket.