Rinnai’s Ultra Series condensing tankless water heater models RUC98i and RUC80i feature a built-in system that allows for both concentric and twin-pipe PVC venting from the same unit. For concentric venting, installers simply remove and discard the exhaust adapter ring. For twin-pipe PVC configurations, the intake cap is removed and discarded. With the twin pipe option, the installer can use 3” or 4” PVC/CPVC pipe for the intake and exhaust. Rinnai.