There are multiple, flexible venting options available with either twin pipe 2" or 3" Schedule 40 PVC/CPVC or 2"x4" 0r 3"x5" Concentric venting. You can also common vent up to 12 units using Rinnai supplied Polypropylene or Schedule 40 PVC/CPVC.
Room air common vent with vertical exhaust
Common venting with sidewall intake with vertical exhaust
Common venting is ULC S636-approved