"Rheem Tankless Water Heater RRTG20151R 4"", 12"" Length Vent Pipe All indoor tankless gas water heaters must be vented vertically or horizontally to the outdoors using Category III stainless steel venting material (No other venting material is permitted). RTG-42PV indoor models require 3"" stainless steel venting. RTG-74PV and GT-199PV i...