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 indoor models require 4'' stainless steel venting. Venting details, including clearances and diagrams, can be found in the ''Use & Care Manual'' of Rheem indoor tankless water heaters.
