Rinnai Gas Direct Vented Space Heaters can use extension venting for up to 13 feet with max 2 elbows (The Bend at the heater does not count towards the max of 2 elbows). Pipe must be accessible and cannot be buried in a wall and must be accessible.
Rinnai extensions exceeding this length should drain to the outside and sloped 3 Degree downward slope to allow moisture to drain out of the pipe instead into the heater, which would damage the unit.
