The wall thimble extension is used to add 6” of length to Wall Pass Through and Wall Thimble for thicker walls.
What is UL 1738? In short, it is a US standard for gas venting systems required in category 3 applications. Category 3 refers to positive vent pressure which is created by exhaust blowers in unit heaters. These vent systems must be constructed of AL29-4C® stainless steel, contain gaskets for sealing, and have a built in mechanical locking band among other requirements. All category 3 vent pipe and fittings offered are single wall.
Item # VN 4650155, 4651155, 4651350