This is from American Metal, Amerivent, installation instructions: (bold is mine)
- ROUND AMERIVENT® TYPE B GAS VENT
- - Model E - Sizes 3" through 6"
- - Model R - Sizes 7" through 30"
- - - Elbows and Offsets
- - - - Whenever Model E or R Gas Vent Pipe is changed from a vertical direction, the Vent system must be resupported before and after the change in direction. The support should provide rigid stability, both vertical and horizontally.
From the Metal-Fab installation instructions: (bold is mine)
- 11. For horizontal and lateral runs, it is recommended that
support be used every 5 feet (1524 mm). Plumbers straps are commonly used. They should be positioned to maintain the 1 inch (25.4 mm) clearance to combustibles.
Simpson states (bold is mine) "Wall Straps every four feet are recommended." and shows them on both the vertical and lateral sections in their related drawing.
Selkirk Metalbestos states (bold is mine) "Lateran runs are to be supported at least every 5 feet."
Just a quick check of some of the more common manufacturers installation instructions.
