Duct Chase Sizes
Question:
Are there charts illustrating allowable HVAC duct sizes that can fit between truss chords for various truss depths?
Answer:
Yes. See SBCA’s TTB – Floor Truss Fact Sheet.
Stuart Lewis, from Alpine Engineered Products, tells us: “For a standard 4x2 floor truss (meaning Warren-style webs with 30-inch panels) with double-cut webs, the maximum round duct in the end panel is 5.625, 6.5 and 7.25 inches for 12, 14 and 16 inch truss depths, respectively. By end panel, I mean the 15-inch long panel formed by the outermost diagonal and the end vertical, where the duct would be against the end vertical of the truss. The outermost 30-inch panel (formed by the two outermost diagonals) will permit round ducts up to 7.375, 8.875 and 10 inches for 12, 14 and 16 inch truss depths, if the truss plate joining the two webs together is narrow enough that it does not extend into that opening. At ends of trusses, it is not unusual for the truss plate to extend into that opening. Since the plates usually get smaller as you move away from the bearing, these sizes should be possible in subsequent panels further from the bearing.
For 4x2 trusses with fan-style webbing, also called modified Warren webbing, which is like the Warren-style but the horizontal run per diagonal is twice the distance and vertical webs are added at web-to-bottom chord joints in order to reduce the top chord panel lengths to 30 inches, the maximum round duct sizes are 7, 8.375 and 9.625 inches between vertical and diagonal webs, and 8, 9.875 and 11.625 inches between two diagonals, for 12, 14 and 16 inch depths, respectively.