I am told the max capacity of the 2 - 7 1/4" x 14 1/2" trunks I have, one running North and one running South from the plenum, is 500 cfm each.
The 3 ton heat pump to be installed needs to move 1200cfm. The trunks can move 2 x 500 = 1000 cfm., so I need to move 200 more cfm.
Is it an option to draw off the additional 200 cfm, by adding additional ducts to the basement, coming right off the central plenum? 1 - 7" (135cfm) plus 1- 5" (65cfm)?
In theory, taking 200 directly from the plenum, that then leaves 1000 cfm to go out through the ducts, so the duct capacity is not exceeded.
Is this standard practice? Or should this be avoided for some reason, and ducts should always be taken off the trunks, and never directly from the plenum?