I am zoning our two stair house into two zones, one upstairs and one downstairs. Our current HVAC unit provides 1600 cfm. I did a Manual J calculation and it says I need appox 800 cfm upstairs and 800 cfm downstairs. I have a 16x10 supply duct that feeds the upstairs and another that feeds the downstairs. Each of these is good for appox 800 cfm.
I would like to install the bypass damper on the upstairs branch. My concern is that if the downstairs branch is closed, it would push the full 1600 cfm upstairs until it reaches the bypass damper. Is it a problem to have a restriction in the duct size (16x10) for the first 8' until I reach the bypass damper? I cannot change this duct size due to existing wires and plumbing.