Just want to know if my cfm for new ducting is on the right track

I replaced the entire ducting system under my house about a year ago. My house is about 1200 sq ft with about 520 sq ft of It under a 22ft high cathedral ceilings. My house had a 14" trunk duct that had been collapsed I guess by a cable installer or something. And the rest was all 6" flex for all the branches. I pulled it all and took the trunk to 16" and replaced all the branches with round metal ducting to each register with dampers at each branch take off. It is an extended plenum system at about 20' for the 16" then to 8" for the remaining 12'. I insulated it all with the r8 thick bear looking wrap and mastic and foil taped all joints and used sheet metal screws. Never did it before in my life but I got a calculator for heat gain and cooling loss and went to town. My unit is 3 tons which it works phenomenally with the ducting I installed. What I would like to do is add 2 bigger registers for the living room in the existing spots. They are currently 6" each I want to upgrade them to 7" and I also want to add another 6" to about 18" from the end of the extended plenum since it only has one branch coming off it at a 6" which is about 3' from where it goes from 16" to 8" .reason I want to do this is because I have cathedral ceilings in this area and need a little better cooling. I am going to upgrade my unit to a 3.5 ton 16 seer unit at the sametime. If the unit specifications say it handles 1200 cfm then as long as the branches are spaced apart the appropriate dimensions from each other, then all I have to do is add each branch up and even with the 2 new sizes and 1 extra 6" branch and run as long as I don't exceed the 1200 cfm for the fan motor on the unit will I be ok?

 

Welcome to the forums.

Just wanted to welcome you. My specialty is in the servicing.... especially the electrical and control end. Can't help you much on the duct work side.

There will be others stopping by.

 

Tyvm I appreciate that a lot