Dampers. Are they good or bad? Originally I planned to install one on each trunk off the plenum. So in the final drawing, damper on the left 14", damper on the right 14" after the Wye, and a damper on the 10".
Best used in the supply runs, to balance the whole system.
Left 14" rigid pipe. My Heat Loss/Heat Gain and Duct Report says the total CFM need for the rooms is 569 CFM and calls for a 12" rigid pipe. I was planning to reduce it to 12" which would give me a bit more physical room when dealing with the squeezed 10" return. See the middle picture in Part 2 post. Would it be wrong to reduce from 14" to a 12".
Not really wrong. But, depending on the number of ells, transitions, bends, etc, may be a tad small.
Final question, I hope. You suggest a 10" off the 14" Wye to supply air to the two downstairs bedrooms. Report says it can be done with an 8" rigid as total CFM need for the two rooms is 200 CFM. Pipe distance for either room will not be over 15" from the plenum.
Its not just linear length that counts. But also all fittings, ells, transition, etc in that run. From the plenum to the register that counts as part of the length.
If you decrease size, and its a bit small, you can't adjust a damper to give it more air.
If its a little larger then needed, you can close a damper to restrict it.