1.5 ton ch&a heat pump and ductwork being insatlled in new construction 900sf home, single story, slab on grade. Ductwork will be in attic with ceiling supply registers. Ductwork design is rectangular galvanized plenum and trunkline with round rigid metal supply lines...no flex anywhere.
Supply branch ducts are to be as follows:
2 of 5" rigid metal round ducts (one for bathroom and one for laundry room)
3 of 6" rigid metal round ducts (one for livingrm, 1 for diningrm, one for bedrm)
2 of 7" rigid metal round ducts (one for mstrbedrm, 1 for kitchen)
Totoal CFM's needed for house is 670
The AHU is centrally located in the house and I will be running two trunklines off the plenum, each trunkline running in opposite direction and each trunkline approx 12' long. My question is...I know that if the AHU was located on one end of the house that I would need to run a single trunkline sized to carry 670 cfm which would be a rectangular trunkline sized at 16" X 8". However, I am running two trunklines off the plenum...one trunkline will carry 385 cfm and the other will carry 285 cfm. I need some opinions here...should I make the 385 cfm trunkline at 10"x8" and the 285 cfm trunkline at 8" x 8" and have two different sized trunklines coming off the plenum? Would it be better to have both trunklines the same size...what effect would slightly oversizing the trunklines have?