Going from 4" to 6" ducts

Lynn

The top of my plenum in the attic has six 4" holes for ducts running to each room. With no trunk and no dampers, I would like to increase the cfm to the farthest two rooms, 12' aned 15' away. If I went with 6" ducts for these two rooms, would I also see an increase in cfm at the registers? Would this drastically affect the balance of cfm in the remaining four holes/ducts/registers?

murray

you can increase the size of the runs .though this sounds like an older instalation(i hope)check the return air ,it might be easier to give the furn more air thru return but 4" runs are really only good for rooms the size of a bathrooms

Tules

Your total supply depends directly on the amount of Returned air.Plus how are you manipulating the supply.
Best case you want to "pull" the supply to the center of the contitioned floorspace.Most installers will introduce two(2) Return air ducts to manipulate comfort space to suite the heated/conditioned sq.footage.
Point is;there is a minimum Return air cfm,but the more the better.visualize if you would=6-4x8 supply vents at 2tn. huffing supply tirelessly but only one Return (prolly 20-22.5"flex)attempting to completely Recover total air waste :c(.
With that supply from ceiling?Two (2) low (chase or cavity)installed Returns would allow the system to breathe freely,and drasticaly reduce "cold"/"warm" areas. Rule of thumb=Returned air>supply

GL with that :c)

Umby

Thanks Tules.

Since I made that post I took out my 4" duct, cut and installed a 6" duct (R6) to the farthest room (my bedroom) exactly 25' away, into a ceiling supply register over an exterior wall. With still no return registers, you have to lean into the door to get it to open! I am debating whether to cut a return register over the door (and into the hallway where the furnace return grate is located), or in the bottom of the door, or both...?