Fix supply & returns

[palex1915]

"Most everything, HVAC wise, in my home is working fine. But, I would like to correct some things that were done before I moved in. I have 2 rooms that are supplied w/ 1 large duct. The end of the large duct run supplies 1 room & a rectangular hole cut 8' away, serves as supply for the other room! There is no return for either room! (or in this part of the house) I want to correct these supply & return issues. Bedroom 1 is 11'2" x 16'8" and is approx. 24' from the main unit. What would be the proper size of the supply duct? Bedroom 2 is 14' x 15'3" and is approx. 17' from the main unit. What would be the proper size of the supply duct? Now the returns. Would 8" flex for the returns in bedroom 1 & 2, be OK ??? ( I have access to free 8" flex duct !!!) Also what would be the proper size of the return grill, with a filter, for these 8" returns? All of my returns have the filters in the living area, so I do not have to go into the attic. Thank you in advance, Paul in Texas."

[Freon]

Are the rooms comfortable now with good air flow into each? If so, then leave the supply ducting alone and add returns or cut the bottom of the doors so they're about 1.5" higher than the floor level.

[palex1915]

The large duct that supplies these 2 room is inside a fir down, which runs the length of one of the rooms. I want to eliminate this fir down. Since there is no return in this part of the house, (this part of the house is an add on) I want to add returns in both rooms. Would the 8' flex returns be OK? What size return grill ( a grill that holds a filter) for these 8' returns?

[Freon]

- 8" duct will allow ~230 cfm. Make sure the ducting is taught and no hard 90 degree bends. If there are hard bends, use 200 cfm.

~100 sq in of filter will handle 200 cfm of air flow (~2cfm per sq in of filter). Pick the best fit dimensions for the filter grill as long as it holds 100 sq in of filter (e.g. 10x10 filter)

If you are in a mouse/rat prone area, go to metal ducting with insulation.

I am not sure what a 'fir down' is.

Each room will need an 8" feed or ~230 cfm per room. The main trunk to give you a total supply of ~460 cfm (230 cfm for each 8" duct leg) will need to be ~12". You will want to have a damper on the last 8" leg to use to balance the air flow to each room.

[palex1915]

Thank you for your reply. I plan to run seperate supply and return ducts to each room. I'm probably not calling it by the correct name, but my definition of a "fir down" is that hollow area, made where the ceiling and a wall meet. Built down approx. 12" and out approx. 12". In my house, this hollow area runs the length of one room, and inside this hollow 12"x12"x14' area, they ran the ductwork for 2 bedrooms. Some older houses have this hollow area above the cabinets in the kitchen. What is the correct term for this hollow area??? Thanks again.