Air Conditioning - Return Installation

Hello,
I am installing a return for my HVAC system and need some help. Here is a quick rundown of the system

1. Split A/C 3ton outside. Heater in Attic
2. 1200 sq ft 1932 home. Lathe and Plaster (ugg)
3. Attic supply ducting 3- 4x10 and 4 - 6x12 registers
4. 6 and 8 inch supply flex ducting
5. Single return is to be centrally located in hallway ceiling. Hallway is 40" W
6. I would like to use a quality filter.

I am trying to be least evasive to the structure. I would like to put it between rafters (14") by using a 14 x 30" filter/return 420 sq."

My question(s) are:
1. Can I install this return easily between rafter which is exactly 14" ? Attic clearance is good.

2. I need to frame up the ends, do I frame first or have the custom return made and frame around it?

Thanks!
-Robert

Things might be a little tight. The filter might be 14x30 but the rack is going to slightly larger. You could surface mount the filter rack, the duct work itself will be much closer to that perfect 14x30 dimension.

A single 14x30 is likely to whistle something terrible. I suggest using two of them with a 12" duct from each. Check the dimensions of the filter grilles before you buy them. If you have exactly 14" between ceiling joists, a 14" filter grille may not fit but if you are good with a circular saw you can shave enough off the joist edge to allow the grille to fit. When I installed mine, I put the grille in then framed.

You can install 14" wide return filter frame between 16" on center studs, but some mild forcing may be required depending on how straight the studs are. Slip the filter frame through, then use sheet metal on all sides to make your return plenum. Use sheet metal screws through the frame and the new sheet metal to secure to the vertical studs. The 14x30 filter is marginal. You should strive for 400 ft. per minimum velocity at filter to avoid noise [3 ton a/c is usually 1200 cfm]. This means 14 x 36 if you can find it [this allows about 20% for metal fin obstruction]. Or you could install double 14x 20 filter frames stacked and make your plenum fit around both frames [be sure to seal between the stacked frames so air won't bypass between them]. Or, you could try the 14x30 and live with the noise, or select a lower fan speed to decrease velocity.

Just don't make the same mistake I did & get a filter grille with oddball size filters. Mine is 14x24 & I have to get them at my local supply house & pay at least triple what I should. Both 14x20 & 14x25 are quite common with 14x20 being easier to find. 14x30 is not extremely hard to find but not quite as common as the others.

This is great info, thank you. The hallway is 40" wide. Can I get close to the edge with a 14x36" or will I be in pushing it? Are there codes i need to worry about?

16" on center will work for a modern 2x4 unfortunatley I have the real 2x4 in this old house.
-Robert