OK - I deviated from using my own build thread for this question with the hope that some of you would have run into this same or similar situation. I'm 99% done with wiring, and I want to run flexible duct for the return in my room - both for soundproofing and because I don't think the electric will pass code as-is (although, unfortunately it was already this way when I opened up the walls).
Here's what I have to work with:
A) picture of the return in the ceiling that feeds up into the HVAC system in the attic. Note that the total inner diameter of that return is either 7.5 or 8 inches and it straddles a ceiling joist.

http://i282.photobucket.com/albums/kk263/markdubphoto/TheaterHVAC-1.jpg
B) side view of that same return from one side of the joist. I plan to cut a chunk out of the upper portion of it so that the flexible duct can be attached. Don't worry, the lower portion will get beefed up with 2X4's similar to a header, although that's probably overkill.

http://i282.photobucket.com/albums/kk263/markdubphoto/TheaterHVAC-2.jpg
C) view of the wall where the return vent existed. Basically, the air through the vent travels up the wall between two sets of 2X4's (non-insulated cavity), makes a 90 degree turn to travel between the ceiling joists (insulated with duct liner) to reach the air return. One potential problem is that a few lines of romex run thru this return space higher up the wall...probably a code issue, correct?

http://i282.photobucket.com/albums/kk263/markdubphoto/TheaterHVAC-3.jpg
Questions for the experienced:
1) Is 3" flexible duct (necessary to fit inside the wall, since I have no outside mechanical space, etc.) enough to serve as a return? i.e., run from the existing vent spot close to the floor, up the wall, and across the ceiling to the return. If it's not enough, would two 3" lines connected to a wye at the return be enough?
2) Which flexible duct should I use, if that matters? e.g., which of the Flexmaster types have you used?
3) Does anyone have pics of how they connected the flexible duct to their existing system and to vents/registers? I'm assuming it's just a matter of getting the right step down connector to the return and liberal duct tape. What about the vent/register close to the floor? Is there a special connector/mating type box for that?

I found a local distributor who can ship just about anything I need, but unfortunately they are not very helpful in actually ID'ing the various parts.
Any help would be greatly appreciated...after I get this resolved, it's insulation and RSIC time!

Can the insulation be removed? The elbow appears to make a right hand turn. Could you not simply remove the elbow and shorten the run previous to the elbow and then reattach the elbow so it no longer straddles the joist?

Flex duct is usually attached with large nylon zip ties, with a couple of sheet metal screws to lock the zip tie so the duct cannot slide off the metal ductwork. Then use silver tape, NOT duct tape. NEVER use duct tape on ductwork.

There'll be some sort of collar available that nails to the studs, before the drywall goes up.

I can get you a picture once my media card reader is returned.

Thanks Tedd. I'm going to see if the setup from the attic will allow me to just move that return a few inches over. I'm mostly concerned about exactly what to get, whether one or two 3" lines will be ok, and how it all goes together.

You need duct work that has at least the same cross sectional area as the original to satisfy the "as designed" air flow. one or two 3 inch round ducts ain't gonna cut it. Are you putting any columns in your theater? Stage and proscenium? if so those might provide enough room for the required size return. You actually might have to use the entire between joist space "as is" if you can't find alternatives.

3) Does anyone have pics of how they connected the flexible duct to their existing system and to vents/registers? I'm assuming it's just a matter of getting the right step down connector to the return and liberal duct tape. What about the vent/register close to the floor? Is there a special connector/mating type box for that?



I don't have any pics handy, but the flexduct can be attached to an existing takeoff (I'm assuming it's round) using an adjustable duct clamp. You can see what I'm referring to here: http://www.wormsway.com/detail.asp?sku=FDC610.

The inner part of the flexduct (the plastic part with the helical wire running through it) gets clamped onto the existing metal duct. You then pull the outer insulation and foil over that and tape it with foil tape (NOT duct tape as was mentioned previously). You can usually find all of this at a local home depot or lowe's.

I'm going to paint in broad strokes here, because without knowing the size of your hvac unit, the size of the room, the method by which the system was designed.. it's impossible to give an accurate description.

That being said, a 3" duct is probably nowheres near enough to furnish a return. The size of a RA duct will almost always be larger than the supply duct to the room (or rooms- see below), and there's no 2" duct, so what size is the supply?

Typically the duct guys will shortcut the installation (right or wrong) and install a RA grill that serves multiple rooms. So even though you have a supply in 10 rooms, you may only have 1 or 2 returns. The returns would be sized to handle the air for several rooms and be substantially larger than any of the supply ducts.

Not knowing if the system was designed properly in the first place, or what changes have been made to room sizes (or installing doors that divide spaces) it's hard to even know if the existing start collar is the proper size.

I designed my hvac (yet to be installed) with rigid piping (except for short runs of flex at the diffusers/returns). The information I got was design for:

900fpm supply
700fpm return

700fpm supply if flex type supply

I used 700fpm supply and 600fpm return just to be safe. My theater will have a 5" supply and a 7" return if I remember correctly, for 12x20x8 with 4 people and no windows, equipment or lights putting heat into the room.

Then you need to choose your grill sizes. Get the spec on the grill from the manufacturer. You need to know how many cfm you're throwing and at what fps. Dennis recommends 250fps at the grill, but I can't find any charts that go below 300fps. I think I ended up with a small diffuser.. maybe 4x8? and two 6x12 returns.. going from memory here.

To sum it all up.. I don't think 3" will be enough. The outcome will be lessened comfort levels and probably increased strain on the hvac system due to increased static pressure.

On the plus side, it's near impossible to make a return too big :) You can always put a damper on it.

Tim

Thanks all. I thought some more about this today, AND Tedd suggested doing this too...I think my approach will be to build two boxes at the floor level - one between each joist - and run two 3" ducts to each box. Then, within the ceiling joist space (lots of room there) I can use some wye's to connect them and merge into the single return. Four 3" ducts should be sufficient, and possibly even better than the existing airflow that occurs in the drywall cavity given pressure differences/overall "tightness." At least this way, I can insulate around them and avoid having such a large resonating empty space in my wall (not to mention the romex running through there).
Thanks again!