As mentioned above, I reluctantly concluded recently that it would be best if I relocated my HVAC supply ducts AND my main return duct.
Here is the default HVAC layout before the old room was demo'd.
Here was my original plan for the HT room, which involved leaving the original supply vent locations in place, splitting and re-locating the return into 3 intakes (main, PJ, and A/V rack), like this:
As the weeks and months of my build progressed, I became increasingly aware of the fact this layout was not optimal. Unfortunately, I realized this well after the point when I had my attic space wide open (old ceiling demo'd), when I could have quite easily made some changes that would have eliminated all the work I'm currently doing to correct my oversights regarding the HVAC. Yes, I could have left well enough alone; however, being somewhat the perfectionist, I didn't want to regret that later on.... so, I am now re-arranging the HVAC to a more sensible layout.
Here's the *New and Improved* plan.
You can see the differences are 1) the main return is being relocated to the rear of the room, in between the A/V rack return and the PJ return; and 2) the rearward supply duct is moved to the front of the room.
The main objective of re-evaluating and improving my HVAC layout was motivated by the idea that the supply vents should ideally be in the front of the room. I began to get concerned that with a side-wall configuration, guests on the same side as the vents might get too cold, and guests on the opposite side might get too warm. Quite frankly it probably wouldn't have been a big deal, but since the pain of making a change is less now than when the room is complete, I decided I should do this now. Once I was firm on that decision, my focus turned to the return vents. Same line-of-thinking. As long as I'm tearing things up for a re-do on the supply side, it seemed prudent to re-evaluate the returns as well. I decided to leave the A/V rack and PJ vents as they were, but move the main return to the back of the room. After all, with the supplies moved up front, I really wanted to be sure the air got pulled fully back across all the seats.
This is probably a good time to re-cap the physics of the return layout. The original return was a 9" duct, and as it enters the sub-plenum that 9" duct is still there. In order to encourage the return duct system to help with cooling my A/V rack and projector, in the attic I split the 9" into one each of 8"/5"/4" ducts. The 8" return duct is the "main" return, the 5" is above the A/V rack position, and the 4" is above the planned projector location.
I can't believe I still don't have the soffits completed. I've said it before... it's more complex than I could have possibly imagined in the beginning. A soffit with nothing in it is no big deal, but when you add duct work, conduit, all kinds of wiring, etc.... well, it quickly turns into a BIG DEAL. Lol.
That said, if the soffits were done then I wouldn't be re-doing the HVAC positions. Double-edged sword per se.
Now that I've started making the HVAC changes, I realized today given the height constraint of my soffit, I was going to have to re-position the 2" conduit that runs from the A/V rack to the front speakers. The issue is the main return duct currently enters the room inside the soffit along the same wall. Given the fact it's an 8" flex duct (which btw is 11" wide when one factors in the insulation around it), and the fact my non-IC lights are planned to be centered 13" from the wall and 11" from the soffit edge... well, let's just say the math doesn't all work out. Yes, I know it sounds like the 11" from the soffit edge would work, however there is a 2x4 in the way (one vertical and one horizontal), which is part of the ladder design that I chose for the soffit frame.
Even after removing the flex duct insulation so the net width would be 8", it still won't work due to - again - those 2x4's. The 2" conduit pushes out a maximum of 4 3/8" wherever it's held in place. That max length is actually for the clamps that hold it in place. The duct itself sits about 3.25" off the soffit framing. Furthermore, whether there's insulation on the side of the flex duct or not, it will necessitate a barrier between it and the recessed light fixtures that are not IC (and even if they were IC it wouldn't help against non-insulated flex duct). So, that means building a backer box for at least 1 or 2 of the light fixtures. I've calculated that to require a minimum of 6" for the box, which will include space inside for cement backer board and a minimum 1/2" between the fixture and cement backer board per UL guidelines. Personally, I'd much prefer a 1" buffer minimum on all sides, so the backer box may grow a little wider. We'll see how it will fit when I get around to building it this weekend.
Atm, my plan is to split the 8" duct into 2x 6" ducts. The 6" ducts will just barely fit between the ladder rungs of the soffit framing. To refresh your memory, my soffit is 9 1/2" tall before adding 1 layer 1/2" MDF + 1 layer 5/8" drywall. So, on the outside the soffit will be 10 1/8" tall. On the inside, it is framed in such a way (with 2x4's) that the minimum clearance in any given spot internally is 6 1/2". That means I'll have a 1/2" clearance to squeeze 6" non-insulated flex duct between those framing members inside the soffit.
Now, how to do this? I built a small box to handle the transition from 1x 8" to 2x 6" duct. Sort of like a small duct muffler. Here's a photo after it was built. I applied drywall tape and then mastic around the exterior connections where it meets the MDF. I later applied UL listed foil tape after attaching the 6" flex ducts. I want to be sure this is air-tight, though quite frankly it's not too big a deal if it isn't as any leaks would simply pull from within the soffit anyway. Given the fact the soffit is already air-tight and sealed from the remainder of the house, it shouldn't be a huge deal if that were to happen. But that said, of course I don't want that to happen.
I did a test fit this evening. So far, so good. This weekend I'll finish re-locating the conduit, positioning this - whatever this thing is (above) - and hooking it up. If all goes well, next week I'll order the return and supply grilles from Dayus.
Reminds me... one other note on the returns. I've determined the returns do not need duct liner and they do not need to be insulated while inside the room/soffit. I'll have insulation for about 2-4 feet or so inside the soffit and close to where it enters the attic, just to be sure there's a little stretch there that is insulated to guard against any possible condensation build up from the change in temps. However, based on my research + real world observations, this is much, much less of an issue versus supply air where it's a real issue given the wider range of air temperature differences (room temp vs. HVAC temp vs. attic temp). If I were dealing with a basement construction project, this whole issue wouldn't be a big deal even for the supplies (most likely), but being a 2nd story HT room it's definitely a concern that should not be taken lightly.
The Supply Vents
Onto the supplies....
I decided since I was going to muck around with relocating the supply vents from one side of the room to the front of the room, I might as well plan for installing some rather long lineal diffuser vents. The act of relocating the supplies meant some new logistical challenges. First, both of the duct runs would need to be extended. I could have left one of them as-is (the one already in the front of the room), but doing so would have made the duct runs uneven in length. So, I decided to take the existing one in the front of the room and extend that to the other side of the room (via the front soffit), and take the one originally toward the rear of the room and extend it to the front along the same side wall. Since I was doing all this, it seemed like an opportune time to consider what I might want in the way of vents (e.g. diffusers) and also take into consideration how whatever I put together would fare if I decided in the future that I needed to install push/pull in-line fans for the supply and exhaust. Essentially, I did not want to have to go through this process AGAIN in the future if I determined at some point the airflow was insufficient.
So, to that end I settled on a 5" wide by 36" long template, which basically yields about a 32-34" max or so length after installing duct liner.
I've made two of those. Butt-ugly, I know. But functional.
It was a bit of a challenge to come up with the size for several reasons. First, it obviously needed to fit into the soffit. With a 7" wide duct and a 9 1/2" max height soffit, that was not a trivial task. Not much room for error. Second, my biggest concern was how the vent would line up with the fabric walls, lighting position, distance from the front screen wall, etc.
With the 5" vent opening that I came up with, the soffit ducts (for the diffuser vents) are 7" wide (allowing for 1" thick duct liner on either side). My plan is for the recessed lights along the walls to be centered 13" from the wall. After accounting for the fabric wall panels, this will give them the appearance of being centered in the soffits. It also means the edge of the light fixtures inside the soffits will be 11" from the wall, which in turn means I could technically position something else up to 10 1/2" from the wall. Except for the fact there's a 2x4 frame member in the way. In fact, it's a bit more complicated than that due to my plans for recessed storage shelving along one side wall.
Bottom line is if I were to position a vent next to a recessed light, I'd have enough clearance to not require a backer box for the light. However, after mulling over this issue for several days, I've concluded it would be better (IMHO) to not center the vent where a recessed light is centered.
The subject has been a bit of thorn-in-the-side for me because the alternative to placing a supply vent adjacent to a recessed light fixture is to place a supply vent adjacent to the front columns. On balance though, I think that's a better option after considering different angles and how they will appear.
A considerable issue in my decision making process was the thought that if I were to position the supply vents and lighting next to each other, it would actually be quite a tight fit. Aside from whether or not it's even possible for them to physically co-exist next to each other (it is), there's the fact that the lineal diffusers I'm planning to use (Dayus) have ~1" lip all the way around the vent. So, that means +2" in width. Then consider how close does one want these supply vents to be to the fabric walls? Well, there's obviously a minimum 1" due to the lip of the vent, but that would place the vent and fabric panel immediately next to one another and likely touching. Aside from potential issues with squeezing in one or the other, there's the fact that I don't think that would look the greatest. And functionally, how good of a job are those supply vents going to do if they're blowing right up against the edge of the fabric walls? Therefore, it seems prudent to push the vent out from the wall a few inches.... which means by default they cannot go next to the recessed lights.
The 5" width of the duct I arrived at while meddling with figures on airflow (CFM, FPM), surface area, and a desire to keep all the ducts as discreet as possible.
The original supply vents were 12" x 6" or 72 sq. in. of (theoretical) area. In reality, it's significantly smaller as the metal of the vents takes up quite a bit of that area. But for comparison sake, let's call it 72 sq. in. The max vent size I've chosen then is 170 sq. in. (5x34). Why so much larger?
I didn't have any issues with noise. The HVAC system for the room was designed rather well it seems to me from the original installer. It has 1x 9" supply duct that branches to 2x 7" supply ducts and 1x 9" return duct. So, the supply/return values are even. I've seen comments elsewhere on this forum speculating that the ratio should be 1:2 for supply:return, but quite frankly I have not seen that in the real-world (I've looked at other HVAC systems besides my home's). That said, it's not a bad idea necessarily either. But since I started with a 1:1 ratio, I don't have any plans right now to make the returns larger.
Anyhow, what does that have to do with my supplies? I wanted 1) linear diffuser slots; 2) quiet; 3) unobtrusive visual design; 4) forward-compatibility in the event I decide to add a blower (in-line fan) in the future (i.e. increased FPM should not result in more noise). All those things pointed toward some kind of soffit diffuser of sorts, between the flex duct and the actual vent. It also means I don't want really wide vents.
The end result I came to was to build soffit diffusers, make them as long as I reasonably could, and then before I make the final cuts in the soffit bottoms, decide how long and wide the supply vents will be initially (but at least I know they CAN be up to around 34" long and 5" wide).