Thoughts on this HVAC silencer?

#1

I'm working on some design ideas for a single room studio. For background's sake, the general plan is room-in-room construction with 2 layers of mass in each leaf. It's about 300 sq ft overall. I was planning to go the route of placing a minisplit in the room and depositing fresh air a few feet above it. I figure I will need about 100cfm of fresh air.

I have access to a small adjacent room that I can place the outer silencers in. As there is space for it, those will be pretty "standard". I'm looking at Gregwor's design as a guide.

I realize that everything depends on everything else and it's hard to ask a question without going into every aspect of the design. But I'd like to focus on the inner silencers here. Due to a number of space constraints, I'm looking at placing these inside the inner leaf joists. In order to accommodate the inside out ceiling, I plan to use 2x12 joists. This provides a decent amount of room for treatment and maybe just enough space for the silencers, although the height is still limited to about 6" after adding 1" duct liner. Joists are 24" apart.

Here is a cross section of a pair of joists. The outer ceiling is "outside in" meaning the drywall is facing in.

My ceiling modules alternate between 7' and 8' long (to span 15'). Consequently, I would make the two inner silencers 7' long, probably a little longer than they need to be, but perhaps a little more effective. The inlet is going to be smaller than I'd like due to minimal space between the walls. I think I can fit a bit of 8" flex duct in there, between inner and outer silencers. I calculated the air speed with each change in area. While air enters the box at 300fpm, it would leave at 200fpm and go into a plenum which slows it down further. With a pair of 10x10 registers at the end, I figure about 70fpm (assuming I did the math right, thinking 2 registers at 10x10 would be equivalent to a single 200 sq in opening.

Here's a diagram of the supply. Does this seem like a reasonably effective plan? I'm trying to avoid soffits in the room and there isn't any space above the ceiling, so this seems like the only decent option to me. One thought I had was to make the silencer shorter and the plenum longer, in order to give the air time to slow down. For example, I could make the whole thing 8' long and make half of it a plenum.

The distance from inlet to register needs to be about 11'. The return would pretty much be identical within a joist at the other end of the room.

#2

That looks about right. Is there any chance you could get the duct into the middle of the box, so the airflow splits out in two directions, sideways, with a register at each end of the box? That's a more efficient setup, since you have two parallel paths for the airflow.

Another question: How much isolation are you shooting for, for this room?

- Stuart -

#3

I've seen the split design and that would be ideal but I don't see how I could do it given the geometry of the room. Even if I ran another duct down the adjacent joist and entered through the side, I think it would compromise the structure. Here's another quick layout of the room configuration. I show the two options - extra baffle on the left and bigger plenum on the right. It's not exactly to scale as those outer boxes look really small here.

I guess I'm not sure exactly where to "end" the silencer. I figured after that extra baffle, I would just be inside a normal joist with a ceiling above, but not completely surrounded by mass like the rest of the baffle is. So I don't really want (need?) to make a heavy silencer box that's 7' or 8' long if I don't have to. Hopefully that makes sense.

As far as isolation, it's kind of a cagey answer but I'll take whatever I can get with two layers of mass and two independent rooms. I'm not too concerned with noise coming out of my space, more with the environmental noise coming in. So I'm not going to get in trouble because of insufficient isolation, just annoyed. Also the whole thing is sitting on a concrete slab so I would imagine flanking noise is a limiting factor (for example if I went with 3 or 4 layers of mass). Oh, and I had a structural engineer tell me no more than 2 layers of drywall on the outer ceiling, so that's a limiting factor as well.

#4

It seems like you don't have a lot of options! You should be fine with that setup: running a simple flex duct through the adjacent joist bay would not work: That would be a direct breach of your inner leaf! You'd have to house it in "box" of similar mass to the leaf... that would be a pain, and not necessary. Your plan looks reasonable.

I did notice that you are thinking of putting a booster fan in one of your ducts? Is that necessary? Is the HRV not able to do the job of moving enough air by itself? You might, indeed, need a fan, and that would be a good location for it, but it might be better to choose an HRV that can handle the load all by itself.

- Stuart -

#5

I'm going to try the HRV on its own and see if it's sufficient. So the fan is an option.

What do you think about the two options in the last diagram? The extra baffle vs a larger plenum? Does it matter which way I go? I'm also thinking in either case the last baffle would mark the end of the silencer (and the extra mass that goes with it).

#6

This thread seems as good a place as any to ask about the outer silencers. I think this design is pretty standard but I wanted to make sure the wall penetration is sufficient to maintain isolation. The silencer would all be 2 layers of mass, as is the wall. It's a bit more complicated this way because I would want the input/output to point directly up, thus avoiding a 90 deg bend in the duct and keeping space to a minimum. It also adds an extra "bend" for the airflow.

The small box area in front of the wall would live in between the outer and inner shells. Does this seem like a good approach?

I calculated the interior area would keep 100CFM at close to 100FPM. The 8" duct would again be the bottleneck, but still under 300FPM.

Oh, and the duct liner is not shown here. I think building this will be a bit like Tetris combined with a Rubik's Cube.

#7

Pequod wrote:Source of the postI think building this will be a bit like Tetris combined with a Rubik's Cube.

You're not wrong there! By the time you've built the fourth one you'll probably have it all figured out

Cheers!
Jennifer

#8

usually you put your silencer/plenum right over the wall penetration and then route flex (or semi-rigid) duct via the ceiling soffit to get the vents positioned for best symmetry (in a CR, live rooms can be more about simply getting cross flows right).

#9

I don't know if it was clear but the big side of this box would live in an external equipment room (where the ERV is). The tiny bit of wall represents the whole outer leaf.

I've always understood the wall penetration needed to maintain the same mass as the silencer, hence the "sleeves" that we often see. This sleeve is just intended to direct the air upwards. In the first post you can see that the inner silencers would live in the ceiling joists, that's why these point up. Post 3 shows the void space where the boxes are connected and where the registers would go.

I just want to make sure this concept would maintain the isolation of the outer leaf. Any reason why it wouldn't?

Soundman2020 - Studio Design Forum