Help with Planning & Building of my Recording Studio

[Studio45]

Hi everyone!

Can't believe it's already been close to a year since I’ve last posted about my build but good thing is I’m back at it!

I’ve had to put the studio on hold so that I could focus on other projects around the house but a big reason was also because I needed to take a step back and put more work and thought into the planning and design phase. I had done a lot of reading on designing an HVAC system for a studio but unfortunately even with all this reading and research, I still have some questions related to that. More specifically with getting Fresh Air into my two rooms as well as figuring out the size of my silencer boxes. Here are a few issues/questions that I have:

1) Starting with the Fresh Air issue, from what I’ve been able to gather, I should aim to have 25% to 30% of fresh air introduced into my duct system. To give a bit of context, the studio is located above my garage and has its own AHU. The rest of the house has its own AHU with built in HRV unit. What I’m trying to figure out is since my house already has a HRV unit I’d ultimately want to try and piggyback off of it in order to get my fresh air into my studio. Is that something that can be done?

2) Since having an HRV means having an extra take-off on the supply and on the return ducts….would there need to be silencer boxes for those too?

3) About my systems Total Equivalent Length, I’ve been able figure out a lot of math for the design but I’m wondering if the EL should include the restriction of the silencer boxes or not. Like does the total EL include the silencer boxes or up until the silencer box input? If the total EL should be with the silencer boxes….I’d like to know how to add that up to the total. This brings up my next question.

4) Knowing how much CFM needed for each room…how can I figure out the sizes of the boxes? I know that the boxes should have at least 3 baffles and should have double the volume of the input so say I have a 6” round metal duct going in the box, the pathway of the box should be double the volume all the way through the output correct? I’m just wondering how and/or what determines the size of the box. Based on my CFM requirements for the LR (185 CFM) and CR (142 CFM ) I was figuring that 4 x 6” round ducts (flex) should suffice per room. (2x supply and 2x returns). This would be pre-silencer boxes. Looks like a 6” round duct should be good for approx. 100cfm.

I’m going to stop here for now and hope that someone will chime in on this and give me some feedback and hopefully that will be enough for me to keep plugging my numbers into my design and start planning the sizing of the silencer boxes and plan to make enough room above my rooms to put them on. I’ve got all kinds of number already if I’m missing anything please let me know and I’ll gladly share.

Cheers and Thanks in advance!!

[Soundman2020]

1) Starting with the Fresh Air issue, from what I’ve been able to gather, I should aim to have 25% to 30% of fresh air introduced into my duct system. To give a bit of context, the studio is located above my garage and has its own AHU. The rest of the house has its own AHU with built in HRV unit. What I’m trying to figure out is since my house already has a HRV unit I’d ultimately want to try and piggyback off of it in order to get my fresh air into my studio. Is that something that can be done?

Probably not. I think you'll find that the house system was designed as a unit, with the pieces integrated together to work hand-in-hand. There's probably a controller somewhere that keeps things flowing smoothly, and trying to tap off extra air out of an back into the HRV would likely upset things. At the very least, you'd be increasing the static pressure load on the HRV, and perhaps robbing the house of the right amount of fresh air flow / stale air exhaust.

At the very least, I would suggest you hire a competent HVAC expert to take a look at your system and tell you if it is possible, but my guess would be no.

2) Since having an HRV means having an extra take-off on the supply and on the return ducts….would there need to be silencer boxes for those too?

The way I design HVAC systems for studios, is to keep the AHU and HRV outside of the studio area completely: outside the outer-leaf, in fact. That way, I only have one silencer duct on the incoming air supply, and one on the outgoing air supply, and I can change anything I want on either side without affecting the other. That's how I'd suggest that you do yours.

but I’m wondering if the EL should include the restriction of the silencer boxes or not.

Yes. Static pressure is affected by EVERYTHING on the air path that your AHU "sees". That includes, ducts, dampers filter, silencer boxes, registers, ... everything. If the air flow passes through it, then you count it. The only time it gets more complex is if you split your air-stream into two parallel and identical paths: in that case, you treat ONLY THAT PART of the flow the same way as you would treat two resistors in parallel.

If the total EL should be with the silencer boxes….I’d like to know how to add that up to the total.

The same rules apply: since silencer boxes are mostly just 90° turns count those and do math. But do also consider that the cross section is much larger inside the silencer box.

Knowing how much CFM needed for each room…how can I figure out the sizes of the boxes?

Divide by required RATE (F^3/M) by the VELOCITY (F/M) and you will get the cross sectional area (F^2). That's the cross sectional of the air flow that you will need at the register. In other words, the total open area of the register. Work backwards from there: you need a change of at least a factor of 2 in the cross-sectional area where the air flow enters and exists the silencer box. In general I try to go up by a factor of two where the duct enters the box, then up by another factor of two getting to the register. In other words, the flow velocity can be two to four times higher in the duct than it is as the register.

should have double the volume of the input

No. Not double the volume: double the cross sectional area.

say I have a 6” round metal duct going in the box, the pathway of the box should be double the volume all the way through the output correct?

The area of a 6" duct is roughly 27 in2, so you would need at least 54 in2 cross-sectional area inside your silencer box. More is better.

I’m just wondering how and/or what determines the size of the box.

Think it through: if the duct has a cross sectional area of 27 in2, and you need at least double that, then that's what determines the size of the silencer! You need an internal cross-sectional area of at least 54 in2, throughout the entire length of the air path: in other words, no matter where you measure the cross-sectional area that the air flow "sees" inside the box, it must be at least 52 in2.

ased on my CFM requirements for the LR (185 CFM) and CR (142 CFM )

Do the math! Don't guess! You can't know what size duct you need unless you also know what your target air flow velocity is, at the register. What is that? What velocity do you want? You know your total RATE (327 CFM), but in order to calculate your duct sizes, register size, and silencer box dimensions, you need to also define the air flow velocity at the register.

I was figuring that 4 x 6” round ducts (flex) should suffice per room. (2x supply and 2x returns).

Did you do the math? You are saying that 2x6" = 54 in2 is a large enough cross section to carry 185 CFM flow rate... does that give you a reasonable velocity in the duct? Also, if you are feeding your silencer box with two ducts totaling 54", then the interior cross section needs to be at least 108 in2. Is that reasonable? What size register would you then need in order to get the target flow velocity that you have in mind?

I can't do the math for you: you didn't say what the velocity is, so I can't help much...

Looks like a 6” round duct should be good for approx. 100cfm.

For what VELOCITY???? That's what you are missing here: you are talking about rates and diameters, but you also need to be talking velocities and cross-sectional areas... If not, then you are just guessing... They all go together...

- Stuart -

[Stadank0]

Was gonna jump in there and try and contribute for once!! Kinda pointless now....

[Studio45]

Probably not. I think you'll find that the house system was designed as a unit, with the pieces integrated together to work hand-in-hand. There's probably a controller somewhere that keeps things flowing smoothly, and trying to tap off extra air out of an back into the HRV would likely upset things. At the very least, you'd be increasing the static pressure load on the HRV, and perhaps robbing the house of the right amount of fresh air flow / stale air exhaust.

At the very least, I would suggest you hire a competent HVAC expert to take a look at your system and tell you if it is possible, but my guess would be no.

Ok that's unfortunate for sure as I was not expecting to install a dedicated HRV just for the studio . I totally understand the reason behind having one for sure but I really thought for such a small studio that I could potentially get my "fresh-air" from my home since it has an central HRV. In fact both the house AHU and HRV were originally sized for the entire house. We just ended up not running any ducts to where the studio is because it was too much trouble to get a supply and return all the way up there.
I figured when the time came to build the studio I would simply get a dedicated AHU so that's what I did but I didn't expect to need a new HRV. I will try and talk it over with an HVAC pro but I have a feeling he won't have a clue as to what I'm trying to do and will likely just tell me I'm good with my original idea :/ I'll look for small HRV units and see how I can integrate this alongside my studio's AHU.

Concerning the Fresh-Air requirements. I'm trying to figure out how much fresh-air I need for the studio and wonder if just I need to do the math for each individual room or for the entire studio? I guess it comes out to the same since the system is for the entire studio?

I think on a very busy day I could fit maybe 8 people in there (say 5 people recording and 3 in the control room) but that will be very rare I’m sure. In most cases (like 80% of the time) I’ll be the only one up there mixing or recording with a friend or 2.

So from what I’ve gathered looks like 15 CFM per person should be adequate and think 3 people (45 CFM) should be the average fresh air requirements and the max should be 8 people (120 CFM).

To properly size the HRV unit, should it be sized based on an average of 45 CFM or simply from the max of 120 CFM? Again, I feel as though the average max will be 5 people with very odd times where it’s a party of 8.

Here's an HRV I'm considering that could potentially suite my needs https://www.venmar.ca/50-air-exchangers-k8-hrv.html

The way I design HVAC systems for studios, is to keep the AHU and HRV outside of the studio area completely: outside the outer-leaf, in fact. That way, I only have one silencer duct on the incoming air supply, and one on the outgoing air supply, and I can change anything I want on either side without affecting the other. That's how I'd suggest that you do yours.

My AHU (and future HRV) is in a storage/HVAC closet that's part of the house so technically I suppose they are outside of the outer leaf? I've got some pictures showing where the AHU is in my previous posts. Note that I still have to frame the outer leaf wall.

Yes. Static pressure is affected by EVERYTHING on the air path that your AHU "sees". That includes, ducts, dampers filter, silencer boxes, registers, ... everything. If the air flow passes through it, then you count it. The only time it gets more complex is if you split your air-stream into two parallel and identical paths: in that case, you treat ONLY THAT PART of the flow the same way as you would treat two resistors in parallel.

Ok! Got it! I've done more reading on this and I've got what I need to figure all that out! I've been able to figure out what my Available Status Pressure is by adding the following:
Trying to design my system with a Total External Static Pressure (iwc) of 0.5wc. I've got 1 x air-filter (0.15) and a total of 4 x registers (0.03 = 0.12) so my ASP should be (0.23 wc). I'll have to recalculate all the equivalent lengths of the duct work and silencer boxes so I can get the final numbers for the friction rate but I think I'm on the right path on getting the math done for this part...I think lol.

Divide by required RATE (F^3/M) by the VELOCITY (F/M) and you will get the cross sectional area (F^2). That's the cross sectional of the air flow that you will need at the register. In other words, the total open area of the register. Work backwards from there: you need a change of at least a factor of 2 in the cross-sectional area where the air flow enters and exits the silencer box. In general I try to go up by a factor of two where the duct enters the box, then up by another factor of two getting to the register. In other words, the flow velocity can be two to four times higher in the duct than it is as the register.

Ok! I think I’ve got that figured out. Knowing I should aim for 200 FPM at the registers here’s the math for both my rooms:

- Live Room: 185 CFM /200 FPM = 0.925 X 144 = 133.2 in2 cross-sectional area
- Control Room: 142.5 CFM / 200 FPM = 0.7125 x 144 = 102.6 in2 cross-sectional area

I understand that from there my silencer box needs a pathway that matches that cross-sectional area with at least 3 baffles? Correct? I’m just confused with how that matches up with the inlet duct.

I’m also not sure what you mean when you say “In general I try to go up by a factor of two where the duct enters the box, then up by another factor of two getting to the register.” If I’m working backwards, does that mean I’d go down instead of up?

Based on an example you’ve put together if I used an 8” round metal duct for 185 CFM as the inlet with a cross-sectional area of 50.27 in2 (entering the box). I would need to make sure the rest of the box pathway would have a cross-sectional area that matches or is as close as possible to my calculated register surface area of (133.2 in2)??

No. Not double the volume: double the cross sectional area.

right! Thanks!

Do the math! Don't guess! You can't know what size duct you need unless you also know what your target air flow velocity is, at the register. What is that? What velocity do you want? You know your total RATE (327 CFM), but in order to calculate your duct sizes, register size, and silencer box dimensions, you need to also define the air flow velocity at the register.

Ok from my math above, target velocity at the registers I would aim for 200 FPM (anything less than 300 is good right?)

Did you do the math? You are saying that 2x6" = 54 in2 is a large enough cross section to carry 185 CFM flow rate... does that give you a reasonable velocity in the duct? Also, if you are feeding your silencer box with two ducts totaling 54", then the interior cross section needs to be at least 108 in2. Is that reasonable? What size register would you then need in order to get the target flow velocity that you have in mind?

I can't do the math for you: you didn't say what the velocity is, so I can't help much...

Sorry, I had not done the math no but I think I'm starting to figure it out slowly now. Working my way back from the registers I can figure out the size of my silencers (ex; for the Live Room the silencer box needs a cross-sectional area of 133.2 in2. (185 CFM /200 FPM = 0.925 X 144 = 133.2 in2)

I'm a bit lost as to how that matches up with the air-flow rate / velocity coming from my AHU. Specs of my AHU shows that at 0.5wc total external static pressure and fan-speed set to medium. I should have a nominal air-flow of approx 646 CFM but from there I'm a little lost and not sure how to put the two together.

Sorry for all the additional questions.... I think once I've got all the HVAC stuff settled I'll be finally ready to start

Thanks again for any help you guys are able to provide!

Cheers!!

[Studio45]

Good day to all! I hope everyone is ready for Xmas (man time flies when you’re planning for a studio haha)

In order to properly describe my HVAC design I thought it would share and post the actual math from my build in hopes that this would help others struggling with the same questions/issues that I have.

Here's a summary of each room. As per Stuart's advice I'm trying to start at the registers and working my way backwards all the way to the AHU.
(Please note that I have sloped ceilings so I've divided the total height by 2)

Control Room
Width: 19' Length: 10' Height: 7'
SQ/FT: 190 sq/ft
Air Volume: 1330 ft2
Required Air Changes per/hr: 6
Total Air Volume to move each hour: 1330 x 6 = 7980 ft2
Required CFM to move/exchange per hour: 7980 / 60 = 133 CFM
Required Fresh Air @ 30%: 133 * 0.3 = 40 CFM
Required Cross-Sectional Area at exit of Silencer boxes (Register) @ recommended 200 FPM: (133 CFM / 200 FPM) * 144 = 96 in2
Minimum Cross-Sectional Area at entry of Silencer boxes: 96 in2 / 2 = 48 in2

LiveRoom
Width: 19' Length: 13' Height: 7'
SQ/FT: 247 sq/ft
Air Volume: 1729 ft2
Required Air Changes per/hr: 6
Total Air Volume to move each hour: 1729 x 6 = 10374 ft2
Required CFM to move/exchange per hour: 10374 / 60 = 173 CFM
Required Fresh Air @ 30%: 173 * 0.3 = 52 CFM
Required Cross-Sectional Area at exit of Silencer boxes (Register) @ recommended 200 FPM: (173 CFM / 200 FPM) * 144 = 125 in2
Minimum Cross-Sectional Area at entry of Silencer boxes: 125 in2 / 2 = 63 in2

From the above calculations, I’ve got a few questions/confirmation.

Q1) Am I right to assume that the cross-sectional area at the entry of the silencer box can be half of the exit? From what I understand bigger is better?

Q2) I’m having a hard time understanding how I can calculate velocities and CFMs of branches coming off of my main supply trunk. I’ve got a main supply trunk that is 8” x 10” (80 in2) and I’d like to install a 8” round-duct take-off from the trunk and then split that into two branches for my two rooms with some sort of Wye or Tee fitting. I’m just not sure how I can figure out the velocity of each branch. Do i simply divide in 2? (Ex; if my AHU has a nominal air flow of 800 CFM and my cross-sectional area is 80 in2 that would give me approx 800 CFM / 80in2 * 144 = 1440 FPM at the end of my trunk going into the tee and two branches, does that mean I’d have around 720 FPM at each branch? If each branch is the same size such as 8” round-duct?)

Q3) I’ve read many times that there should be a silencer box for each wall leaf penetration (outer and inner). My AHU is outside of my outer leaf wall so does that mean I HAVE to have one for each leaf regardless? Considering I’m aiming for 40db of isolation MAX if I’m lucky 50db. I was also thinking that if I needed to reduce velocities even more (if my silencers are enough) that I could build my own sort of custom Tee fitting as a silencer box and from there tee off to the other silencers of each room. I’ve seen a few designs from Stuart that do something similar to that where the main trunk goes to a big box then split off to other silencer boxes.

Again, words can't describe my appreciation for this forum and the amazing members that contribute to it. Without you guys home based studios would be almost impossible.

Happy Holidays to everyone! Make sure to spend some quality time with all your loved ones and not so much on your studio planning during the holidays haha

Cheers,

Francis,

[Gregwor]

Q1) Am I right to assume that the cross-sectional area at the entry of the silencer box can be half of the exit? From what I understand bigger is better?

Yes.

Q2) I’m having a hard time understanding how I can calculate velocities and CFMs of branches coming off of my main supply trunk. I’ve got a main supply trunk that is 8” x 10” (80 in2) and I’d like to install a 8” round-duct take-off from the trunk and then split that into two branches for my two rooms with some sort of Wye or Tee fitting. I’m just not sure how I can figure out the velocity of each branch. Do i simply divide in 2? (Ex; if my AHU has a nominal air flow of 800 CFM and my cross-sectional area is 80 in2 that would give me approx 800 CFM / 80in2 * 144 = 1440 FPM at the end of my trunk going into the tee and two branches, does that mean I’d have around 720 FPM at each branch? If each branch is the same size such as 8” round-duct?)

I use an ASHRAE iPhone app called ASHRAE DFDB. It was pretty cheap and it will tell you your velocities and loss coefficients for your branches/fittings/etc.

Q3) I’ve read many times that there should be a silencer box for each wall leaf penetration (outer and inner). My AHU is outside of my outer leaf wall so does that mean I HAVE to have one for each leaf regardless? Considering I’m aiming for 40db of isolation MAX if I’m lucky 50db. I was also thinking that if I needed to reduce velocities even more (if my silencers are enough) that I could build my own sort of custom Tee fitting as a silencer box and from there tee off to the other silencers of each room. I’ve seen a few designs from Stuart that do something similar to that where the main trunk goes to a big box then split off to other silencer boxes.

Like a door or a window, an HVAC penetration through a leaf must maintain the same level of isolation as the rest of the leaf. The level of isolation the silencer box provides is referred to as insertion loss. There is no single solution to achieve desirable levels of insertion loss like there is with transmission loss (air tight mass). Instead, to achieve insertion loss, we are trying to squeeze as much mojo into it as possible. This means lots of 180 degree turns, thick duct liner, and abrupt cross sectional changes. Unfortunately for us studio builders, we have to cover the entire audio spectrum. For things like generator engines that have a limited operating RPM, it's pretty easy to tune a silencer with a simple tuned stub. 20-20kHz is a different story.

Having said that, if you have two rooms that share an outer leaf you can totally make one penetration and have a single outer leaf silencer box that Y's off to each room. Then, each room would have it's own inner leaf silencer box.

I hope that helps!

Greg

[Studio45]

Happy New Year everyone!! Hope you all had some downtime with your loved ones this holiday season. If you're a bit like me you couldn't help yourself and spent some time trying to make progress on your studio planning/build haha.

Thanks for replying Gregwor! Much appreciated!

Like a door or a window, an HVAC penetration through a leaf must maintain the same level of isolation as the rest of the leaf. The level of isolation the silencer box provides is referred to as insertion loss. There is no single solution to achieve desirable levels of insertion loss like there is with transmission loss (air tight mass). Instead, to achieve insertion loss, we are trying to squeeze as much mojo into it as possible. This means lots of 180 degree turns, thick duct liner, and abrupt cross sectional changes. Unfortunately for us studio builders, we have to cover the entire audio spectrum. For things like generator engines that have a limited operating RPM, it's pretty easy to tune a silencer with a simple tuned stub. 20-20kHz is a different story.

Having said that, if you have two rooms that share an outer leaf you can totally make one penetration and have a single outer leaf silencer box that Y's off to each room. Then, each room would have it's own inner leaf silencer box.

From doing a bit more reading on this I think I'll be okay. My silencers and ducts will live in between the inner and outer leaf walls so I think I'll be okay with just one silencer per register. There will only be 1 outer leaf penetration for the main supply and return trunk coming out of the HVAC closet and then a inner leaf penetration from the silencer box register exit into the room. Once I have my HVAC plan confirmed, I'll update my Sketchup plan with all these details so you can see what I mean. Unless a quick example is needed to clarification.

I use an ASHRAE iPhone app called ASHRAE DFDB. It was pretty cheap and it will tell you your velocities and loss coefficients for your branches/fittings/etc.

Done! I've downloaded the app and think it's awesome!! I've been able to use it to figure out the pressure loss and velocity of my main trunk all the way up to where I'll be branching out two separate runs for each room. Unfortunately, I'm still stuck trying to find the numbers for each branch . I guess I don't fully understand how to use the app properly yet for that purpose and I'm really hoping someone can chime in and help me figure out how I can get the remainder of my numbers.

Here's where I'm stuck....

Currently at the end of my main trunk I've got a 10" x 10" rectangular duct that's carrying 800 CFM from my AHU at a velocity of 1152 FPM and the total friction loss at that point is 0.44 in wc. What I'm trying to achieve is a few feet before the main trunk ends I'd like to branch out straight up two 8" round ducts spaced about 2ft apart that would each carry 200 CFM to each room and from there go to my silencers.

Here's where I get stuck and struggling with:

1) I can't seem to find the right fitting in the ASHRAE DFDB app that's the same as a straight rectangular to 8" round take-off. I think it might be fitting SD4-2 which is a Rectangular to Round fitting?

2) Assuming that the SD4-2 fitting is the correct one for a branch starting takeoff, when entering the numbers in it's asking for a Flow Rate (Q, CFM) and I'm wondering if I should be entering the Flow Rate of the main trunk (800CFM) or should that be the Flow Rate of the branch (200CFM)? Entering the 800CFM doubles my velocity to 2292FPM.

3) I know my current trunk velocity is a bit high @ 1152 FPM but am I wrong to assume that the velocity would drop by branching out two separate 8" round duct?

Note that I do have a backup plan in case I need to reduce the velocity. I can increase the size of the trunk again if needed (ex; adding a 10x16 vertical 90 degree elbow could reduce it down to 720FPM, or I can go even bigger if needed and then branch out from there instead somehow.

I've really been trying and scratching my head on this one and hope someone can shed some light on this.

Either way thanks for taking the time to read this. Any feedback/help is greatly appreciated!

Cheers!!

[Soundman2020]

and ducts will live in between the inner and outer leaf walls so I think I'll be okay with just one silencer per register.

I'd be really interested to see how you accomplished that. Please post a diagram. I'd be especially interested in how you managed to decouple one single silencer from both leaves at once, and how you managed to get the sleeves through the leaves without leaving any flanking path!!!

There will only be 1 outer leaf penetration for the main supply and return trunk

How can you have one penetration for two ducts, while still maintaining isolation?

Unless a quick example is needed to clarification.

That would be good, yes. I have a strange feeling that you are forgetting something important....

all the way up to where I'll be branching out two separate runs for each room. Unfortunately,

Why do you need to supply duct and two return ducts in each room? I'm not questioning that it is sometimes necessary: Rather, I'm questioning to see if you know WHY you are doing that....

FIY, when I have to split airflow, I do it inside one single silencer.... Save money, time, effort, and space. Easily allows you to cut the air flow velocity by a factor of four, at the register, as compared to the duct, if you design it carefully...

Currently at the end of my main trunk I've got a 10" x 10" rectangular duct that's carrying 800 CFM from my AHU at a velocity of 1152 FPM

Whaaa????? Hog big are you rooms, that you need such a high capacity? Did you add on a few extra rooms at some point? That's enough air for somewhere between 7,000 and 10,000 cubic feet of room volume... about right for a 1200 square foot home.... Is your studio really going to be that big? Or is this AHU for the entire house?

and the total friction loss at that point is 0.44 in wc.

What AHU did you find that will deliver 800 CFM at 1200 FPM into a static pressure load of 0.44 in.wc.?

What I'm trying to achieve is a few feet before the main trunk ends I'd like to branch out straight up two 8" round ducts spaced about 2ft apart that would each carry 200 CFM to each room

The tool you will need to accomplish that can be bought at Hogwarts. It's called a "magic wand"....

Seriously, that cannot happen. You have 100 in2 of duct carrying 800 CFM at 1200 fpm. The area of two 8" round ducts is indeed about 100 in2, but those are ROUND ducts, not rectangular... And in any case, even if they did act the same as rectangular cuts, they would each then be carrying 400 CFM at 1100 fpm. Ahat do you plan to do with the other half of your airflow? There's 400 CFM that you are not accounting for here... That's where you'll need the magic wand: to make it disappear somehow...

And you'll need it again at the registers, to magically reduce the flow velocity by a factor of 400 %.... you have to get that 1200 fpm down to 300 fpm...

3) I know my current trunk velocity is a bit high @ 1152 FPM but am I wrong to assume that the velocity would drop by branching out two separate 8" round duct?

There's a very simple relationship between flow RATE, flow VELOCITY and cross sectional area. If you have, for example 200 CFM of air flowing through a duct that has a cross sectional area of one square foot, then the air is flowing at a velocity of 200 fpm. Logical. If you reduce the size of the duct to half a square foot, then the velocity MUST double. Air cannot be created or destroyed inside the duct (unless you can find that magic wand...), so the same VOLUME of air moving at the same RATE must go FASTER if the cross section is SMALLER.

Note the relationship between the units of measurement: If you divide CUBIC feet per minute by FEET per minute, your resulting units are SQUARE FEET. ("feet x feet x feet" divided by "feet" leaves "feet x feet", and "minutes" divided by "minutes" = 1). So flow RATE divided by flow VELOCITY is always equal to the cross sectional area. Keep that relationship in mind, and turn it around any way you need it, you can calculate whatever you want.

Of course, that is the simple relationship, and it ignores friction, static pressure, and other losses, but it's a very useful rule of thumb.

Note that I do have a backup plan in case I need to reduce the velocity. I can increase the size of the trunk again if needed

OR you could just enlarge the cross sectional area inside the silencer box, then enlarge it again at the registers....

I seldom use ducts as large as you are talking about: 8" occasionally, but more commonly 6". If I need more flow, I run two in parallel, or I switch to rectangular metal ducts. You are talking about ducts that are 10" or even 16".... ad you say you plan to fit your silencers inside the wall cavity between your inner-leaf and outer leaf??? That implies that, assuming you do use a 16" duct, plus the thickness of the wood, plus the thickness of the duct liner, plus the air gaps, your wall cavities will need to be about 22" deep... Is that what you are planning?

Something isn't working out right here....

- Stuart -

[Studio45]

Hey Stuart! Thanks for the response!

Let me start off by saying I apologize for the lack of drawing examples....I'm spending too much time on details that don't matter when trying to show what I'm trying to do. I'll try to make things a little simpler from now on.

I'd be really interested to see how you accomplished that. Please post a diagram. I'd be especially interested in how you managed to decouple one single silencer from both leaves at once, and how you managed to get the sleeves through the leaves without leaving any flanking path!!!

I'm trying to do sort of the same thing that was done on Frank's Studio by putting the silencers above the ceilings of each room. I believe the boxes are fastened to the ceiling and then the sleeve with the register at the end extends through the ceiling. This is the only penetration I can see through the inner leaf ceiling. There's a bunch of acoustic sealant all around that sleeve.

Here's an example of what I'm trying to do. Please note this is far from being exact and is only to show what I'd like to do. You can see the main trunk right in front of the outer leaf wall with the window. There's a structural box (not touching the outer leaf wall) that is build over the main trunk where the inner leaf wall is on top. (trunk does not touch the inner leaf wall) You can also see the round ducts I'm trying to run that's in between the two inner leaf walls of the CR and LR. I drew some simple boxes on top of the inner leaf ceiling to show where I'd like to put the silencers. I know I would need either two silencers (supply and return) for each register or maybe have huge silencers split to two registers and only have two silencers total per room.

HVAC-EX-1.png

How can you have one penetration for two ducts, while still maintaining isolation?

Sorry, I meant two penetrations...one hole for the supply trunk and another for the return trunk. The two penetrations would be on the same wall that's part of the house/hvac closet. Here's an example of the ducts going through the closet wall and where the trunks are located in between the walls. It's hard to see but both supply and return trunks are penetrating the HVAC closet wall. Again, note that the duct size on the drawing is not the actual size but the idea is to leave enough room in between the CR and LR inner wall to run my supply and return ducts above my two rooms.

HVAC-EX-2.png

HVAC-EX-4.png

Why do you need to supply duct and two return ducts in each room? I'm not questioning that it is sometimes necessary: Rather, I'm questioning to see if you know WHY you are doing that....

FIY, when I have to split airflow, I do it inside one single silencer.... Save money, time, effort, and space. Easily allows you to cut the air flow velocity by a factor of four, at the register, as compared to the duct, if you design it carefully...

Again sorry my fault for not explaining properly. What I'd like to do is run 1 supply and 1 return for each room not two for each. Ultimately I'd like to do what you're proposing and split the airflow to two exits/registers into 1 single silencer. This way I would have two silencers per room versus four. The design part might be tough but I believe I could figure it out with a bit of help I saw the silencers for Frank's build and I would love to design something similar.

Whaaa????? Hog big are you rooms, that you need such a high capacity? Did you add on a few extra rooms at some point? That's enough air for somewhere between 7,000 and 10,000 cubic feet of room volume... about right for a 1200 square foot home.... Is your studio really going to be that big? Or is this AHU for the entire house?

Alrighty, this is another thing I'm a little lost with and not exactly sure if I'm using the right numbers. When I refer to 800CFM I'm using what my AHU is rated for (Nominal). Now I know the AHU has 3 different speeds...and looking at the specs manual for the AHU I'm seeing a table with all kinds of different CFM ratings based on External Static Pressure. (Ex; at Medium Speed and Static Pressure of 0.5 in/wc it's showing an airflow of 646CFM and at Low Speed it drops to 558CFM). Perhaps these are the CFM rates I should be using instead of 800CFM? My rooms are pretty small CR= 190 sq/ft and LR=250 sq/ft. so I know I don't need 800CFM...in fact I only need half of that. The AHU is dedicated for the studio only and is a 2 ton unit. I'm hoping I simply didn't use the right CFM for my friction loss calculations

What AHU did you find that will deliver 800 CFM at 1200 FPM into a static pressure load of 0.44 in.wc.?

As stated above, I most likely messed up my calculations by using 800CFM as the airflow for all the ASHRAE DFDB fittings and ducts. At least that's what I'm really hoping :/

The tool you will need to accomplish that can be bought at Hogwarts. It's called a "magic wand"....

Seriously, that cannot happen. You have 100 in2 of duct carrying 800 CFM at 1200 fpm. The area of two 8" round ducts is indeed about 100 in2, but those are ROUND ducts, not rectangular... And in any case, even if they did act the same as rectangular cuts, they would each then be carrying 400 CFM at 1100 fpm. Ahat do you plan to do with the other half of your airflow? There's 400 CFM that you are not accounting for here... That's where you'll need the magic wand: to make it disappear somehow...

And you'll need it again at the registers, to magically reduce the flow velocity by a factor of 400 %.... you have to get that 1200 fpm down to 300 fpm...

Rrrrrridiculouuussss!!! lol

Thanks for this explanation! This makes sense now and is what I started suspecting in terms of having to deal with the leftover CFMs that I'm not using. It needs to go somewhere and is not helping me in any way. I'm hoping my AHU can be configured to produce a much lower CFM rating. I better stop hoping and start praying instead haha

There's a very simple relationship between flow RATE, flow VELOCITY and cross sectional area. If you have, for example 200 CFM of air flowing through a duct that has a cross sectional area of one square foot, then the air is flowing at a velocity of 200 fpm. Logical. If you reduce the size of the duct to half a square foot, then the velocity MUST double. Air cannot be created or destroyed inside the duct (unless you can find that magic wand...), so the same VOLUME of air moving at the same RATE must go FASTER if the cross section is SMALLER.

Note the relationship between the units of measurement: If you divide CUBIC feet per minute by FEET per minute, your resulting units are SQUARE FEET. ("feet x feet x feet" divided by "feet" leaves "feet x feet", and "minutes" divided by "minutes" = 1). So flow RATE divided by flow VELOCITY is always equal to the cross sectional area. Keep that relationship in mind, and turn it around any way you need it, you can calculate whatever you want.

Of course, that is the simple relationship, and it ignores friction, static pressure, and other losses, but it's a very useful rule of thumb.

This is very helpful thanks a lot!! I will refer to this in the future for sure!

OR you could just enlarge the cross sectional area inside the silencer box, then enlarge it again at the registers....

I seldom use ducts as large as you are talking about: 8" occasionally, but more commonly 6". If I need more flow, I run two in parallel, or I switch to rectangular metal ducts. You are talking about ducts that are 10" or even 16".... ad you say you plan to fit your silencers inside the wall cavity between your inner-leaf and outer leaf??? That implies that, assuming you do use a 16" duct, plus the thickness of the wood, plus the thickness of the duct liner, plus the air gaps, your wall cavities will need to be about 22" deep... Is that what you are planning?

Something isn't working out right here....

The main reason my main trunk is reduced to 10x10 is to save space along the walls and maximize the room size. Here's an example of what I was thinking to increase the size in between the inner leaf walls. In this case below, I'm going from a 8" Wide by 10" High trunk to a 8" x 20" duct. This is just an idea but this would reduce the velocity by quite a bit I believe since the cross-sectional area would be 160 sq/in versus 80 sq/in. If my calculations are correct in this example @ 800CFM the velocity would drop from 1440FPM to 576FPM. This is just an example but I suppose could potentially work for me should my airflow rate be 800CFM.

HVAC-EX-3.png

Stuart my man, you are a blessing! I know I might be repeating myself in every post but the bottom line is it's the truth! I would be making so many mistakes without your help and not even Dumbledore's magic wand could fix them

Thanks again for your patience with all this HVAC crap...it's just insane how complex the design part can be. Maybe it's just me that's stupid lol but even the pros don't go this much into details I'm sure of it otherwise they wouldn't make any money...either that or HVAC systems would cost half the price of a total house lol.

Cheers!

Francis,

[Gregwor]

Francis,

So you already bought your AHU? If so, you should have figured out your design and then bought the AHU that matches those design specs.

190+250 sq ft with what I'm guessing is an average of 8ft ceilings (with your tilted walls) so that puts you at ~ 3520 cubic feet.

Needed flow rate = (at LEAST 6 air changes per hour X 3520 cubic feet )/60 min = 352 CFM

For your fresh air, you'll need ~30%, so 352 CFM x 0.3 = 105.6 CFM

So for your main trunks, you need a 10" RD duct or equivalent rect.

Your branches you need CR 190 CFM = 8" RD and LR 250 CFM = 9" RD or equivalent rect.

So for your CR with 8" RD branches, that puts you at 50.27 in2 cross sectional area. For your LR with 9" RD branches, that puts you at 63.62 in2.

So, for inside your silencer boxes, you need at least double those CSA's. So for your CR you need 100.53 in2. For your live room silencer boxes, you need 127.23 in2

Now, for your main trunk and inside of your silencer boxes, you should have duct liner. So, that's going to increase the footprint.

Also, you have to make sure that your total CFM is under 300 feet per minute. That might mean increasing your silencer box CSA more or at least increasing the final turn in it to allow for a large outlet.

Lastly, make sure your outlet sleeve length equals a certain distance from any direction change or CSA change and the mouth of your grille/register. That distance is between 3 and 5 times the diameter of that which is feeding the grille/register. This is to prevent any distortion or turbulence noise at your grille/register.

As an example of for your LR silencer box:

If you need 127.23 in2, an inner dimensions could be 11.5" x 11.5" path.

If 11.5" = x, you can use the following to determine other dimensions:

Silencer Box Depth.jpg

The total depth of your box will be
2x + 8
2(11.5) + 8
23 + 8
31" wide

Silencer Box Width.jpg

For the total width of your box
4x + 11.5
4(11.5) + 11.5
46 + 11.5
57.5" deep

For the height,
It's simply x + 4
11.5 + 4
15.5" deep

If you want to Y your silencer like you suggested, you will basically have to double the width. That would put your box at 115" wide.

Now, you mentioned you want to use one box per leaf. The problem with that idea is that your sleeve will "see" all of the sound inside of your room and transmit it to the outer leaf due to no de-coupling. And vise versa. This almost negates the concept of building a room in a room.

Furthermore, since you should have 1 silencer per leaf on both your supply and return ducts (a total of 4 for your two rooms), the sound passing through your supply duct for example, would have to go around a total of 6 baffles (if you follow the rule of thumb of having at least 3 baffles per silencer). If you Y split your silencer and only use 1 as you mentioned, the silencer path would only include 3 baffles. Yes, your silencer would have 6 baffles in it, but the sound going from in your room to outside would only pass 3 of them.

What I'm getting at is that you should do it properly. I find it crazy to compromise your isolation because you don't want to include two more boxes.

Now, back to your AHU, if you haven't bought one yet, make sure you can zone it. That means you need a true variable speed fan in it. Some unit's can be programmed for max CFM output and they have sensors that detect the static pressure. This sensing will then regulate the fan speed allowing it to properly zone. Having said that, your drawing doesn't leave a spot for the motorized dampers.

I'm not sure that I have access to my blank static pressure template I've made. I'm pretty confident it is proper, but either way, it seems to yield more accurate results than anything else I've found online. I'll try to share it on your thread if I remember to do so. Maybe PM me and remind me if I haven't done it in a few days. One last note on the static pressure: ASHREA suggest 2 safety factors to include in your calculation when choosing the correct fan. For safety factor 1, to increase your static pressure by 15% and for safety factor 2, to increase it by another 10%.

After you draw up your plan completely, you can then calculate the static pressure and know what AHU to buy.

I hope this helps a bit. I know the pain of trying to wrap your head around HVAC. I talked to a handful of journeyman HVAC dudes who couldn't answer any of my questions. So, everything I know about the topic is basically self taught after hundreds of hours of research. So to say I'm 100% correct in my knowledge would be a lie. But, I think what I've told you should at least get you on the right track!

Greg

[Studio45]

Wow! Thanks Greg!

I will review all this information over the weekend (since we don't have a hockey team to watch anymore ) with a few drinks in the hopes of making good progress with my design. This is so much appreciated!! I'll be making a toast to you this evening lol

So you already bought your AHU? If so, you should have figured out your design and then bought the AHU that matches those design specs.

Yeeahh, the AHU was purchased a few years ago when I started the planning for the studio. This was before I learned I had to completely re-think the hole system in details but it was sized by a pro HVAC company and based on the two room dimensions along with heating/cooling requirements with some head room to play.

The HVAC guy had also started putting in a main supply trunk (you can see pics on the first couple pages of this thread) and got him to stop everything as I needed to figure all this stuff out before going to far. So yeah I do have some ducts already that I'm hoping I can make use of but won't be a big deal if that's all I need to replace.

So for your main trunks, you need a 10" RD duct or equivalent rect.

Based on your quick calculations, I think I'll be okay to use the main trunk i've got already. It's a reducing plenum type starting from the AHU at 16x16 down to 12x10, 10x10 and finally to 8x10. Which is 80in2...a tad bigger than a 10" round duct (78.54 in2).

Now, for your main trunk and inside of your silencer boxes, you should have duct liner. So, that's going to increase the footprint.

I'll be wrapping the entire main trunks with roxull insulation but on the outside versus the inside. I've already started looking at the 1" duct liner for the silencers. Any recommendations? I saw that CertainTeed makes TOUGHGARD R DUCT LINER and also Johns Mainville who make Linacoustic R-300 and Linacoustic RC which I think I could maybe find somewhere around Ottawa region.

Also, you have to make sure that your total CFM is under 300 feet per minute. That might mean increasing your silencer box CSA more or at least increasing the final turn in it to allow for a large outlet.

Right! This is something I'm struggling with. For the AHU I've been assuming that my CFM rate is 800 since my AHU is rated at that but I have a feeling that's not the right figure. I have no problem increasing the silencer CSA. Sorry what do you mean by "increasing the final turn in it"? I can certainly make the outlet bigger since I'll be exiting at the bottom of the silencer with some sort of sleeve.

Lastly, make sure your outlet sleeve length equals a certain distance from any direction change or CSA change and the mouth of your grille/register. That distance is between 3 and 5 times the diameter of that which is feeding the grille/register. This is to prevent any distortion or turbulence noise at your grille/register.

Sorry I'm totally lost here. The length of the sleeve would need to be 3 to 5 times the diameter of the exit? So if my exit is 12"x12" the sleeve would have to be 36" long mininum??

If you want to Y your silencer like you suggested, you will basically have to double the width. That would put your box at 115" wide.

Now, you mentioned you want to use one box per leaf. The problem with that idea is that your sleeve will "see" all of the sound inside of your room and transmit it to the outer leaf due to no de-coupling. And vise versa. This almost negates the concept of building a room in a room.

Furthermore, since you should have 1 silencer per leaf on both your supply and return ducts (a total of 4 for your two rooms), the sound passing through your supply duct for example, would have to go around a total of 6 baffles (if you follow the rule of thumb of having at least 3 baffles per silencer). If you Y split your silencer and only use 1 as you mentioned, the silencer path would only include 3 baffles. Yes, your silencer would have 6 baffles in it, but the sound going from in your room to outside would only pass 3 of them.

What I'm getting at is that you should do it properly. I find it crazy to compromise your isolation because you don't want to include two more boxes.

I understand what you mean. I've been paying close attention to Frank's (Stadank0) build which Stuart designed and I was thinking I could maybe try and do the same but perhaps I'm not getting a full understanding. Maybe Stuart can chime in and explain furthermore. Here are a few pictures that Frank was nice enough to share on my thread and it kinda shows the massive silencer box and the sleeve that's surrounded by large amount of acoustical caulking. I get that it's not 100% decoupled but I figure if Stuart gave him the green light that it must be somewhat acceptable? By all means if the best way to do it is by adding a few boxes I'll do it no questions asked.

Stuart's Description
In that one you can clearly see the "sleeve", which is part of the silencer box, that will extend down through the inner-leaf ceiling, to the register. That sleeve is also isolated from the inner-leaf: the hole that it pokes through is a bit larger then the sleeve, so there's a gap all the way around, and that gap is filled with abundant highly flexible, soft caulk, to create the seal. Here's a view of that caulk around the sleeve, just before the ceiling module was raised in place around it:

SB-EX-1.jpg

SB-EX-2.jpg

SB-EX-3.jpg

Again Greg thanks so much for chiming in and helping out! I really can't express how appreciative I am of all your contributions to my planning.

Francis,

[Gregwor]

I'll be wrapping the entire main trunks with roxull insulation but on the outside versus the inside.

In order for it to do it's job properly, you need to install DUCT LINER inside of your rectangular duct. Of course, you should place insulation around the outside as well if you're able!

I've already started looking at the 1" duct liner for the silencers. Any recommendations?

I do not.

For the AHU I've been assuming that my CFM rate is 800 since my AHU is rated at that but I have a feeling that's not the right figure.

That is the wrong figure. I've given you the correct figures above (assuming my height estimate was correct).

I have no problem increasing the silencer CSA.

Providing you have the space to do so, awesome!

. Sorry what do you mean by "increasing the final turn in it"?

I mean, after the last baffle, you might need to increase the inner dimensions so that you're able to have your outlet/sleeve the correct size. Once you start drawing it up in SketchUp, you'll clearly see what I mean

Sorry I'm totally lost here. The length of the sleeve would need to be 3 to 5 times the diameter of the exit? So if my exit is 12"x12" the sleeve would have to be 36" long mininum??

No, not your exit. It's length would be equivalent to 3 - 5 times the dimension it WAS. So, as an example, say at your last cross sectional area change, it was indeed 12"x12", then yes, a 36" long sleeve would be needed. However, if you can afford the static pressure drop, you could shrink the final part of your silencer to something smaller like 8"x8" and then have a 24" sleeve. Or even smaller. Realistically, the more drastic the cross sectional area changes, the more effective the attenuation.

Insertion Loss Graph from Engineering Acoustics 2009.png

I get that it's not 100% decoupled but I figure if Stuart gave him the green light that it must be somewhat acceptable? By all means if the best way to do it is by adding a few boxes I'll do it no questions asked.

I'd put money on it that the outer and inner silencer boxes are 99+% decoupled from one another. Ideally, you want to de-couple the silencer boxes from their respective leaves even. And the acoustic caulk is there for 2 reasons:
1. to maintain isolation between the penetrating sleeve and the drywall/osb
2. to provide an awesome seal while maintaining the mass of the drywall/osb

Hopefully that answers some more of your questions!

Greg

[Soundman2020]

based on the two room dimensions along with heating/cooling requirements with some head room to play.

... and therein lies the problem! HAVC should never be oversized or undersized too much, for several reasons. As you have already discovered! HVAC should be sized correctly for the job.

and finally to 8x10. Which is 80in2...a tad bigger than a 10" round duct (78.54 in2).

Yes, but 80in2 of round duct is NOT the same as 80in2 of rectangular duct! Round duct has lower surface area, and therefore lower friction loss. You need to check the actual conversion factor, based on the rectangular dimensions:

https://www.engineeringtoolbox.com/equi ... d_205.html

Remember I said that the explanation I gave about the relationship between velocity, rate, and surface area was simplified? This is one of the "simplifications"... I did hint at this in the post, but probably not in enough detail...

I'll be wrapping the entire main trunks with roxull insulation but on the outside versus the inside.

You seem to be missing the point: The purpose of duct liner in a studio is NOT for thermal reasons, but for acoustic reasons (OK it is for thermal reasons as well, but mainly acoustic). If you put insulation on the outside, that is only thermal: it helps reduce heat loss through the walls of the duct, but doesn't do much for the acoustic issues INSIDE the duct...

You can probably skip the internal duct liner for any ducts that you run in the cavity between the leaves, but not for ducts that are outside the outer leaf, or inside the inner leaf.

This is something I'm struggling with. For the AHU I've been assuming that my CFM rate is 800 since my AHU is rated at that but I have a feeling that's not the right figure.

Check the manual for the AHU: Most have at least three different fan speed settings, and each one produces a different CFM / fpm combination. Some have more than 3 settings. As long as one of those settings is close to what you need for your room, that's fine. If not, then you have two choices: 1) sell that unit and buy one the right size, 2) dump the excess air flow to some other place: either throe it away back to the outside world (very inefficient!) or use it in another part of the building. Since Dumbledor is dead, he can't help you here!

Sorry I'm totally lost here. The length of the sleeve would need to be 3 to 5 times the diameter of the exit? So if my exit is 12"x12" the sleeve would have to be 36" long mininum??

Once again, use the round equivalent, not the rectangular dimension. And 800 CFM going through a 12x12 (one square foot) opening, is going to produce 800 fpm.... Yes, I realize that you'll be splitting some of that to another room, but even if you sent half to another room, you still have 400 fpm... So you'll also have to dump that excess flow some place! And after you do that, I'd suggest splitting the airflow inside the silencer, to a pair of outlets that total around 120 in2, so 60 in2 each, which is roughly the equivalent of an 8.4" round duct, x 3 = 25 inches. So your "through-wall sleeve" should be about 24" long, total, from the furthest point inside the silencer box, up to the register. That's not too bad: assuming the interior depth of the silencer box is 10" (because you plan on using 10" duct, so it has to be at least deep enough for the duct to attach!), and that you have 2x10 joists, plus a 1/2" gap between silencer and joist, that already gives you 20" of depth, so the sleeve would only need to extend another 5" or so below the joists.

That's all theoretically optimal: if you have a lower flow rate, you can go with a shorter sleeve. If you hide the register inside some type of acoustic treatment as I often do, then you can use a shorter register because the treatment itself diffuses the air flow, and helps to quieten any noise.

I've been paying close attention to Frank's (Stadank0) build which Stuart designed and I was thinking I could maybe try and do the same but perhaps I'm not getting a full understanding. Maybe Stuart can chime in and explain furthermore. Here are a few pictures that Frank was nice enough to share on my thread and it kinda shows the massive silencer box and the sleeve that's surrounded by large amount of acoustical caulking. I get that it's not 100% decoupled but I figure if Stuart gave him the green light that it must be somewhat acceptable?

It works! And do note that Frank has duct liner on the INSIDE of his sleeves... not the outside...

Here's the actual noise floor measured in his room, from a test a couple of days ago:

FRANK--noise-floor--ls-t088.png

Hard to pin down, because it is so flat, but somewhere between NC-15 and NC-25, depending where on the graph you measure it. For the low end (always the hardest part), it's much closer to NC-15. Not quite as good as I had wanted, but Frank had to cut some corners in his HVAC, due to unexpected issues that I won't go into. But even at NC-20 across most of the spectrum, it's acceptable for a studio.

Here's the actual NC curves, for comparison:

NC-curves-graph.jpg

Note that for some unknown reason, REW does not allow you to smooth the noise floor curve, so that is UNSMOOTHED data, but the NC curve calls for it to be smoothed to one octave, which would make that line much flatter. And that was measured dBC, instead of dBA (what it should be for NC), so it greatly exaggerates the low end: in other words, the real noise floor is even lower than that REW graph shows. So the REAL curve (dbA, smoothed one octave), would be a fairly flat line at around 21 dB. Compare that to the NC curves, and you'll see that it is basically NC-20 below 2 kHz, and NC-15 below 1 kHz.

I'd put money on it that the outer and inner silencer boxes are 99+% decoupled from one another.

Yup! In the middle image, you are seeing three silencer boxes. The one on the right supplies air to the CR (inner-leaf), the one on the left is the return from the CR (inner-leaf), and the one in the middle is the silencer through the OUTER leaf. That connects to the right-hand inner-leaf box with flexduct.

...and the acoustic caulk is there for 2 reasons:
1. to maintain isolation between the penetrating sleeve and the drywall/osb
2. to provide an awesome seal while maintaining the mass of the drywall/osb

Yup.

- Stuart -

[Studio45]

Hi Greg & Stuart!

I've had a chance to review all this great information and I'm very happy overall because I wasn't totally off track....just confused with certain numbers.

I have slightly different lower numbers because of the ceilings. I was told since the room is basically a triangle that I should take the total height and divide by two, so in my case ceiling height is 7'.

Control Room
Width: 19' Length: 10' Height: 7'
SQ/FT: 190 sq/ft
Air Volume: 1330 ft2
Required Air Changes per/hr: 6
Total Air Volume to move each hour: 1330 x 6 = 7980 ft2
Required CFM: 7980 / 60 = 133 CFM
Recommended Branch Duct Size: 8" Round-Duct
Required Fresh Air @ 30%: 133 * 0.3 = 40 CFM
Silencer Internal CSA @ recommended 200 FPM: (133 CFM / 200 FPM) * 144 = 96 in2
Silencer Inlet CSA: 96 in2 / 2 = 48 in2

LiveRoom
Width: 19' Length: 13' Height: 7'
SQ/FT: 247 sq/ft
Air Volume: 1729 ft2
Required Air Changes per/hr: 6
Total Air Volume to move each hour: 1729 x 6 = 10374 ft2
Required CFM: 10374 / 60 = 173 CFM
Required Fresh Air @ 30%: 173 * 0.3 = 52 CFM
Silencer Internal CSA @ recommended 200 FPM: (173 CFM / 200 FPM) * 144 = 125 in2
Silencer Inlet CSA: 125 in2 / 2 = 63 in2

With all this above and based on your help:

Total cubic feet of the studio: 3059 ft2
Required airflow rate for my main trunks would be: (6 air changes per hour X 3059 cubic feet )/60 min = 306 CFM

If I was to keep my current main trunk as is as a rectangular 8"x10" duct I would have a velocity of: 306 / (80in2 /144) = 546 FPM with a Friction Loss of 0.061 wc/100ft
If I was to change my current main trunk from 8" x 10" to a 10" round duct I would have a velocity of: 306 / (78.54in2 / 144) = 561FPM with a Friction Loss of 0.054 wc/100ft
If I was to change my current main trunk from 8" x 10" to a 10" x 10" rectangular duct I would have a velocity of: 306 / (100in2 / 144) = 440 with a Friction Loss of 0.035 wc/100ft

Yes, but 80in2 of round duct is NOT the same as 80in2 of rectangular duct! Round duct has lower surface area, and therefore lower friction loss. You need to check the actual conversion factor, based on the rectangular dimensions:

Using the simple formula (CFM = FPM / CSA), seems like the velocity would be lower using my current 8" x 10" rectangular duct (546 FPM versus 561 FPM) but understand that the friction loss would be greater than using a round duct (0.061 versus 0.054).

Question: Would the friction loss difference have that much of an impact on my total system?

Again, the current trunk system that's already been installed is a reducing plenum type that starts from 16" x 16" down to 12" x 10" down to 10" x 10" down to 8" x 10"

I guess my options are: A) Keep the current configuration or B) Transition from my current 10" x 10" to a 10" round duct. Note that I have no problem going with option B, I just want to make sure I wouldn't be going through all this trouble and extra cost for something that in the end wouldn't make that much difference.

You seem to be missing the point: The purpose of duct liner in a studio is NOT for thermal reasons, but for acoustic reasons (OK it is for thermal reasons as well, but mainly acoustic). If you put insulation on the outside, that is only thermal: it helps reduce heat loss through the walls of the duct, but doesn't do much for the acoustic issues INSIDE the duct...
You can probably skip the internal duct liner for any ducts that you run in the cavity between the leaves, but not for ducts that are outside the outer leaf, or inside the inner leaf.

Yep, I kinda figured it wasn't for thermal reasons when used for studio applications. Since the entire duct system will be in between the inner and outer leaf..I will be skipping the duct liner in the main trunk, but will put liner in all the silencer boxes and sleeves.

Check the manual for the AHU: Most have at least three different fan speed settings, and each one produces a different CFM / fpm combination. Some have more than 3 settings. As long as one of those settings is close to what you need for your room, that's fine. If not, then you have two choices: 1) sell that unit and buy one the right size, 2) dump the excess air flow to some other place: either throe it away back to the outside world (very inefficient!) or use it in another part of the building. Since Dumbledor is dead, he can't help you here!

Here are those details from my AHU. Note that my unit is the 08 model.

MX-AHU-DETAILS.png

If I set the fan speed on low looks like 558 CFM with an external static pressure of 0.5 would be the lowest it could go. I suppose this would mean I would need to design the system and aim for 0.5? Do you guys think 558 CFM would be too much? If that's the case, I will add branches to dump the excess air into the room next to the HVAC closet. It's an unoccupied section of the house that will be used as a storage area.

Question on how to use the ASHRAE DFDB app....for Equivalent length of all the fittings. When asked to input a CFM value for the fittings calculation...should I be using the 558 CFM that my AHU will be pushing or the 306 CFM that's required? That part confuses me still.

Another question about the fresh air and HRV requirements of the studio. As calculated by Greg if my fresh air requirement is 30% and is represented by 306 CFM * 0.3 = 92 CFM. Does this mean the branch ducts going to the HRV unit has to be sized on 92 CFM and most importantly do I add that to my total CFM requirements of the AHU? Just curious to know if that is totally separate from AHU.

... and therein lies the problem! HAVC should never be oversized or undersized too much, for several reasons. As you have already discovered! HVAC should be sized correctly for the job.

Right and I can see how this can be a big problem if not sized properly. I think I'll be okay based on all the information I've got so far. My best option if it becomes an issue is to dump the excess into the adjacent room next to the HVAC closet.

I'd suggest splitting the airflow inside the silencer, to a pair of outlets that total around 120 in2, so 60 in2 each, which is roughly the equivalent of an 8.4" round duct, x 3 = 25 inches. So your "through-wall sleeve" should be about 24" long, total, from the furthest point inside the silencer box, up to the register. That's not too bad: assuming the interior depth of the silencer box is 10" (because you plan on using 10" duct, so it has to be at least deep enough for the duct to attach!), and that you have 2x10 joists, plus a 1/2" gap between silencer and joist, that already gives you 20" of depth, so the sleeve would only need to extend another 5" or so below the joists.

That's all theoretically optimal: if you have a lower flow rate, you can go with a shorter sleeve. If you hide the register inside some type of acoustic treatment as I often do, then you can use a shorter register because the treatment itself diffuses the air flow, and helps to quieten any noise.

Excellent! I will plan to do exactly that and split the airflow inside the silencers. I plan to secure the silencers on the support beam as seen in the pictures of my previous post. That will give me enough room to have a long enough sleeve to meet this requirement.

As for the decoupling of the silencer boxes from the inner leaf ceilings of each room, as stated above I plan to secure the silencers on the support beam above each room with straps and probably some sort of rubber strips. They will not be touching the inner leaf ceiling of the rooms so I suppose I could consider this decoupled as the only material that will touch the sleeve of the silencer and the inner leaf ceiling (drywall mass) will be the best caulking I can find.

Thanks again for everything guys! this starting to make more and more sense to me and will be able to progress with my HVAC installation.

Francis,

[Soundman2020]

Do you guys think 558 CFM would be too much? .

You need 300. You have 560... Do the math.

There's another option here: Just live with the higher flow rate! The "6 changes per hour" thing is the recommended rate, but if you don't mind a breezy room, you can have a higher rate... Roughly double, in your case. So as long as you then double the sizes of everything to keep the flow velocity at hte registers below 300 fpm, then that's fine.

If that's the case, I will add branches to dump the excess air into the room next to the HVAC closet. It's an unoccupied section of the house that will be used as a storage area

You would also need a return duct from that same storage room, back to the AHU. You need to think of this as a closed-loop system, except for the fresh air intake and stale air exhaust. The rest of the air recirculates through the AHU.

You could also do a bypass on the system, with a duct that just loops the supply right back to the return, a little way upstream. Inefficient, and frowned on by many HVAC people, but possible. However, that's a lot of extra air to bypass!!

Or you could sell that unit, and buy one half the size, which would have lower running costs, be quieter anyway, and would also provide the right amount of cooling and dehumidification.

By the way, you DID calculate your sensible heat load and latent heat load completely, right? And you check that it matches the BTU capacity of the AHU fairly closely, right? What is the total load that you will be dealing with in your rooms, in BTU/hr?

HVAC isn't just about moving the right amount of air: it's also about cooling that air by the right amount, and removing the right amount of humidity. Not just "-V--", but also "H-AC"! I have a feeling that the capacity of your unit might be too large for the space, which implies very short duty cycles, which implies sudden large swings in temperature and humidity....

- Stuart -