John Sayers' Recording Studio Design Forum Archive

I am trying to learn how this HVAC system will work, and making sure I calculate everything properly so I don't create any major issues with the design. This HVAC stuff is completely foreign to me, so bare with me.

The duct size on the ERV unit I'm going to use is 5". Since the unit is in the room and the fresh air duct and the stale air return will also be in the room and not going through the inner leaf, is it best to just use 5" ducts all the way around the interior?

When I calculated the cross sectional diameter from the specs of my mini-split unit and the specs from the ERV, it looks like I should be using 8" ducts. So Will I use 8" ducts to bring the air in from outside and into the silencer box, then the other end of the silencer box goes into the ERV with a 5" duct and then into the room with a 5" duct? and the same with the return. The air from the room goes into the ERV via the 5" duct, the another 5" duct to the silencer box. From the silencer box (through the wall) to the outside will be an 8" duct?

When I'm calculating the static pressure, is it the total duct length (plus elbows, registers, take-offs, flanges, etc.) of each port of the ERV combined? Or do you calculate each port separately? For example, the ERV has 4 ports (fresh air from outside, fresh air to inside, stale air from inside, and stale air to outside). Am I finding the static pressure separately for each port (and making sure each port is within the specs of the unit), or am I taking the result of each port and adding it all together, and making sure THAT number is within the specs of the unit? Or is it something else? Since there are 2 fans (one bringing in fresh air, and one taking out stale air), am I calculating the static pressure for each fan?

I found a formula by Gregwor for building the proper size of the silencer boxes, but it all depends on the side ducts that I'm using. At this point I've read so many damn build threads that they're starting to mush all together, and it's hard for me to find the ones that had a lot of the better info.

Jag94 wrote:I am trying to learn how this HVAC system will work, and making sure I calculate everything properly so I don't create any major issues with the design. This HVAC stuff is completely foreign to me, so bare with me.

The duct size on the ERV unit I'm going to use is 5". Since the unit is in the room and the fresh air duct and the stale air return will also be in the room and not going through the inner leaf, is it best to just use 5" ducts all the way around the interior?

When I calculated the cross sectional diameter from the specs of my mini-split unit and the specs from the ERV, it looks like I should be using 8" ducts. So Will I use 8" ducts to bring the air in from outside and into the silencer box, then the other end of the silencer box goes into the ERV with a 5" duct and then into the room with a 5" duct? and the same with the return. The air from the room goes into the ERV via the 5" duct, the another 5" duct to the silencer box. From the silencer box (through the wall) to the outside will be an 8" duct?

When I'm calculating the static pressure, is it the total duct length (plus elbows, registers, take-offs, flanges, etc.) of each port of the ERV combined? Or do you calculate each port separately? For example, the ERV has 4 ports (fresh air from outside, fresh air to inside, stale air from inside, and stale air to outside). Am I finding the static pressure separately for each port (and making sure each port is within the specs of the unit), or am I taking the result of each port and adding it all together, and making sure THAT number is within the specs of the unit? Or is it something else? Since there are 2 fans (one bringing in fresh air, and one taking out stale air), am I calculating the static pressure for each fan?

I found a formula by Gregwor for building the proper size of the silencer boxes, but it all depends on the side ducts that I'm using. At this point I've read so many damn build threads that they're starting to mush all together, and it's hard for me to find the ones that had a lot of the better info.

You would usually have the HRV outside of the room (I appreciate in your case you have to have it inside) the duct would go to the silencer box which would increase the CSA by at least double, but more if possible. It is the increase of CSA which lowers the velocity, which leads to a nice silent air flow.

If you were to do it the other way round (8" - 5") you would be increasing velocity, which may mean a noisy air flow. You can use this: https://www.engineeringtoolbox.com/duct ... d_883.html
to help you figure out what velocity you can expect at each point in your system.

To give you an example of how it should work in my room:

I need ~150cfm of fresh air coming in to my space.

By using a duct sizing table that means I need a 7" duct. Since that's hard to find, I can use 8" which will feed my silencer box.

My silencer box will then quadruple the CSA, which equates to around 12" duct coming out of the box and feeding my register in the room.

This will give me a velocity of around 190 ft/min.

If I want to further decrease this then I can split this duct using a Y splitter, down to 2x 10" ducts for example, which will mean each one provides 75cfm each (150/2) which gives me a velocity of about 137 ft/min each.

If I maintained the 12" diameter for the split as well, then 2 x 12" ducts @ 75cfm each would have a velocity of 95.5 ft/min.

The velocity needs to stay below 300 ft/min, ideally closer to 100 ft/min.


Regarding static pressure, I have read that it's not worth calculating for the first 100' or so of duct, if your system is bigger than that then I would maybe take it into account.

I'm not an expert, but I hope this helps a little bit.

Paul

Wow, thank you for the detailed response, Paul. That makes a lot of sense. I'll keep researching, it appears I have a lot of work to do. haha.

Update and some questions.

I finally, FINALLY got started on building the interior leaf frame today. I thought this day was going to come a lot sooner, but beefing up the outer leaf took WAY longer than I had imagined. It's starting to feel like I am making progress. One thing that is tough, is since the room is so small, I'm constantly having to move things to make space to work. I spend so much time relocating materials and tools that I lose a half day of productivity. Now that the walls are being built, a substantial amount of space will start to open up in the room. By the time I'm ready for drywall, It's going to be really easy to move around in there.

I think I'm starting to get a good idea of this HVAC nonsense. Here is what I have so far, with update and more accurate numbers. Please tell me if I'm either missing something, or am wrong about something.

Interior room dimensions:
14'10" x 14'1" x 8'8"
= 1,810 cubic feet

With 6 room changes per hour = 10,860 cubic feet/hour

10,860 / 60 = 181 CFM for heating and cooling.

My mini split is very capable of handling this CFM.

181 x 40% = 72.4 CFM for the ERV

The ERV I'm now using can also handle this, even with a static pressure of 0.4 (I think maximum I'm looking at 0.2 for static pressure).

OK. So in determining duct size and silencer box size, here is what I have. I'm using Gregwor's silencer box formula design.
If I use an 8" round duct (to both bring the air in from outside, exhaust to outside, as well as bringing air to room from ERV, and from the room via ERV)...

8" round duct = Cross Sectional Area of 50.27 square inches. For a silencer box, I want to at least double the CSA inside the box (quadruple it would be better, but I can't afford that much space). So doubling the CSA gives me 100.53 square inches.

The square root of 100.53 = 10.26464. I'm going to upsize that to 10.5" (which is now 110.25 square inches).
10.5 is now my value for X in the silencer box above.

To calculate the velocity (feet per minute) of the air,

110.25 / 144 = .765625 square feet

(40% fresh air in CFM) / square feet
72.4 CFM / .765625 square feet = 94.563265 feet per minute.

This is good, because it's well under 300 fpm.

Using the formulas in the silencer box design above, I end up with a box dimension of...

53.5" x 28" x ??

I can't seem to figure out the value of the depth of the box. I am researching all over john sayers site to find more info on this silencer box design to see if I can find how to calculate the depth dimension, but otherwise, am I on the right track here?

Jag94 wrote:Update and some questions.

I finally, FINALLY got started on building the interior leaf frame today. I thought this day was going to come a lot sooner, but beefing up the outer leaf took WAY longer than I had imagined. It's starting to feel like I am making progress. One thing that is tough, is since the room is so small, I'm constantly having to move things to make space to work. I spend so much time relocating materials and tools that I lose a half day of productivity. Now that the walls are being built, a substantial amount of space will start to open up in the room. By the time I'm ready for drywall, It's going to be really easy to move around in there.

I think I'm starting to get a good idea of this HVAC nonsense. Here is what I have so far, with update and more accurate numbers. Please tell me if I'm either missing something, or am wrong about something.

Interior room dimensions:
14'10" x 14'1" x 8'8"
= 1,810 cubic feet

With 6 room changes per hour = 10,860 cubic feet/hour

10,860 / 60 = 181 CFM for heating and cooling.

My mini split is very capable of handling this CFM.

181 x 40% = 72.4 CFM for the ERV

The ERV I'm now using can also handle this, even with a static pressure of 0.4 (I think maximum I'm looking at 0.2 for static pressure).

OK. So in determining duct size and silencer box size, here is what I have. I'm using Gregwor's silencer box formula design.

Gregwor's Silencer Box.png

If I use an 8" round duct (to both bring the air in from outside, exhaust to outside, as well as bringing air to room from ERV, and from the room via ERV)...

8" round duct = Cross Sectional Area of 50.27 square inches. For a silencer box, I want to at least double the CSA inside the box (quadruple it would be better, but I can't afford that much space). So doubling the CSA gives me 100.53 square inches.

The square root of 100.53 = 10.26464. I'm going to upsize that to 10.5" (which is now 110.25 square inches).
10.5 is now my value for X in the silencer box above.

To calculate the velocity (feet per minute) of the air,

110.25 / 144 = .765625 square feet

(40% fresh air in CFM) / square feet
72.4 CFM / .765625 square feet = 94.563265 feet per minute.

This is good, because it's well under 300 fpm.

Using the formulas in the silencer box design above, I end up with a box dimension of...

53.5" x 28" x ??

I can't seem to figure out the value of the depth of the box. I am researching all over john sayers site to find more info on this silencer box design to see if I can find how to calculate the depth dimension, but otherwise, am I on the right track here?

The CSA of the silencer box is the depth x width, then the length is X multiplied by 4.

You only need to provide 6 changes per hour of 15cfm of fresh air per person or 30% of your room volume due to you using a mini split.

Paul

Paulus87 wrote:
The CSA of the silencer box is the depth x width, then the length is X multiplied by 4.

You only need to provide 6 changes per hour of 15cfm of fresh air per person or 30% of your room volume due to you using a mini split.

Paul

Sorry Paul, I think you lost me here.

I think I'm missing something with the formulas on the silencer box. I only see length (4(x) + 11.5), and width (2(x) + 3), but I don't see depth. I found a thread last night that mentioned the depth is x + 4... so if 10.5 is my x, that makes it 14.5, correct?

Also, I was under the impression that the ERV will refresh 20-40% of the air (which is why you're saying 30%), but I was using 40% because there will be times when I have 5 or maybe 6 people in the room, and wanted to make sure the ERV could handle 75-90 CFM.

Does that make sense? Am I wrong in thinking that way?

Jag94 wrote:

Paulus87 wrote:
The CSA of the silencer box is the depth x width, then the length is X multiplied by 4.

You only need to provide 6 changes per hour of 15cfm of fresh air per person or 30% of your room volume due to you using a mini split.

Paul

Sorry Paul, I think you lost me here.

I think I'm missing something with the formulas on the silencer box. I only see length (4(x) + 11.5), and width (2(x) + 3), but I don't see depth. I found a thread last night that mentioned the depth is x + 4... so if 10.5 is my x, that makes it 14.5, correct?

Also, I was under the impression that the ERV will refresh 20-40% of the air (which is why you're saying 30%), but I was using 40% because there will be times when I have 5 or maybe 6 people in the room, and wanted to make sure the ERV could handle 75-90 CFM.

Does that make sense? Am I wrong in thinking that way?

That all sounds good, I was just making sure you were aware so that you don't end up with bigger boxes than you need (not that bigger boxes is a bad thing, just overkill when space is already at a premium).

What I mean regarding the depth is in order to know what your CSA of the silencer box is you have to come up with a figure for the depth otherwise you cannot calculate your CSA. If you know what your CSA needs to be then it is easy to reverse calculate it so that you end up with your width and depth figures.

Paul

Got it. Thanks. I feel much better now.

I am not conversant with the whole thread and I see you are in good hands. But a question/comment may be helpful.
EPS foam seems to be thermally efficient but I reckon has no acoustic benefit.

DanDan wrote: EPS foam seems to be thermally efficient but I reckon has no acoustic benefit.

Hey DanDan,

Yes this is correct. The purpose of the EPS foam up in the ceiling, as well as the garage door is not for sound isolation purposes (per say). It's to make sure that there is one continuous leaf. The wall that i built with OSB on the back side needs to butt right up against the garage door, with no gaps or spaces. The EPS allows me to fill those gaps and spaces making it one thick leaf. So while it doesn't have enough mass to help with isolation, it does help bridge the wall with the door so there isn't a 3 leaf situation. Same as up in the roof rafters where I used it.

Plus with the added benefit of thermal insulation!

Update comin' in hot!

Since lifting the isolation wall in front of the garage door, I installed another layer of drywall in the new garage door wall, backer rod around the perimeter, and caulked. I secured them in place with wooden cleats.

I forgot to take a picture of this, but you can see it in a picture coming up.

Next up was starting to build the interior frame... FINALLY!

I thought I was going to be doing this months ago, but the exterior beef up took ****ing forever.

I didn't take as many pictures of this as I should have. I'll take more once all 4 walls are stood up and put in place. Right now, all 4 walls are built, 3 of them are standing, and the east wall (where the entry door will be) is still on the floor because I can't lift it by myself. A buddy is coming in a few days to help.
You can see in the background the drywall added to the garage door beef-up wall.
The current entry way door is only 28" wide. If you've ever carried any piece of musical gear into a doorway, you know how annoying narrow doorways are. So I have added studs wider than the current frame, built a new frame, and will be cutting the stucco to open the doorway. I haven't taken pictures of this yet, but I will and add them after I cut the stucco (which I'm not looking forward to, even though i bought a nice new Makita angle grinder!).

The rough opening of the new interior frame matches the new wider/taller frame on the exterior, so both doors will be the exact same size. This was tricky, but I think I did a pretty good job of getting the measurements right. I'll post more pictures once I stand up that wall.

I hired Rod Gervais to consult on my HVAC system that I've been asking hundreds of questions about on all these forums. I used all the information I learned/gathered over the last 4 months (thanks to all of you) and created a design. He hasn't gotten back to me yet, but hopefully he doesn't tell me it's all wrong and I have to start over.

As soon as we finalize the plan, I'll start putting the necessary holes in the exterior wall for the ducting.

After that will be getting the rough electrical plans, and making sure i have all the proper wires coming in and exiting the exterior. Once I place the interior frame and bolt it in place, it's going to be a lot harder to poke holes and work with the exterior... so I want that all done first. I'm hopeful that I can start sheathing the interior in the next couple of weeks (OSB > Green Glue > 5/8" drywall > Green Glue > 5/8" Drywall). I feel like I'm getting close... like I can't see the light yet, but I know it's there. I'll try to do a better job of updating as I go.

Oh yeah, forgot to mention, the hot roof has been installed, and looks great. It's also noticeably cooler in there already!

It's look excellent so far - well done.

Really nice heavy duty framing, 3 king studs and metal brackets for your door should definitely be enough me thinks

Well done on hiring Rod, he is the HVAC guru. Would be interesting to see what he says about all the advice you've received here as well as seeing the design he has come up with - hopefully you'll be able to help the rest of us and clear up any misinformation we may have given you!

Paul

Paulus87 wrote:It's look excellent so far - well done.

Really nice heavy duty framing, 3 king studs and metal brackets for your door should definitely be enough me thinks

Well done on hiring Rod, he is the HVAC guru. Would be interesting to see what he says about all the advice you've received here as well as seeing the design he has come up with - hopefully you'll be able to help the rest of us and clear up any misinformation we may have given you!

Paul

The door framing was interesting. I knew that my doors were going to be heavy, so I wanted to make sure I had adequate framing to support them. It just so happened to work out that the 2' spacing for the studs landed in the perfect places for me to have several king studs... or as I labeled them on the top/bottom plates, Jack, Queen, King, and Ace studs. lol. The header is pretty cool too. I got a 2x8, cut two pieces to length, separated it by a 2x3 (which creates the perfect width of 5 1/2" for the 2x6 framing), and I used EPS foam in between to insulate the header. I found that design in two of the framing books I read.

Yeah I'm excited AND scared to hear what Rod has to say. He wanted a lot of money to do the design himself, but i couldn't afford him. He offered an hourly consulting fee to basically look it over and give small tips if needed. I'm hoping I'm in the ballpark. lol.

Little Update:

So I'm currently waiting on a few HVAC parts to be shipped, so I'm sort of at a standstill. There are a few minor things I'll be doing in the meantime, but I can't get any major work done until I get the first part of the HVAC started... and that's putting holes in the exterior to run the ductwork.

Good news though. I heard back from Rod on my HVAC design, and his response was... "The HVAC is looking pretty good", and "There is nothing in your HVAC design that concerns me". This was a HUGE relief. Once all the parts get here, I'm really confident I can get this done right.

I do have some electrical concerns though. I have a pretty good understanding of the electrical, but I was hoping maybe someone could help clarify some of the details, as well as see if I'm missing something really important. I've read 2 books on electrical wiring, plus the section in Rod's book, plus several papers on isolated ground systems, so I feel like I'm in good shape.

Ok, my house has a 200 amp main panel. There is a grounding rod right there at the panel connected with 4AWG stranded wire, and driven deep into the ground. There is a 2nd grounding rod at the front of the house that is bonded to the water pipes. Currently, there is a sub panel in the garage (studio), that is supplied by a 40 amp breaker being fed underground via conduit with four 10AWG conductors (2 hot, 1 neutral, 1 ground) from the main panel. I want to replace the 40amp breaker with a 60amp breaker, and swap the 10AWG conductors with 6AWG conductors.

Question 1: is this OK? And does the size of the ground conductor matter? Should it also be 6AWG, or can it be smaller?

Inside the studio, There will be just 1 penetration in the wall where I bring in the power conductors. Everything will be surface mounted (sub panel, outlets, switches, lighting, etc). I plan on having one phase of the panel be ONLY for studio/audio equipment, and all of those outlets will be isolated ground receptacles. They will be run in EMT conduit, so the conduit acts as the safety ground, and the hot, neutral, and isolated ground conductors will be run inside the conduit together, and will all be home runs back to the sub panel. The other phase of the sub panel will be for everything else, i.e. lights, hvac, standard plugs, etc.

As for the isolated grounds, I understand each isolated ground conductor needs to be sent to a separate ground bus bar that is not connected to the ground bus or neutral bus in the sub panel. They also need to be connected back to the ground bus at the main service entrance.

Question 2: Can I install a 2nd small panel in the studio to house the isolated ground bus bar, then send a single isolated ground conductor back to the main service entrance via the underground conduit to connect to the main ground/neutral bars in the main panel? So I will essentially have 5 conductors in the underground conduit from the house to the studio, instead of 4? And if so, what size should that ground conductor be?


Question 3: Am I correct that I do NOT need another ground rod outside the garage/studio? As long as the isolated grounds are bonded to the main service ground/neutral bar (which is bonded to the grounding rod at the service entrance), i'm good?

This sketch should illustrate everything.
Thanks!

Separate question:

In the gap between the exterior and interior leafs, I plan on filling that gap with insulation. Should i put something on the concrete floor so that the insulation is not touching the concrete? Could moisture from the concrete saturate the insulation?