HVAC Silencer Design

#1

Hey all,

I am of course turning my garage into a rehearsal/recording studio. I have read Rod's book at least 3 times ( :horse:

) and have scoured several website forums (including gearslutz, john sayers forum, and THIS one!), and I feel like I have a very good idea on how to do this. Also a big thanks to Stuart who responded to my questions on one of the other forums, which brought me to this one where I continue to learn more.

I'm in the "exterior beef up" phase of my build, but that is coming to an end this week (hopefully!). I had worked with a structural engineer to make sure the existing exterior structure was strong enough for the added mass, and he helped me make custom gussets for the rafters to support the weight and raise the overall ceiling height of the finished room. I am working with a different structural engineer for designing the interior frame (I didn't have the best experience with the first SE, so I decided to go with someone else). The design will be a completely decoupled frame to achieve the 'room within a room'.

Anyway, I'm still about a week or two away from building the internal frame, but I really need to figure out the specifics of my HVAC now so I can properly prepare. HVAC is easily my weakest area, so I'm trying to get as much information as possible to figure this out. My room is rather small, so I am using a single room HRV for the exchange of fresh air, and a ductless mini split for heating/cooling. Here are the details:

Estimated dimensions of finished interior room:
15' 3" x 14' 3" x 8' 6"
(approx 11,100 cubic feet)

Ductless Mini-Split CFM:
High speed - 294 CFM
Medium speed - 212 CFM
Low Speed - 147 CFM

HRV CFM:
High speed - 71 CFM
Medium speed - 53 CFM
Low speed - 35 CFM.

11,100 / 60 = ~187 CFM for cooling heating.

187 x 4 % = ~73 CFM for HRV

Here's the rub. The HRV is designed to be installed IN the room. The intake and exhaust vents are on the unit itself. It has two 5" ducts on the back of it that are SUPPOSED to just go straight through the wall (using duct extenders) and the air is pulled/pushed right through those ducts. Obviously this will not work for sound isolation purposes. Here are some links to the unit so you can see what I'm talking about. HRV1 HRV2

My plan was to build a rack mount of sorts to hold the HRV up (either directly below or to the side of the mini-split unit), allowing the ducts on the back to go straight into silencer boxes that are running along the floor INSIDE THE ROOM. These will be fairly large, and will be made extremely thick and heavy... like the same as my walls. OSB/drywall/drywall. Then inside the wall, there will be ducts carrying the air through the exterior leaf.

I guess my questions are this.

1: Will putting the silencer boxes inside the room even work?
2: If so, I'm having a real tough time figuring out how large they need to be, how big the openings on both ends need to be, and how big the ducts need to be inside the wall. I figure the 5" ducts from the unit itself are fine, and will be fairly short as they feed the boxes. Would the ducts on the other end inside the wall be 5" as well? Am I slowing down the velocity of the air enough?

Any help or ideas would be greatly appreciated!

#2

Greetings Jag94,

I will, of course, defer to the more experienced builders here (and to Stuart, of course), but here is my two cents:

I too have started the research on HVAC for my upcoming build. If possible, I am going to use a ducted mini split (these are usually installed above the ceiling). With a ducted system, the HRV or ERV can be installed in-line rather than having a separate hole in your carefully sealed studio.

I am not familiar with the brand of HRV you linked to (and I don't know if you've already purchased your unit), but I think I will be going with a Soler & Palau ERV (summer humidity is a big concern here in Philly, of course). They are well-established company (they make big commercial units as well as small residential ones), well-reviewed, and don't cost much more than what you're getting from HD. More important, is it can be installed wherever you want to put it (read: the ceiling, I'm guessing). I'm pretty sure that S&P no longer makes HRVs, only ERVs. Some more searching will help you find the proper HRV.

All the best,

Paul

#3

Hey Paul,

Thanks for the extremely quick reply. I also realize I need to fix the formatting of the links i tried to add. lol.

I am not tied to an HRV. In my area (Los Angeles) I can use either an HRV or an ERV. I chose that particular unit because it can be installed inside the room, and from all my research, its the only single room unit I've found that can produce the CFM I need. I don't want to install a unit in the ceiling because I am worried about not being able to service it. Also since the garage is so small, and detatched, I don't have another "area" or room to put it outside of the building and have just the ducts feeding the room. So I need a unit that can be installed in the room, and have the silencers on the inside of the room.

#4

Greetings Jag94,

Ah yes, I see your predicament. I must have misunderstood that you needed a fully in-room unit. My apologies.

One thing is certain: because your room is nearly square, you'll need to reserve the corners for substantive acoustic treatment. To that end, placement of your HRV(ERV) and silencers should be planned carefully. I'm sure that others here will provide much better guidance than I can, as I'm just a beginner at this too (with a modicum of construction experience).

All the best,

Paul

#5

Hi there "Jag94", and a big WELCOME! To the forum...

Also a big thanks to Stuart who responded to my questions on one of the other forums, which brought me to this one where I continue to learn more.

I'm glad you found us! There's a lot more than can be said and done here, than in other places. Hopefully you are finding all the info you need, but if you can think of any way to improve the forum, please let me know! I'm always trying to make it better, ore useful, more helpful, more complete.

(I didn't have the best experience with the first SE, so I decided to go with someone else).

Sorry to hear about your less-than-ideal experience with the structural guy. Hopefully the new one will work out better.

The design will be a completely decoupled frame to achieve the 'room within a room'.

Excellent!

Anyway, I'm still about a week or two away from building the internal frame, but I really need to figure out the specifics of my HVAC now so I can properly prepare. HVAC is easily my weakest area, so I'm trying to get as much information as possible to figure this out.

You might already have seen these threads, but it's probably worth taking another look: why your studio needs proper HVAC. and Studio HVAC: All about mini-split systems, HRV's and ERV's You might find some of the info you need in there.

Estimated dimensions of finished interior room:
15' 3" x 14' 3" x 8' 6"
(approx 11,100 cubic feet)

Nice! That's a pretty good size room. Is this a control room only? Or will it also serve other purposes? However, there's a problem with your volume calculation: 15' 3" x 14' 3" x 8' 6" is 1,847 cubic feet, not the 11,100 you have there. So you would need about 190 CFM to get 6 room changes per hour. Which is fine: Your mini-split can handle that on the medium setting, and would probably do reasonably well on the low setting. Plus, you have the high setting for more severe situations.

187 x 4 % = ~73 CFM for HRV

Welllll. the answer is close, but the equation is a little wrong!

4% of 187 is about 7.5! :shock:

I think you intended to write "40%". That's what you are looking for: about one quarter to one half of the room circulation rate is what you usually need for the make-up fresh air supply, and 40% is a good bet. and 40% would, indeed, be 74.8 CFM. Call it 75, to make it easy, but 73 is also fine.

Here's the rub. The HRV is designed to be installed IN the room.

That's unusual. HRV's are not exactly silent! I would not want one in my room. Maybe you should look for an alternative? This is what studio HRV's commonly look like:

: sample-HRV unit.jpg (43.81 KiB) Viewed 35571 times

: sample-HRV unit.jpg (43.81 KiB) Viewed 35571 times

Basically, just a box with 4 duct attachments. I'd suggest looking for something like that.

1: Will putting the silencer boxes inside the room even work?

I can't see how that would work. The noise generated by the unit itself would not be silenced: the silencers would only be acting on other sounds coming from outside... and on sounds going the other way too, of course! So if you stand OUTSIDE the room, then you would not hear the HRV...

But inside the room is a different story.

To be honest, I'd suggest that you look for a more conventional HRV.

- Stuart -

#6

SoWhat wrote:Source of the post I too have started the research on HVAC for my upcoming build. If possible, I am going to use a ducted mini split (these are usually installed above the ceiling). With a ducted system, the HRV or ERV can be installed in-line rather than having a separate hole in your carefully sealed studio.

I'm very partial to ducted systems myself! It's just a better solution, all around, since all of the "noisy" stuff is outside of the studio. Even though some ductless mini-splits are very quiet these days, they are still not silent. For studios, I prefer to keep both the mini-split itself and also the HRV or ERV outside, with silencers, so that there is no HVAC noise at all inside the studio.

I don't want to install a unit in the ceiling because I am worried about not being able to service it. Also since the garage is so small, and detatched, I don't have another "area" or room to put it outside of the building and have just the ducts feeding the room.

It would be helpful if you would post a diagram of what you are planning to do, showing where you are thinking of locating the various parts of your HVAC system. Photos of the garage as it is right now would also help: maybe we might notice somthing you haven't seen so far...

So I need a unit that can be installed in the room, and have the silencers on the inside of the room.

To do that, you would probably need to build a very large box to completely surround the unit, then build your silencers as part of that "large box".

A few years ago, I designed a speaker soffit system with a built-in HVAC system, for a client in New York who had a very small room. His only option for HVAC was one of those portable "air conditioners" that have a couple of ducts on them:

That was the only possible solution for his room. So I designed a complete soffit system with that unit built into the center section, between the speaker soffits, and the silencers also built into the soffits:

That's the final outcome. You can see the small glass panel (thick!) in the middle section, so he could control the unit using the infrared remote control, but apart from that, it is completely hidden... and silent! The interior of that soffit system is rather complicated to say the least, since it has three complete HVAC silencers incorporated, plus the HVAC unit itself, and also the acoustic stuff to make the soffits work as soffits! It was a nightmare to do that design, and took quite a while to get it right. There's even a small plastic receptacle built into the lower part of the right soffit, for the condensate that rains out of the unit.. and of course, he needs to empty that a couple of times per day. But it works!

So you could so something like that, but do be warned: It ain't easy! That monstrosity is a lot more complicated than it looks, on the inside. If you happen to be a fan of the Dr. Who sci-fi series, I guess you could say that the soffit is an acoustic Tardis: it's bigger on the inside!

Here's a photo during the construction, where you can see some of that... but the best parts are already hidden there!

So it is possible, but NOT easy. If there is any other way of doing it, such as siting the HRV outside of the room, that would be a much better option.

- Stuart -

#7

Estimated dimensions of finished interior room:
15' 3" x 14' 3" x 8' 6"
(approx 11,100 cubic feet)

Nice! That's a pretty good size room. Is this a control room only? Or will it also serve other purposes?

This is more of a rehearsal room than a recording studio. I am a drummer and I will be practicing anywhere from 4-8 hours a day. I will be recording drums in there as well, but the room will be acoustically treated for the drums to sound good in the room.

However, there's a problem with your volume calculation: 15' 3" x 14' 3" x 8' 6" is 1,847 cubic feet, not the 11,100 you have there. So you would need about 190 CFM to get 6 room changes per hour. Which is fine: Your mini-split can handle that on the medium setting, and would probably do reasonably well on the low setting. Plus, you have the high setting for more severe situations.

Ahhh! I was doing so many calculations, that I forgot to mention the steps. The 11,100 number was supposed to be 1,847 cubic feet times 6 (for 6 room changes per hour). Then divide 11,100 by 60 and you get the 187 (or 190) CFM for the AC/Heating.

187 x 4 % = ~73 CFM for HRV

Welllll. the answer is close, but the equation is a little wrong! 4% of 187 is about 7.5! I think you intended to write "40%". That's what you are looking for: about one quarter to one half of the room circulation rate is what you usually need for the make-up fresh air supply, and 40% is a good bet. and 40% would, indeed, be 74.8 CFM. Call it 75, to make it easy, but 73 is also fine.

Yes, I meant 40%, not 4%, haha.

That's unusual. HRV's are not exactly silent! I would not want one in my room. Maybe you should look for an alternative? This is what studio HRV's commonly look like: sample-HRV unit.jpgBasically, just a box with 4 duct attachments. I'd suggest looking for something like that.

Yeah I know, I've seen and researched a TON of HRV's and ERV's, and the "in room" units are rare. However, I really have no other way of placing the unit outside of the building.

1: Will putting the silencer boxes inside the room even work?

I can't see how that would work. The noise generated by the unit itself would not be silenced: the silencers would only be acting on other sounds coming from outside... and on sounds going the other way too, of course! So if you stand OUTSIDE the room, then you would not hear the HRV... But inside the room is a different story.

I'm not so worried about the noise of the HRV or the AC unit inside the room. My concern is keeping the noise of me playing inside the room and not getting out. On the occasions when I do need to record, I can turn down the units, or turn them on low while I record, then turn them back up after. It's going to be mostly be just me in the room 90% of the time, so I'm fine dealing with that sort of situation.

It would be helpful if you would post a diagram of what you are planning to do, showing where you are thinking of locating the various parts of your HVAC system. Photos of the garage as it is right now would also help: maybe we might notice somthing you haven't seen so far...

I will post some photos tonight. I'll show the mockups, as well as what it looks like right now (mid beef up).

To do that, you would probably need to build a very large box to completely surround the unit, then build your silencers as part of that "large box".

Yeah, that's what I was envisioning. A large box that holds the HRV unit itself, that has flex duct feeding large/long silencers that penetrate the wall, and then ducts in the wall to carry the air outside. I'll try to draw something up in sketchup to give you an idea of what I want to do.

You might already have seen these threads, but it's probably worth taking another look: why your studio needs proper HVAC. and Studio HVAC: All about mini-split systems, HRV's and ERV's You might find some of the info you need in there.

Yeah, I read through both of those, and they were extremely helpful. I felt it gave me a much better idea of what I need to know... it's just hard figuring out how to take that information and make it work for my situation, which is quite different from all the build threads I've read.

Thank you for reading my gibberish and taking the time to respond, Stuart. It is very much appreciated.

Joe

#8

Hi Jag94, the ventilator you have linked to is a single room ventilator, that is why it goes in the room you are in. At the same company, look instead (on this page) at residential or commercial ventilators, where the unit is placed not in the same room - it will ideally be placed in a room or space next to your drum room, keeping the noise outside your studio.

Like Paul, I have not heard of the Micra brand before. Having read so many fora I feel sure that you will have come across Mitsubishi, Broan, Daikin, Toshiba, LG and Samsung as the regular culprits. There is usually a good reason why professional studio designers use these brands and it might be foolish to wander too far off the beaten track.

#9

Ok so here is a very rough sketchup of my idea. The HRV unit will sit on the wall next to the mini split system. In my particular case, the HRV vent that blows fresh air is on the right side of the unit, so that fresh air will come out and immediately hit the mini-split system to then be conditioned.

There will be two ducts coming off of the HRV unit, each going to it's own baffle box. I didn't draw both ducts because it was taking a long time and I just wanted to get the idea across. The baffle boxes will then feed into the wall, where ducts will take the air from the baffle box to the outside (and vise versa). Again, I didn't draw in the ducts in the wall because it's extremely tedious.

To reiterate, I am not so much concerned about the sound of the hvac units in the room. I care that I am getting fresh air (and stale air is being taken away), I care that the air coming in gets treated (heat/cooled), and I care that my drumming does not make it out of the room through the ducts.

Obviously I would prefer if the baffle boxes weren't this big, but if they need to be in order to get the isolation I need, then so be it.

Let me know if there is any other information you need to understand what my concept is.

#10

Greetings Jag94,

If you can re-orient your design so that the boxes are in the center of the wall (or if you can do what Stuart posted in that photo with the AC unit), you'll be able to treat the corners much more easily for LF, which in turn will make your isolation much better. Since this is a drum room, you'll all the LF treatment you can muster.

All the best,

Paul

#11

Hey Paul, thanks for the tip. That makes a lot of sense. I guess I just need help figuring out how big the boxes need to be, and what size ducts I should put in the walls to feed to the outside. All of the build posts I've read on all of the forums usually have an HRV/ERV in another room where they run the ducts to/from, or they just use a simple fan with intake/exhaust ducts through silencer boxes. I haven't seen anything like what I'm trying to do, so I'm a little stuck on how to proceed.

#12

Sorry, Jag94, I cannot seem to find the layout, the plan. Is your room the entire building, meaning are all 4 walls external walls?

#13

Hi Starlight,

Yeah, sorry, I haven't uploaded the full design and build progress (yet). I've done it on a few other sites and have been so busy with the construction phase, that I just haven't gotten around to doing it here yet.

It's a fairly small garage (14'x15'x8.5'), so it will be just one room. It's more of a live room/rehearsal room than a mixing room. But it will be just one room, no booths or separate rooms.

#14

Jag94 wrote:Source of the postI haven't uploaded the full design and build progress (yet). I've ... been so busy with the construction phase ...

Sorry, I got the impression from the OP that you are in the design phase. All the best with the build, it sounds like it is going to be a great place for drumming.

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