Sound Isolation Closet - Return Air Question

M

MarkFerraro

Member
I finally finished my assembly of the CV 1800 in a sound closet.

The closet was built free standing inside my shop. The walls consist of 5/8 inch Sheetrock (both inside and outside the closet)on 2x4 framing with fiberglass insulation in the walls and ceiling. I hung double solid wood doors with exterior weather stripping. For return into the shop, I framed out two return plenums. Both have 12 inch x 12 inch grills with filters. One grill is set high, with an baffle set midway between the studs that directs the return air down the inside of the wall and then allows a chase for form up the other side so the air goes into the high set grill, runs down the inside of the wall, passes through an opening at the bottom and then runs back up the inside of the wall to exit the outside wall through an unfiltered grille.

I am stunned at how much air pressure there is in the closet, trying to get out of the closet. Right now I am only using one of the 12x12 return grilles.

I'm also stunned at how quiet the whole arrangement is running. My shop is in a basement. The entire shop floor is 2 layers of 3/4 inch plywood laid at right angles over 1 inch of solid foam paneling. There are no wooden sleepers under the floor.

I am pleased as punch at how quiet the cyclone is running.

But I do have some questions:

1: Should I open up the second 12x12 return air grille? This grille is set by the floor, while the other grille is set 9.5 feet above the closet floor. the idea was the return air would draw both high and low temperature air out of the closet and expel it, thus aiding in the cooling of the motor.

2: With the one high set exhaust grille open, the air pressure on the doors is tremendous. If I set the bolts on one door that forms the jamb for the second door, the second door cannot be closed if the cyclone is running. I'm 300 pounds and I cannot shut the door. If the doors are closed and bolted and the cyclone is turned on, lots of air is leaking out of the second door because I have only one throw bolt holding the door closed at the top. I'm adding one or two bolts to the second door: one at the bottom and maybe one in the middle.

I guess I was thinking that a 6 inch round intake pipe couldn't over pressurize the closet or impede air flow if there was a single 12x12 exhaust. I included the second exhaust just in case, but now think it might be needed. Any facts or opinions as to how much air exhaust volume we have to allow for? I'm sure the nominal 12x12 grille is actually something like 10x10 in actual air flow ability.

So to recap my question:

Is a single 12x12 return air chase/plenum enough volume or should I open the second 12x12 return air chase/plenum?

Thanks in advance,
 
Mark,
I do not understand the return duct description. Could you send some pictures or drawings to help. I think the duct resistance is the problem. I just calculated about 92 lbs of force against a 32 x 80" door for each inch of water column return pressure. The return pressure sounds really high.

Bababrown
 
More Information

More Information

Attached is a pdf on the design of the air return system. The drawings are terrible, as I have no artistic talent what so ever. To clarify - there is one empty bay in the wall that has no insulation. Between the studs that makes up the bay there is a 1/4 inch piece of plywood that has been attached to cleats at each stud. The plywood, stud and top plate have been sealed with caulk. At the bottom of the bay the plywood baffle has been stopped 3-4 inches above the bottom plate. This is supposed to allow air to move around the baffle and go up the outer chamber to the grille at the top of the wall, where the air can exit back into the workshop.
 

Attachments

  • MFCVClosetBaffles.pdf
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Mark,
Okay, now I get it. You are getting too much back pressure because the cross section of the return ducts is too small and the length is long. Assuming 16" stud spacing the duct cross section is 1.625' X 14.5" or some 23.6 square inches. An 8" diameter pipe outlet has a cross section of 50.2 square inches and is typical of what people use. I see at least two solutions. You could use a baffle at the inside top and feed down the full stud cross section to an outside baffle at the bottom. Or you could build another duct over top of the stud spacing for the double path approach. In any case make the cross sections at least 50 square inches. Hope this helps.
bababrown
 
I'm using an 6 inch inlet pipe, as my shop is compact. Longest run is only 25 feet or so. But I get the idea. The 8 inch outlet is the filter assembly????

I will open up the second air grille/baffle. That should give me a total outlet area of around 46-47 sq. inches.

Thanks for the math lesson.

Mark
 
Baffle material

Baffle material

Mark,
What did you use for the baffle material? Sheetrock?
After adding a second return how is the noise level?
David
 
Baffle Material

Baffle Material

David -

I had to cut out the 5/8 inch sheet rock to get to the internal baffle that was 5/8 inch sheet rock screwed and caulked to cleats on the studs in the bay.

After cutting out the internal baffle, I repaired the wall with the 5/8 inch sheetrock, taped and mud. I left a 6 x 14 opening at the bottom of the wall. Then I ripped a 2x4 and pocket screwed it to the wall as sistered studs and covered the studs ( with a ripped piece of 2x4 to seal the bottom of the chase) with 3/8 inch plywood. I used screw washers and screws to make sure I didn't puncture the plywood and to add a bit of extra grip to cut down on any vibration. I left the top of the sistered stub bay open.

So I basically created a U shaped return air plenum/chase that starts near the ceiling, goes down the inside wall, thru the opening, up the outer wall and out a 12 x 12 air grille. So the return air has about a 14 foot journey till it reaches the shop.

I mostly hear the rush of air. With the solid wood doors, the two layers of 5/8 sheetrock and the insulation in the walls and ceiling, it actually runs very quietly. The closet is also isolated from any connection to the basement walls and sits on a floor that is made up of 1 inch of foam panel and two layers of 3/4 inch plywood.

Besides the rush of air returning to the shop thru the two outlet grilles, I do hear a low register hum, but it sounds like a powerful motor running very smoothly and in the distance. I don't have a high pitched sound that is very common in other DC installations/companies. I wonder if the extra tall wooden dust collector/clean out I built for the filters to sit on is actually working as a sound damper.

It it built with 3/4 inch plywood and sits in full contact with the filters and the plywood floor. There is a picture of it in my gallery.

Hope this helps.
 
Question

Question

Mark I am not sure what you mean by ripping 2x4 to make the second chase on the inside wall. Does this mean that the first chase down the wall is smaller than the return going up the outside wall which I assume is constructed of 2x4 wall? Thanks for clarifying. Chuck
 
Chuck -

I ripped a 2x4 down its length to create two 3/4 x 4's. Since its only purpose was to hold the 3/8 inch plywood, I didn't need much structural strength. All I needed was the nominal 4 inch depth. It is plenty rigid once the 3/8 inch plywood was screwed to the two sides and the bottom piece.

The chase mounted on the outside of the wall in the closet is as deep as the chase inside the wall that leads to the outside of the closet.

Yes, I know. I'm cheap. I had taken apart an old racking system built out of 2x4's and simpson strong tie connectors. Seemed more prudent to use one 8 foot 2x4 than 2. Never know when another project might call for used lumber. Like the raised garden boxes I just made. They made great battens to fasten the 2x10's into simple boxes. I guess since I met and shook Norm Abram's hand, the Yankee thriftiness rubbed off? Plus I sold my Yukon XLT and bought a PRIUS V. Getting lumber home now is a bit more complicated.
 
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