Air return for DC in closet

[mark mcfarlane]

I pan on putting my DC in a large closet and use 25' of flexible duct to return air to the shop. Inlet on DC ( not purchased yet, probably a 3HP Oneida system) is probably going to be 7.

Do you think a 10" return duct is enough, thats around 80 sq inches.

[Greg Parrish]

I pan on putting my DC in a large closet and use 25' of flexible duct to return air to the shop. Inlet on DC ( not purchased yet, probably a 3HP Oneida system) is probably going to be 7.

Do you think a 10" return duct is enough, thats around 80 sq inches.

Mark, I would probably call Oneida on that one as they know their machine best. The 3hp is great but they may even recommend a different machine or motor size based on that intended usage.

ps - if you can wait to purchase Oneida usually has a sale around July 4th as I think that's when I bought mine. Saved quite a bit by buying during one of their sales too.

[Wayne Lomman]

Mark, you will need more than that. I can't give you an actual size but you have to avoid any possibility of back pressure which will restrict the efficiency of the machine. Cheers

[Lee Schierer]

I think if I were going to do this I would build an opening that would hold at least a 20 x 20 furnace air filter and put a Merv 11 or higher filter in it. This would catch anything that gets past your DC system filters.

[Andy Giddings]

Mark, see attached from the ClearVue forum where someone with a CV1800 is asking the same question. Advice seems to be 8 inch diameter is enough for their blower which is 5HP 1442CFM rated. As you'll already be cleaning the air down to 0.5 micron I don't see a need for additional filtering after that. You will need a plenum if you are doing this to reduce noise http://www.clearvuecyclones.com/forum/forum/clearvue-cyclones/noise-control/1301-sound-isolation-closet-return-air-question

[mark mcfarlane]

Thanks Greg,Wayne, Lee and Andy. July is about the right time frame :). I will talk with Oneida.

I plan to use something like this (https://www.amazon.com/dp/B00B065MIS/ref=wl_it_dp_o_pC_S_ttl?_encoding=UTF8&colid=Y5GG4NXGMSVF&coliid=I1ET6VLSR06MGP) on each end of the run although I may go with insulated boxes and pipe. The insulation should add some damping. It seems like I can effectively test the back pressure by opening and closing the machine room door and either listen, or measure air flow in the machine-side ductwork.

[Jim Becker]

The 10" equivalent is about what I used for my return, but mine isn't 25' long. For an air return, "bigger can be better" and as long as it's "bent", direct sound transmission will be mitigated. Here's how my return is constructed:

http://img.photobucket.com/albums/v646/a-j-adopt/Woodworking/air-return.jpg

[mark mcfarlane]

Thanks Jim. I thought I'd get a similar baffle effect just by running through 10" flexible duct. I can zig-zag it and only travel 10 feet, and maybe put the output register in front of my overhead air cleaner. Two wall registers, 25 feet of duct. Will take about an hour to install before the drywall goes up.

Seems like an easier install than what you did, but probably more expensive, maybe $150 for mine. I plan to insulate the very short (3') shared wall with my shop, which precludes putting the return in the insulated wall.

[Jim Becker]

The exact "how" you do it matters less than making sure you have the volume available for flow and don't have a direct path for noise transmission. The flexible duct will do it!

[Vince Rosypal]

The Home theater guys over at AVSForum like the dead vent system that the soundproofing company talk about in this article
http://www.soundproofingcompany.com/soundproofing-articles/how-can-i-ventilate-a-sealed-soundproof-room/

[John K Jordan]

Depends on the volume and velocity of air you want blowing out of the return duct.

My 5 hp ClearVue cyclone sucks in air from a 6" duct, about 28 sq inches. I duct back into the room through a 144 sq in opening (with a filter and grate, just for fun), about five times the area of the input. There is no significant back pressure on the filters judging by the lack of force when opening a sealed door. All works well.

BTW, I built a plywood return duct snaked up through the trusses with angles and turns (stealth bomber stuff), sprayed the inside with rubber coating, and insulated outside with fiberglass batt. I wanted to minimize the sound coming through the duct and the ceiling. It worked.

This is my working sketch of the duct, grill for the return is in the ceiling, framing shown here before I put the ceiling up.

357556 357557

JKJ

[mark mcfarlane]

Thanks Jim, Vince and John.

I think I'll probably start with a simple flexible duct system and if I need more attenuation I can build something more substantial later. I don't need dead-quiet DC, just enough attenuation to make it more comfortable working in the shop.

I can't use the ceiling joist space, the joists run perpendicular to the direction I need to go, and will have R45 insulation blown in between the joists.

[Ole Anderson]

And those of you opting for a more, non-ducted route, line at least one side of the return with Safe-N-Sound mineral wool to minimize reflected sound.

[Kris Whitman]

358293

Here is a pic of my folded exhaust closet top. Obviously this was taken before I added the foamed 3/4" plywood top. My cyclone has a 7" inlet and exhaust which equates to 38.5 inches. I made my exhaust route 15"x15" or 225" which is just shy of 6 times as large as the cyclone exhaust. The lower right part of the picture is where the motor to the cyclone resides. By making the inlet to the folded exhaust originate by the motor my thinking was it forces all the air past the fins on the motor. I am just now closing up the closet so I do not know how effective it will be. The closet is standard stud construction with 5/8" sheetrock on the inside walls with Roxul insulation between studs then 5/8" sheetrock on the outside then another layer of OSB to match the shop walls. The door is a foam filled metal door with a good seal. I was tempted to buy green glue and add a second 5/8" plywood on the inside and outside but decided that a 15"x15" opening on top of the closet kind of made that idea pointless.

Time will tell if I wasted my time but I guess closing in the cyclone and not offering a direct path exhaust has got to improve the noise level. How much is the question. Time will tell. I'll let everyone know how it did once the closet is complete.

358294

Kris

[Jim Andrew]

Mark, if you really want to get your money's worth on blown insulation, just order the amount of insulation for the r factor you want and square footage from your local lumberyard, and have them bring out the blower at the same time. You will need one helper, someone to put the insulation in the hopper, and you just keep blowing till you get all the material in. I found that contractors tend to stop at a certain thickness, which is considerably less than what you paid for, as the stuff settles, and the manufacturer factors that in. Then call the yard and have them pick up the blower.

[mark mcfarlane]

Thanks Jim for the tip. I'll probably be using open-cell spray foam in the walls and ceiling.

[Jim Becker]

Thanks Jim for the tip. I'll probably be using open-cell spray foam in the walls and ceiling.

Why open cell? (just curious...I'm a fan of closed cell foam with R7 per inch, natural moisture barrier and structural stiffness, which is what we used for our 2200 sq ft addition in 2008)

[mark mcfarlane]

Why open cell? (just curious...I'm a fan of closed cell foam with R7 per inch, natural moisture barrier and structural stiffness, which is what we used for our 2200 sq ft addition in 2008)

Open cell is what my builder recommended, and used on his own home. It's cheaper and has better acoustic adsorption than closed cell, which should be helpful for the recording studio part of the new building. Still not as good acoustically as dense cellulose, but my 10:12 roof and resulting very small eaves present some challenges for insulating above the exterior walls with anything other than foam.
It might partly be a Texas thing. We have different issues for insulation, e.g. hot outside and cold inside most of the year, compared to PA.

Our climate also presents interesting questions such as which side of the insulation does the vapor barrier belong? For attics, we don't use a vapor barrier. An unconditioned attic in Texas is 40F in the winter and 140F in the summer, but most of the year it is hotter in the attic.

It was about 100 in the attic today: I was pulling cable in the attic for the new build.

Closed cell does seem to be better for everything except acoustic absorption, but also at a $ cost. The best solution for me acoustically and energy wise would probably be 1.5" of closed cell and 1.5" of dense cellulose in the walls, Similarly in the ceiling...

[Chris Fairbanks]

When I talked with clearvue about my setup they said 3x inlet diameter is the recommended exhaust output. Less than that and back pressure could cause you problems. Also remember that is 3x with no length or resistance. If you are running 25' of something that is going to add a bunch of back pressure and if it is flex it's going to be a hell of a lot worse than smooth wall pipe. Maybe put a gauge in the room and to the outside to make sure there is no back pressure in the room.

[mark mcfarlane]

Thanks Chris.

[Jim Becker]

Mark, one advantage of open cell in your wall cavities is that they can "spray it full" and then trim it off flush without compromising the product. That means the bays can be completely full which is something you don't necessarily get with closed cell. The softer material combined with a full fill of the bays is likely helpful with the acoustic situation you have for sure!

[mark mcfarlane]

For the record, a 10" return through 25' of ducting was not nearly enough for the 5HP Gorilla Pro I ended up installing. I can't even get close to closing the door to the machine room with the DC running. More ducts are in the future.

[Jim Becker]

Wow...that thing must be moving some air!

[mark mcfarlane]

Wow...that thing must be moving some air!

It seems to be working pretty well, and I haven't taped up a couple of joints that aren't quick-connect style.

There are two 90 degree bends in the return flex ductwork, and the bends aren't perfect smooth 90s, so there is some back pressure.

I can't easily do the 'common' in-wall returns since I put foam insulation in the 2*4 wall adjoining the shop. I'd have to remove the wall board.


...Then again, that may be much easier than going into the attic at this point in time and installing large register boots, and it would be much cheaper, I'd just need to buy 2 grates and offset them 7 feet or so. I don't think the 2*4 wall is thick enough to do any insulation or baffling, the cross sectional area between studs is going to be 3.5" * 12" or so, its a short wall with a few studs and some condensate drains...

3.5*16 = 56 sq in (if I even have a 16" span, more likely 11" or so...)

10" duct is 78 sq in

[Jim Becker]

Ceiling returns would work just fine...just keep them away from the area you plan on doing your finishing work. And doing more than one turn to increase the area should also be fine to increase the air flow capability. Up and over deals with direct sound transmission. So crawling in the ceiling may just be worth it.

[Steve Peterson]

For the record, a 10" return through 25' of ducting was not nearly enough for the 5HP Gorilla Pro I ended up installing. I can't even get close to closing the door to the machine room with the DC running. More ducts are in the future.

Can you cut it in half and use two 12.5' sections of duct?

[mark mcfarlane]

Can you cut it in half and use two 12.5' sections of duct?

Not without moving the cold air return boot and repairing the current drywall hole that is about 18' away. The cost of the ductwork isn't that big a deal....

[Frank Pratt]

I have a 5HP collector & made up a 10" x 30" return air duct that is lined with fiberglass duct insulation. I has a 90* bend & a dogleg built into it which does a very nice job of reducing the noise.

[Alan Lightstone]

Just making sure I understand this, as I'll be putting my Oneida 5HP cyclone in a closet for soundproofing.

I need sufficient opening from the main room to the cyclone to put out to make up for the large quantity of air it is sucking in. This is typically 3-5 times the cross sectional area of the inlet ducting as stated above - hence the large folded returns, meant to be large enough to allow that air to return to the main room, but absorb as much sound as possible. For mine, the inlet diameter is 8" or 50 sq in. 3 x that would require a diameter of 14" to provide the necessary area.

Is this correct? Does it really need that big an opening?

[mark mcfarlane]

Just making sure I understand this, as I'll be putting my Oneida 5HP cyclone in a closet for soundproofing.

I need sufficient opening from the main room to the cyclone to put out to make up for the large quantity of air it is sucking in. This is typically 3-5 times the cross sectional area of the inlet ducting as stated above - hence the large folded returns, meant to be large enough to allow that air to return to the main room, but absorb as much sound as possible. For mine, the inlet diameter is 8" or 50 sq in. 3 x that would require a diameter of 14" to provide the necessary area.

Is this correct? Does it really need that big an opening?

Alan, my guess is that 14" diameter will be adequate if you use hard pipe. If you use flex pipe like I did, its very hard to keep the full cross sectional area through 90 degree bends, so 14" might not be enough for an Oneida 5HP.

I'd ask Oneida. I wish I had asked them.

I'll try to run my vac tomorrow and see how far I can get the closet door closed before it pushes back, that should give me some idea of how much more ducting I need,

[mark mcfarlane]

I just did a little experiment to determine how much air return my 5HP DC really wants.

The test was run with single 6" duct open (jointer). The 5HP Oneida Dust Gorilla Pro has in 8" inlet, which transitions to 7" after about 10', which transitions to 6" after 10 more feet, then there is about a 20' 6" run to the jointer:

- turned on the DC in its 7'*6' closet
- slowly closed the heavy exterior door until I felt pressure
- let the pressure in the closet naturally reopen the door.

Not perfectly scientific, but perhaps adequate to ensure there was no back pressure. I repeated this half a dozen times, sometime closing just little more than where the door opened previously, and got basically the same result.

My 8' door wants to be open 5". If I push it closed to 4.5" it goes back to 5" and holds. 5*96=480 sq inches, plus a bit more for the pie slice at the top and bottom of the door. Add to that my existing 10" duct, which is probably 8" in the bends, and we have something around 550 sq inches.

When I opened up 4 blast gates (a combination of 6", 4", and 3") I could hear the DC reving up as I opened each blast gate, the closet door wants to open 7.75 inches, which is 744 square inches of return.

That's a lot more than the 153 sq inches given by a perfect 14" duct.

Unfortunately, my DC is along an outside wall and there isn't very much room in the attic to work considering the roof pitch.

This makes me curious how much return area others have, and if they actually verified there was no back pressure.

Frank's Pratts 10*30 rigid duct gives 300 sq inches (- area of insulation). Frank does your closet door close without any pressure when the DC is sucking without any signs back pressure?

Am I missing something in this experimental design?

[Frank Pratt]

Frank's Pratts 10*30 rigid duct gives 300 sq inches (- area of insulation). Frank does your closet door close without any pressure when the DC is sucking without any signs back pressure?

Am I missing something in this experimental design?

I haven't actually tried, but the air velocity coming out of the return air is pretty low, so I don't imagine that there is much restriction in that duct. I'm sure it could be made smaller & still be fine, but I wanted to make sure I only did it once.