Air return for DC in closet

[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,