HVAC - # of Returns for 24x13'8 room - Page 2 - AVS Forum
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post #31 of 42 Old 05-30-2007, 04:39 AM
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Sorry I left out cause it is heavier & cause it is lighter. I did not think I had to be that clear and assumed people here had common sense. Hot air rises & cold air drops I thoght would be enough.
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post #32 of 42 Old 05-30-2007, 06:04 AM
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Quote:
Originally Posted by Dennis Erskine View Post

As it rises, the temperature drops and at the point where the temperature and dew point are the same, the cloud (visible moisture) forms. As the air cools (when it rises), and moisture droplets become larger, they fall. We call it rain (and we need more of it).


So you're saying we want it to rain in our Home Theater?

j/k
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post #33 of 42 Old 05-30-2007, 10:59 AM
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...absolutely. All you need are 40,000 foot ceilings.

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post #34 of 42 Old 06-12-2012, 06:24 AM
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Thanks for discussion as my HT is gutted and in process of moving air from attic to basement and probably doing seperate trunk for upstair and seperate trunk and zone for basement so i can make my HT air"perfect" by having hvac guys follow ur best practices if i can figure out is is most right.....LOL

http://www.avsforum.com/t/1415166/bluff-rock-cinema-buildnow-under-way#post_22181478 Bluff Rock Cinema build and whole basement renow for fun and games
From moldy/musty mess to magnificent wonder(i hope)
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post #35 of 42 Old 06-12-2012, 10:42 AM
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So I getting ready to finalize quotes and plans for HVAC and here's what I think I took away from this thread.

My room is 13x23, with the PJ mounted about 16-18 feet back.

1. Keep CFM <150 at the supply side (so far quotes are for 2 6" flexduct supplies, I'm told these are 100 CFM each.)
2. Supplies in the front, but away from screen - up high (Where should these be room wise? In the soffits up front, one on each side? Can they be behind the false wall?)
3. Returns in the rear - up high and oversized (so far the quotes are spec'ing 1 single 8" or 10" duct)
4. Minimize duct entrance into sealed room - use soffits for maneuvering.

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post #36 of 42 Old 06-12-2012, 12:06 PM
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I think the critical HVAC number for acoustic purposes is to limit the velocity of the air flow to <250 ft/min. The volume (CFM) should be sized to provide the required number of air changes per hour (probably about 6).

Might be better off to specify the temp range and noise level (for example, NC 21) than to try to design the HVAC system and spec that.
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post #37 of 42 Old 06-12-2012, 12:23 PM
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Quote:
Originally Posted by blipszyc View Post

So I getting ready to finalize quotes and plans for HVAC and here's what I think I took away from this thread.
My room is 13x23, with the PJ mounted about 16-18 feet back.
1. Keep CFM <150 at the supply side (so far quotes are for 2 6" flexduct supplies, I'm told these are 100 CFM each.)
2. Supplies in the front, but away from screen - up high (Where should these be room wise? In the soffits up front, one on each side? Can they be behind the false wall?)
3. Returns in the rear - up high and oversized (so far the quotes are spec'ing 1 single 8" or 10" duct)
4. Minimize duct entrance into sealed room - use soffits for maneuvering.

I have a similar sized room. I plan on having 2 supplies venting from side soffits next to the screen - avoid putting them in front of screen. Add some 90 degree bends behind some mass (soffits) for better low-frequency sound isolation. Don't put supplies behind false wall, others have had problems with proper air movement with this approach. You can optimize returns by having two of them, one high and the other low. This allows you to choose which one should be open depending on the heating/cooling needs of the room. You might also want to consider a variable speed inline return duct fan to give you some insurance against having insufficient air flow. Here's a sketchup of my current plan: http://www.avsforum.com/t/1383183/235-build/60_60#post_21950547
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post #38 of 42 Old 06-12-2012, 12:33 PM
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I would suggest at least 4 8" Supplies and one return at least 12x18....  More is usually quieter also.

 

My room 14x24.

 

One 12x18 return

Two 8" flex duct supplies

All Equipment and projector in room and its HOT.  I need more supply. :(


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post #39 of 42 Old 06-12-2012, 03:51 PM
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HVAC is science and math just like acoustics.

Lots of suggestions, but no real details on the room. Does it face south with 100 sq.ft. of glass? Is it in the basement, second floor? Are you seating 3 or 13?

Just like acoustics, you can take a best guess based on what has worked for others. Honestly, a lot of what I have seen done works, but at what expense. Slot diffusers are expensive, so did you actually need 8 feet? Do you need 2-slot or 4-slot diffusers?

Oversizing the diffuser reduces throw and can decrease efficiency. It's like washing your car with no nozzle on the hose. The whole car gets wet. Eventually. Best to have the proper size meeting the NC and velocity without making it larger than need be.

Did you really need to run a 10" duct that was difficult and time consuming, or would a 8" have done the job?

Figuring out the cooling load for a single room with only the projector should be a piece of cake (I suspect at least one person posting in this thread can do it for you). Are you doing all the work yourself, or are you going to spec it for a HVAC contractor? [rhetorical question] If you are involving a contractor I would make him figure out the load. Once you have that you can come here and get some great advice on sizing.

But everything depends on what the heat gain is... IIRC you can figure about 1000 btu for the projector and 350 btu per person. Then you need to figure the heat gain for the room based on where it is. Once you have total btu you can determine required cfm and progress to duct size and diffuser size. Two 6" flex ducts? Maybe, but that's pushing the limit for 100CFM, especially in a theater. That's if you need 100CFM and if the length of the run is acceptable. I don't think you'll get 100 CFM through 100 feet of 6" flex.

As for returns (sorry, was your OP and neglected to answer it), I never heard of an issue oversizing it as long as you have a damper to balance it. The return register really can't be too big, AFAIK, other than expense and the size of the hole you're punching through your aquarium.

Not a HVAC professional.. but seen my share.

Tim
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post #40 of 42 Old 06-13-2012, 06:12 PM
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Interesting thread, in summary (have I got this right?)

Supplies – larger the better - 8” is good and two (front and high and away from screen)

Returns – at least the same size as the supplies and two (rear and high and away from seats)

Do the plenums need to be the same size as the ducting? Or larger? Say for supplies you’ve got 8” going into a box containing a 4” x 24” bar diffuser. Should the plenums be say 8” x 8” x ~24” internally (so say two layers of 18mm ply/osb with GG plus say 50mm (2 inches) of duct liner that makes it about 14” x 14” x 30” external plenum size?)

Do both returns need to feed into the HVAC unit or can you have one powered by an inline fan?

I had planned 2 x supplies, 1 return going into the unit and 1 inline fan return ducted out (with 1 powered or maybe passive fresh air supply in addition)

Also, what should be the minimum length of ductwork for isolation purposes?

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post #41 of 42 Old 07-21-2012, 06:49 PM
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Quote:
Originally Posted by Elill View Post

Interesting thread, in summary (have I got this right?)
Supplies – larger the better - 8” is good and two (front and high and away from screen)
Returns – at least the same size as the supplies and two (rear and high and away from seats)
Do the plenums need to be the same size as the ducting? Or larger? Say for supplies you’ve got 8” going into a box containing a 4” x 24” bar diffuser. Should the plenums be say 8” x 8” x ~24” internally (so say two layers of 18mm ply/osb with GG plus say 50mm (2 inches) of duct liner that makes it about 14” x 14” x 30” external plenum size?)
Do both returns need to feed into the HVAC unit or can you have one powered by an inline fan?
I had planned 2 x supplies, 1 return going into the unit and 1 inline fan return ducted out (with 1 powered or maybe passive fresh air supply in addition)
Also, what should be the minimum length of ductwork for isolation purposes?

Would love to hear some answers to these questions as well.

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post #42 of 42 Old 07-22-2012, 03:24 PM
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Also interested.

Is it a terrible idea to tie into the return system bc of noise concerns? Making the only quiet option a dead vent?
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