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Old 03-05-2012, 12:09 PM   #16
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how big isit......16 x25,,,16 x 20...20 x 20..
Don't think that the size of the grill matters at this point, based on the unit model and assuming .5" external pressure its moving (or try to move) about 1000 cfm (on MED speed) through (1) grill. The stud cavity doesn't have enough free area to be used as a singular return. Must add some RA capacity in my opinion.

You have a total of 2 ft2 of grill going into about .5 ft2 of stud cavity where you need approximately 2 Ft2 of duct/stud cavity to maintain an acceptable air velocity.

Mark

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Old 03-05-2012, 05:10 PM   #17
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20X20 filter grille is good for about 570 CFM at a face velocity of 300 FPM, and about 960 CFM at 500 FPM.

At a 70 rise it needs 879 CFM, for a 50 rise it needs 1,120 CFM.
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Old 03-05-2012, 05:40 PM   #18
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20X20 filter grille is good for about 570 CFM at a face velocity of 300 FPM, and about 960 CFM at 500 FPM.

At a 70 rise it needs 879 CFM, for a 50 rise it needs 1,120 CFM.
Thats all fine an dandy, but if this RA is going into a stud cavity the velocities would be upward of 1500 FPM resulting is too high of a delta P diminishing the volume dramically.

We need to know where the RA is.
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Old 03-05-2012, 05:43 PM   #19
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Thats all fine an dandy, but if this RA is going into a stud cavity the velocities would be upward of 1500 FPM resulting is too high of a delta P diminishing the volume dramically.

We need to know where the RA is.
Mark
see alot down south..20 x20 filter box with 12 or 14" flex hooked to it... framed in hallway..
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Old 03-05-2012, 05:52 PM   #20
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see alot down south..20 x20 filter box with 12 or 14" flex hooked to it... framed in hallway..
Yes this is what I am thinking now, I haven't seen much of that around here, RA are usually in stud cavities.
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Old 03-05-2012, 05:58 PM   #21
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Yes this is what I am thinking now, I haven't seen much of that around here, RA are usually in stud cavities.
yea my thougth too... but many are now just installing filter box with flex duct connected to back...had one last week where 3m pleated filter was colasped into the duct from dirt....12" round return..
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Old 03-05-2012, 06:02 PM   #22
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I imagine you see all kinds of neglect, folks think that they turn it on and it runs, that is until it doesn't run any more.

Then they call the fix-it man

and then they post on a DIY web site, like, is this too much to pay for a

service call.......

I can't imagine working in the service industry, what you guys see and hear on a regular basis would make a grown man cry, I am sure.

Mark
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Old 03-05-2012, 06:06 PM   #23
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I imagine you see all kinds of neglect, folks think that you turn it on and it runs, that is until it doesn't run any more.

Then you call the fix-it man

and then you post on a DIY web site, like, is this too much to pay for a

service call.......

I can't imagine working in the service industry, what you guys see and hear on a regular basis would make a grown man cry, I am sure.

Mark
yes Iam crying now just got off the phone with a no heat took me 15 min to explain how to turn off switch for furnace that I painted RED for her 2yrs ago...
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Old 03-06-2012, 07:15 AM   #24
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Did 'ya use the old "light Switch"
analogy?

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