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Old 06-25-2010, 01:25 PM   #1
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Plenum Min/Max size?


Slowly planning my system. I'll be putting an AH in my FL attic. I have not done the manual J yet, but looks like 2 tons. I am looking at a Ruud. The unit will be 20X40X18". Is there a minimum or Maximum size for a plenum? JIm

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Old 06-25-2010, 05:26 PM   #2
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Plenum Min/Max size?


Are you talking a supply plenum. or an extended plenum duct system.

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Old 06-26-2010, 12:15 AM   #3
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Plenum Min/Max size?


I am more interested in the return air plenum, 16 inch flex. Fig A would run the length of the attic and be OK. Fig B would run the width of the attic and the unit would be much more accessible. I think!
I'm not sure B would work, it is a low trussed attic, I cannot find my drawing but the max height I believe is 39". I plan on making a cardboard mock up and seeing what would be best.
Is it acceptable to put the return air filter at the AH instead of the ceiling?? My present return air grill/filter holder in the ceiling is an odd size (roughly 14X32)and the ceiling is textured so I really don't want to replace it. I would want to leave the present return air grill in the ceiling and run the return duct to the AH from it.
I do not plan on doing this till late fall or early winter, might be awful cold in PA. JIm
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Old 06-26-2010, 12:19 AM   #4
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Plenum Min/Max size?


Forgot the attachment!!
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Old 06-26-2010, 05:51 AM   #5
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Plenum Min/Max size?


Which ever location gives you the larger air filter is fine to use.

Which way you position the air handler, is more of an access to the air handler for future service thing.

You'll have a few inches of space under the air handler to account for. Since you need to have a secondary drain pan under it. So figure 4" more on the height of the air handler when its in position. Probably will make Fig A the position you use.

Both are fine to use, if you have the room/clearance for them.
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Old 06-26-2010, 07:27 AM   #6
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Plenum Min/Max size?


FOUR INCHES! Am I glad you said that! I'm looking at the present pan and going by that. It is about 1 1/2" high. JIm
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Old 06-26-2010, 07:42 AM   #7
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Plenum Min/Max size?


The pan is only 1" high. But you should have your unit/air handler raised off the bottom of that pan so it clears the pan sides by 1 to 2". Makes connecting the plenum to the unit much easier.

I use 4" pump ups to do it. Makes it easier. They are generally for raising a heat pump outdoor unit off the pad for drainage. But work great at raising an air handler/furnace, out/off of the drain pan. Makes duct work installation much easier.

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