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Old 07-07-2012, 03:36 PM   #1
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Conditional Attic Space


Hello,
I'm looking for advice regarding conditioning my attic. The attic is 1300 cubic feet volume with about 3 to 4" of spray foam insulation in the rafters; the attic floor is uninsulated with conditioned rooms below. The attic is only 4' high at the peak, so it's not a large space from a volume perspective. My HVAC unit is within the attic space. Currently I run a dehumidifier in the attic to keep humidity down during the summer. My goal is to use the existing HVAC system instead of the dehumidifier.

My thoughts are to provide a return in the existing return plenum which is already in the attic. The plenum is about 3'x3'x4', so there is plenty of room to add some type of return inlet.

For supply air I was considering tieing into one or two of the existing 6" ducts to the rooms below and running a couple of ducts to outlets within the attic. The AC unit currenty supplies the following ducts, all run within the attic space...
(3) 6" x 20' long, (3) 6" x 15' long.

First question is would this work? What about sizing? Should I install dampers to the return and supply lines? What size should the return in the plenum be? What size and how many supply lines should I run?

Thanks.

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Old 07-07-2012, 05:56 PM   #2
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Conditional Attic Space


The answer depends on what type of heat you have. If it's an 80% eff. furnace then no return in the space,there's already not enough free air for required proper combustion. A 90% furnace with outside air intake or electric heat would be fine though.

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Old 07-07-2012, 07:57 PM   #3
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Conditional Attic Space


Hopefully you spray foamed the gable ends also. Run a separate supply for the attic. Don't take air from a supply for any of the below rooms. A single 6" is all you probably need. And 6X12 return grille in the return plenum. This will gt you to near human comfort level.

This is based on the furnace being a direct vent.
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Old 07-07-2012, 08:47 PM   #4
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Conditional Attic Space


Thanks for the input. Heat is electric, so that should not be an issue. Yes, the gable ends are also sprayed with foam.

Should I put some type of damper on the return? Should I be concerned about creating a vacuum in the attic? I already have a slight moisture problem and hate to make it worse by removing too much through the return.

What if I had no return at all? My thinking is that then I may cool the attic, but would not remove moisture. Is that correct?

Thanks again for the help.
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Old 07-07-2012, 09:05 PM   #5
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Conditional Attic Space


Quote:
Originally Posted by gandlbecker View Post
Thanks for the input. Heat is electric, so that should not be an issue. Yes, the gable ends are also sprayed with foam.

Should I put some type of damper on the return? Should I be concerned about creating a vacuum in the attic? I already have a slight moisture problem and hate to make it worse by removing too much through the return.

What if I had no return at all? My thinking is that then I may cool the attic, but would not remove moisture. Is that correct?

That is correct.

Thanks again for the help.
You really only need a small supply, and return. You can use a return grille with a damper in it to adjust the amount or return your pulling. A 4" round supply will be enough to condition the space for humidity control. but better off with a 5".
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