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Old 12-19-2013, 01:20 PM   #1
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Plenum Separation Issue


I have had a problem for awhile that I didn't understand until recently. The ductboard plenum attached to my A/C coil with aluminum tape immediately starts pulling away one one side until there is a gap and air is blowing out of it.

I finally figured out that the pressure from the blower was causing this, because once I braced the plenum temporarily on the end, the tape stayed in place and everything held together. The A/C coil is mechanically fastened to the furnace with a metal strap and screws, however I'm not sure how you would appropriately do this going from metal to ductboard.

The unit is a horizontally mounted single speed (no way to adjust the fan speed that i'm aware of) furnace with coil attached. I have two units and it only happens on one, however the other smaller one has a much slower fan speed.

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Old 12-19-2013, 04:30 PM   #2
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Plenum Separation Issue


Post pics.

Your duct work is probably under sized for the air flow.

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Old 12-19-2013, 05:48 PM   #3
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Plenum Separation Issue


Quote:
Originally Posted by beenthere View Post
Post pics.

Your duct work is probably under sized for the air flow.
i'll take some pics a little later and post them but you are probably right. bottom line, they added a second unit a long time ago right after the house was built. i know this because the lines and electrical run on the outside of the brick up to the attic (and because my home inspector told me so). i'm guessing that the original unit couldn't keep the upstairs and downstairs balanced. it's south texas and almost every house above 2500 sq. ft. that is 2 stories has two units. anyway, i'm also guessing that they took all the upstairs ducting and moved it over to the second, smaller unit that they installed later.

now, it appears that the original unit is oversized for downstairs and goes along with what you are saying. i'm not positive on this, but there may be a low speed setting on the blower. another guess is that to do it RIGHT, they would have had to replace the original and add two new ones, but leaving the original was a lot cheaper.

Last edited by NitroNate; 12-19-2013 at 05:53 PM.
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Old 12-19-2013, 06:22 PM   #4
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Plenum Separation Issue


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Old 12-19-2013, 06:26 PM   #5
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Plenum Separation Issue


Wrap a piece of tape around the plenum. That should keep it from separating. not many supplies for a large furnace. Setting to a lower blower speed will help. be careful not to set it too low though.
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Old 12-19-2013, 06:40 PM   #6
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Plenum Separation Issue


ok thanks, i will check on the blower speed.
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Old 12-20-2013, 06:08 AM   #7
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Plenum Separation Issue


Find the wiring diagram and or the manual for the A/H and look at the motor location and see if there are different speed taps on the motor.

Most of them are very easy it's moving a motor wire from on connector to another and it's usually color coded for the different speeds.

When you re tape the duct board make sure you wipe off all dust and dirt so the new tape sticks good, run the tape in a couple directions if needed.

One more thing don't run the fan with the diffusers closed it puts more pressure on the takeoffs and plenum.

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