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Old 07-31-2011, 03:35 PM   #1
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Supply duct sweating

I notice last night the supply duct to the kitchen (comes off the main trunk about 5 feet from the register) is sweating and drops of water are falling on the basement floor. This is in what I call the mechanical room which is about 10 by 10 and unfinished. The furnace, water heater, sump pit and ejector pit are in this room. This is the first time I have noticed it. I run a dehumidifier in the basement and humidity is around 50 to 60%.
Can I replace the metal ducting with insulated flex duct and fix this? My concern is the rest of the basement metal ducts which are above the finished drywall. I wouldn't want those sweating.

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Old 07-31-2011, 03:44 PM   #2
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Supply duct sweating

Dewpoint is what is causing this. Happens when cold air meets warmer air. Look at that can of soda or beer, or glass of iced tea. Same concept.

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Old 07-31-2011, 04:09 PM   #3
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Supply duct sweating

just buy some duct insulation for the metal duct. they make it in all the standard duct sizes. its like flex duct with the flex duct inside. they also make a wrap around. any good plumbing and heating supplier should have what you need.


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Old 07-31-2011, 05:01 PM   #4
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Supply duct sweating

insulating the duct will help quite a bit.
You should also look for air leaks in the ductwork.
Insulate the house baseplate area
Since you already have a dehumidifier you might also need to increase fan speed.
I wonder if you have moisture seeping in through the basement floor and/or walls?
I'm also wondering if your ductwork system is too small which might be slowing the air movement causing the duct to get so cold that it causes condensation with only 50-60% RH.

Last edited by REP; 07-31-2011 at 05:05 PM.
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Old 07-31-2011, 05:30 PM   #5
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Supply duct sweating

The duct insulation makes sense, thanks. The basement stays dry. I have been finishing it and drywall dust and the like remain a powder. So much so I won't sweep it (it becomes airborne if I do). I vac it up each time there is some. Also I just tiled the bathroom down there and the grout dried pretty much over night.

Oh and the duct work joints are all taped.
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