Condensation on duct work.

[Todd.Brock]

Hey all,



I am getting condensation on one run of ductwork. For reference it's a typical suburban home split system manufactured in 1988. One run serving the first floor is getting condensation. And then there is one run that appears to be heading upstairs to serve the up stairs that has condensation. All the other ducts in the basement appear to be dry. I have a dehumidifier about five or 6 feet from the duct that is condensing. The humidistat reads 49% humidity and 70°F. Any thoughts on what could be causing this? The duct is not blocked. It is open as well. How should I proceed?

 

[bonneyman]

Ducts need to be insulated if they run in an attic or unoccupied space.

 

[Todd.Brock]

I guess that would be important info. It's in the basement.

 

[Jamie V]

It's pretty basic info. You have air in the basement that has a certain amount of moisture in it (49% humidity) then the duct is at a certain temp. You need to either insulate that duct or condition the space so the humidity % goes down.

 

[Todd.Brock]

Doesn't it seem odd that one duct is sweatin like a hog and the trunk line it's attached to and everywhere else is not? I can't say I ever remember it sweating. This is the second summer being here. But I opened a vent on the trunk line and moved the dehumidifier on that side. It still said it was 51% percent humidity in the air.

 

[SketchyBikes]

You're reaching the dew point of the air on the duct. If you look at your conditions on a psychometric chart you can understand what it happening. Google it for more details. You can plot your conditions and know.

 

[SketchyBikes]

also is that duct plugged up? No where for the cold air to go.

 

[Todd.Brock]

I messed around with the damper and it fell off. There is condensation all the way to the register. I will have to see if it's clogged. I couldn't imagine WTH would be in there. I will say that the duct is COLD compared to the other ducts.

 

[pseudorealityx]

Yeah... it's just like a cold glass of water. If the temperature of the metal falls below the dew point, the water in the air condenses on the cold surface.

Dehumidifying the space can help... but at ~50% RH, there's not a lot of range left to pull it down much further.

Insulating the duct is the better solution.

 

[Highbeam]

The duct is being cooled by a flow of cooled air on the inside. If you want to warm up that duct to make it stop sweating then limit the flow of cold air inside the duct.

Close the damper partially or fully to divert more cold air to the other registers.

Yes, this doesn't really solve the root problem but it keeps that duct from sweating.

 

[Basskiller]

I would get some mastic and seal up the open seams. Then wrap with insulation.

 

[S4cruiser]

We recently purchased a ranch style 1960s house and it had a crawlspace moisture problem...it was the uninsulated galvanized duct work condensating and pooling on top of the moisture barrier.

Easy fix - ripped all that **** out, replaced insulation, redid all the original duct work with insulated runs and replaced the moisture barrier.

No more moisture problems! Wasn't cheap though...

 

[magicrat]

Wow i do hvac and never seen that b4....its like it is not even insulated on the inside....The ez option is to insulate the outside or air condition that space as well. If ambitious you could open ductwork and use thicker insulation