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Thought i'd start a new question to see if I can get any new ideas. Overview:

E. PA location. 2 year old new construction home, approx 1200 sq ft, open concept first floor (so all supply and returns run through exterior walls). Basement is unfinished and unconditioned. We have a heat pump and central ac (2 ton). The basement is dry.

Last summer we noticed one specific supply duct in the basement at the front of our house condensating at the corners of the wall where it goes up to the main floor of the house, when it's hot and humid outside. (This duct runs up and bends and goes through the floor in both front bedrooms to floor registers, 2nd floor). All of our other ductwork in the basement is dry and has never done this. The dripping isn't constant, it seems to come and go.

Things we have tried so far: -We run a dehumidifier in the basement, set to 45%. -We change filters seasonally and they are never really that dirty. -I got a thermometer w/a humidity sensor and the temp is steady at 67 with 44% humidity in the basement. -I have tried adjusting the dampers with no change. -We had our blower fan speed checked and it was on low (we moved it up to high, still dripping, although it's not any better or worse than before). -We've looked in the vent at the top of the duct that drips the most and the duct is clean with no signs of water inside (felt dry, no signs of dust pooling where water may have been, no white chalky stuff, etc). -Condensation seems to get worse when we run the clothes dryer in the basement, which is a new discovery last night. -We have tried adjusting the thermostat anywhere between 71-74 and it drips regardless. -We have checked the vapor barrier by cutting the living room wall open where the duct is (on the worse side). There was insulation behind the duct facing the correct way but we stuffed it full with additional insulation just in case). There was no sign of moisture anywhere, either on the duct or on the back of the drywall.

I had to live w/a torn up living room for months until I was able to get it respackled, repainted, etc so I am hoping for the least invasive problem and repair, if possible. Do any of these things sound like they could be it or fix the issue? -conditioning the basement air (we want to finish the basement anyway, which why this issue is a current priority). -wrapping the duct (how do we know this isn't just a bandaid and will seal condensation inside?) -Could our hvac unit be oversized? -Possible issue with our ac unit? (It is still under warranty)

Any other thoughts or opinions are appreciated.

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The condensation will form on the outside of the duct the same as it forms on a glass of ice water.

The dryer must be allowing some of the moist air to escape into the basement that’s why the problem gets worse as you just noticed.

Even with your home at a 76 set point when the AC is running I would expect a 20 degree temp drop from the evaporator coil so the duct may be 46 or even colder causing the condensation on a duct that was not insulated or sealed from the humid air outside or possibly even in the basement.

Sealing the area that the duct travels inside the walls at the top and bottom is usually enough to prevent more than a minor amount of moisture condensation.

Since you are still having issues foam sealing or wrapping the duct is enough. I prefer foam insulation as it is easier to spray than trying to wrap a duct in an open wall. Reducing the amount of metal that is exposed to humid air.

So wrapping the duct really works but can be quite difficult, not really a bandaid a cure that is not always needed but if air can travel in the joist bays between floors a necessity.

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Adding to @ Ed Beal's advice, I would add at least 1 cold air return located at the floor level to help remove or lessen any stratified air in the basement. Also, check near the duct that has condensation for any outside air infiltration which could cause the condensation. Check the temperature across the A/C coil. A temperature drop of more than 20 degrees may indicate an air flow or system problem. Lower the dehumidifier setting to 35%. The fact that the problems gets worse when the dryer runs shows that there is a problem with outside air infiltrating the basement of that the dryer's discharge vent is not correct.

@Alwayscold, Add a return grill connected to the cold air return at or near the basement floor.

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  • This was my first thought. Given it is only one duct doing this, there must be a source of warm moist air near that duct. First thing I'd look to is outside air entering the basement or the wall near the duct. Dehumidifier is not solving the problem, it is just masking it, making it harder to solve. Clothes dryer could be the issue, if its vent is very near the affected duct, but I'd suspect outside air first, some crack or gap in the outside wall near the affected duct.
    – jay613
    Jun 28 at 12:42
  • The dryer is at the other end of the basement, furthest from the drippy duct. @d.george, did you mean add one cold air supply vent or a return in the basement or first floor? Another question actually now that i'm thinking about it, in an old house we did not have any returns in the entire house and we ended up putting a louvered door at the top of our basement steps to help with circulation. We do have a floor return right by our basement door in the living room (it's the only return on the entire first floor). Maybe this isn't enough? Should we keep the basement door open?
    – AlwaysCold
    Jun 28 at 14:56

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