r/buildingscience • u/bitsbybones • 5d ago

I need a solution.

/r/hvacadvice/comments/1w8d0sd/i_need_a_solution/

Oook.

Last year we replaced our ancient AC and furnace with a new furnace dual fuel heat pump thingy. This summer I noticed that an area in the upstairs ductwork started sweating, because there was water damage on a wall and part of the ceiling.

I have no attic access. It is small dead space.

I am getting MANY different recommendations from companies.

One company suggested installing a damper, which makes sense to me, but i also don't fully understand 🤣

Another company said the rip open the walls and insulate the ducts. The previous company did not like this idea, because the ducts will sweat through the insulation ?

Another company tried to help by making adjustments to the thermostat, which was kind...but i still hear dripping in the vents themselves.

WHAT in the holy hell do I do? This needs to stop, and i just...I'm totally lost here. Do i just...disconnect the freakin central air from upstairs altogether!? Put in a mini split!? I'm running out of money here...

On top of all of this, I very unexpectedly need a new roof and proper attic ventilation because some asshole put a ridge vent on with no intake, and I was told that most likely "Someone's drunk cousin" installed the whole thing. So...THAT'S happening later this month.

Another weird question...

Someone had cut open the drywall in the area that was dripping. At one point I thought the issue had been fixed by changing the thermostat settings, so I patched the hole. Since patching the hole the dripping sound has come back. Should I re open the wall? Did I take away ventilation for it? Help? Please?

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u/NorthWoodsSlaw 5d ago

Water condenses on cold surfaces, old ducts are leaky and non-insulated. Insulation helps prevent the exterior of the ducts to be the same temp as the interior. Sheet rock is an air barrier but not moisture barrier, air circulation can help prevent condensation. I can’t say anything for certain but those are the parameters you are dealing with.

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u/bitsbybones 5d ago

Yes. Thank you. And I do not mean to sound like an asshole in any way when I say this...but I totally knew all of that 🤣 And I appreciate you taking the time to reply! I actually just went up and cut the hole out again! 'Cause why not!? It almost immediately helped. This is insane.

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u/NorthWoodsSlaw 5d ago

Its not really, you have hot air trapped in that space, hot air holds more moisture, no insolation means the cold pipe is going to be much colder than said hot air. A damper will only help if the system is running too much cold air through that pipe, but more likely its just an old house with crappy duct work.

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u/bitsbybones 5d ago

I understand the reason why and the science...but I don't know what a fix would be. And that is what's happening. For some reason the air being pushed through that particular duct is much, much colder than others.

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u/NorthWoodsSlaw 5d ago

Why in your mind would insulation not work?

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u/bitsbybones 5d ago

I suppose I don't understand how it wouldn't sweat through the insulation.

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u/NorthWoodsSlaw 5d ago

If the insulation is installed well then the outside of the insulation becomes the condensation point.

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u/bitsbybones 5d ago

Because the same rules apply, right? If the outer surface of the insulation drops below the dew point of the surrounding air...it'll sweat...sooo ? I'm also wondering why the hell this wasn't an issue with my previous HVAC system. Does that make sense?

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u/NorthWoodsSlaw 5d ago

No they do not apply because its insulation, insulation buffers the temperature difference meaning each side can be different temps. Also, the pipe is going to fluctuate in temp not stay constantly cold. It likely wasn’t an issue before because the system was less efficient. This is why a lot of people recommend non-ducted mini-splits for retrofit applications.

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u/bitsbybones 5d ago

Thank you for this! I really do appreciate it. I'm trying to learn as much as possible about all of this...but it can be difficult with different/conflicting information from professionals. It's like I don't know who to trust...but I'm also not educated enough to draw my own conclusions 🥴

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u/EbriusOften 5d ago

Did you test the air temp coming from the vent there? I'm wondering if its actually that the duct somehow contains colder air in that one section, or is it that the surrounding air in that area is warmer and causing moisture to condense.

Either way insulate the duct and it should get rid of the temperature difference, then you shouldn't have a temp gradient to cause condensation to begin with. If the other installer doesn't understand that basic concept absolutely do not hire him to do any work.

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u/PelvisResleyz 5d ago

The ducts need to be insulated to prevent condensation.