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What causes moisture inside hvac ductwork? - it's visible on the inside of the ducts when we remove vent covers

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  • what is your humidity in the home
    – DIY75
    Commented Sep 17 at 19:39

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Is it one vent or only one side of the home or all the ducts? The location could direct you to what is causing this.

Air is being drawn into the system from a high humidity area. The moisture in the air should be drawn out by the evaporator. Then the moisture expelled through the drain. The resulting cold air should be dryer. If there are droplets in the AC ducts, there is too much humid air getting into the air handler.

The first place to look is the air return plenum at the air handler. Then each return branch if there are any. Look for openings, cracks, tape that is loose, parts coming apart. If returns are in the attic look for openings there, or even gaps chewed by rodents.

Then look to see if there is a source of moisture close to the air return such as a bathroom that is used often for showers without an exhaust fan being used. ( I actually had a shower wall that was leaking water behind tiles and that moisture was being sucked into the air handler on the other side of the wall.)

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  • scary , hidden leak behind the tiles
    – DIY75
    Commented Sep 17 at 22:47
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The moisture becomes visible with cold air been converted to drops.

That is what happens in air ducts.

It all depends where you live. In Florida that is possible, in Arizona you have no such problem.

Your indoor humidity is high. Investigate sources for that. Beside the outdoor humidity where you can not do much.

The indoor humidity sources could be the shower. If the mirror in the bathroom fogs, the exhaust is not sufficient or not working, In the kitchen the cooking produces a lot of humidity, and the exhaust is not working. Opening the dishwasher before the dry cycle is completed will release a lot of humidity.

Investigate the drip pan under the AC coils, is it full ?

You might be a candidate for indoor dehumidifier to help the AC manage the humidity.

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    Sorry, but I live in Fla. Have for almost 40 years. Never had humidity condensing in the ducts in any normal situation. It doesn't just happen because Florida is humid.
    – RMDman
    Commented Sep 17 at 22:32

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