A handful of the rooms in our house have mold on the A/C vents/registers. A couple of them are actually pretty caked with it.

How big of an issue do I have on my hands? Full duct replacement in my future?

Depends. Pull the vents and see if you see any accumulated in the duct. Could just be moisture accumulating in the vent itself.

I just dealt with this.

Without seeing yours here is my best guess. It is most likely just around the registers due to attic air touching the cool air. Take off the registers and notice the gap between the metal and the drywall. That gap is creating the environment for mold. In my case, there was up to a 2-inch gap where I could see straight into the attic.

Spray a mold cleaner into any affected areas and on the registers to clean them. If using something with bleach be careful of what is underneath you.

There are a number of ways to seal the gaps. I used Great Stuff expanding foam with a gun and it was pretty simple. A little messy but the gun helps. When it dried I trimmed it down and reinstalled the registers.

I have also seen people use aluminum tape to do this which may be less messy.

May also add a foam window seal strip around the outside of the inner part of the registers so ceiling air doesn't seep into the vents easily.

Hopefully, this solved our issue...or I'll see ya back here in a year.

-If duct work is old, Replacing it is like making the whole home healthier again. ( but def find why there's mold 1st)

Also an air purifier to help eat what up whatever it is may be a good bet. We try to install Reme Halo air purifiers every time we see anything not clean in ducts/ vents.

Lobster Twins said:

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Without seeing yours here is my best guess. It is most likely just around the registers due to attic air touching the cool air. Take off the registers and notice the gap between the metal and the drywall. That gap is creating the environment for mold. In my case, there was up to a 2-inch gap where I could see straight into the attic.

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Says the guy who just dealt with this, who is probably right. Mold comes from moisture. Moisture comes from condensation. Condensation comes from warm air hitting something cold, like a glass of iced tea sweating. As with anything, start simple and start cheap, then go from there.

EcoZapp.Makes.Crisp.Air said:

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-If duct work is old, Replacing it is like making the whole home healthier again.

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Says the guy who sells HVAC equipment.

often it is not mold but simply dirt and fuzz that gets caught up in the little air foils as the air exits the register. Add a bit of moisture that may or may not exist from the coils and you can have a bit of a mess that is not mold. A brush, soap and water as necessary and some Killz in a spray can typically will take care of the problem.

FWIW I have a couple of ceiling fans that are near supply registers and the fuzz foils got into the motor housing and slowed/stopped the fans from working and required vacuuming and cleaning. .

UnderoosAg said:

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EcoZapp.Makes.Crisp.Air said:

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-If duct work is old, Replacing it is like making the whole home healthier again.

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Says the guy who sells HVAC equipment.

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Yes, because people breath that air inside of homes

We scuba in my house.