mildew around AC vents

What causes the black mildew on air conditioning vents and on the adjacent ceiling? Why do only some of the vents in my newly-built house get this while others don't?
What's the best way to get rid of the mildew and to prevent it from recurring?
thanks
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Newly built ? Lack of insulation for one reason. What does the builder say
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flyer wrote:

If it is a newly built home, I suggest you go back to the builder and have them address the issue.
Mold will grow just about anywhere it is damp. A/C dries the air, but it can cause condensation on the vent pipes is they pass though an area that is moist enough.
The problem with trying to solve your problem from here is we have no way of seeing exactly what is going on. Any good tech should be able to walk in and in a minute to two identify the cause and offer a solution.
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Use your bathroom mildew removers or bleach solution gently so you don't take off the paint. Then use some caulk to make your vents air tight around the edges.
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flyer wrote:

Cold air hitting warm surface. Direct the vent away from the ceiling. Clean the mildewed surface with your favorite cleaner with a bit of bleach (assuming it is painted or vinyl coated wallpaper).
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Most of the time it is from the vent being improperly insulated. In other words, there is bare metal exposed to excessive heat in the attic space. When cold air hits the hot metal, condensation is the result. The area around the vent(s) needs to be insulated. If you are still "under warranty" with the builder, I would contact hi, and explain your problem.
I wouldn't clean it until the builder sees it. After that, and, if it's not too bad, a simple bathroom cleaner (mildew remover) can be used.
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