Why Tampa AC Vents Sweat and Turn Black

What is happening

A sweating register is condensation. Air leaving the supply is cold, often in the mid-50s. When humid air touches the metal grille or the duct boot behind it, it reaches its dew point and water forms, the same way it does on a glass of iced tea on a lanai. During Tampa's rainy season, which the National Weather Service dates from about late May to early October, dew points sit above 70 degrees, and it takes very little to cross that line.

Where the humid air comes from

• The attic, through gaps where the duct boot meets the ceiling drywall
• The room itself, when the house is under negative pressure from leaky return ducts and pulls in outside air
• A door or window left open with the AC running
• An oversized system that cools fast and shuts off before it has removed much moisture
• Supply air set very cold, with the fan on high or a thermostat set to the low 70s or below

Why the ring turns black

Dust is always moving past a register. Where the surface is damp, dust sticks and stays, and that damp dust is what mold grows on. The dark ring on the ceiling around a supply vent is usually a mix of the two. Wiping it off does nothing about the moisture, so it comes back.

What you can check yourself

The boot seal

Take the grille off (two screws, usually). If you can see attic insulation or daylight around the edge of the duct boot, that gap is pulling attic air in. Sealing it with mastic or foil tape is a common fix.

The drain line

A clogged condensate line can raise humidity and flood the pan. If the air handler is in the garage, look for water around it or a float switch that has tripped.

The thermostat

Running "fan on" rather than "auto" in summer re-evaporates moisture from the coil back into the house. Switching to auto often helps.

When to call someone

If more than one vent is sweating, if water is dripping onto floors, or if the black growth comes back within weeks, the cause is in the system or the attic, and a duct company or AC contractor needs to look. A cleaning will remove the growth from the grilles and the first few feet of duct. It will not stop it returning until the water is dealt with.