AC vents sweating or dripping water

Why AC vents sweat

Air holds more water vapor when it's warm. When humid air touches something colder than its dew point, the vapor turns into water droplets. An AC register can be well below the room's dew point, because the air coming out is often in the 50s Fahrenheit. In a dry house that's fine. But if the room air is humid, or if air from a hot, humid attic is reaching the back of the register, droplets form on the grille and the metal duct boot behind it, then drip.

Three conditions make it worse. High indoor humidity, from showers, cooking, a fan left on, an oversized AC that cycles too briefly, or a humid climate, gives the air more water to drop. Low airflow, from a dirty filter, closed or blocked vents or a weak blower, makes the supply air colder than it should be. And gaps between the duct boot and the ceiling drywall, or missing or wet insulation on ducts in the attic, let hot attic air touch cold metal. Fixing the cause, not just wiping the vent, keeps water out of the drywall and insulation.

Why the vents drip

Let's fix it

1. Check the filter and open every ventFind the air filter at the indoor unit or behind a return grille, slide it out and look at it. If it's gray or you can't see light through it, replace it with the same size printed on its edge; see how to change an HVAC air filter. Then walk the house and open every supply register, and move furniture, rugs and curtains off supply and return vents. Closing vents in unused rooms raises pressure in the ducts and can make the remaining vents colder and wetter. Good airflow keeps the supply air from getting too cold.
   Done

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2. Set the fan to AutoLook at the thermostat's fan setting. If it says On, switch it to Auto. With the fan on all the time, water sitting on the indoor coil after a cooling cycle evaporates back into the air, which raises indoor humidity. On Auto, the fan stops with the compressor and the coil drains. Also keep the cooling setpoint reasonable; very low settings run longer and can make supply air colder. If your system has a dehumidify or humidity setting, turn it on as the manual shows.
   Done

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3. Lower the humidity in the houseA small indoor hygrometer, usually under $15, shows the humidity. The EPA suggests keeping indoor humidity below 60%, ideally around 30% to 50%. If it's higher, run bathroom fans during and about 20 minutes after showers, use the range hood while cooking, and keep windows and doors closed while the AC runs. A dehumidifier helps in a damp basement or humid climate. Dry laundry in a vented dryer, not indoors. If the house stays humid even though the AC runs, the system may be oversized or need adjustment, which an HVAC tech can check.
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4. Dry and clean the vent and ceilingTurn the AC off at the thermostat. On a ladder, remove the register: most ceiling grilles come off by removing two or four screws, while some floor and wall registers lift out. Dry the grille and the metal boot behind it with a towel. Look at the drywall around the opening. Wipe small mold spots with water and a little detergent, and dry the area completely. If the drywall is soft, crumbles or sags, or a stain keeps growing, water is getting into the ceiling and a pro should look at it. A stain-blocking primer covers a dry stain later; see the guide for a water stain on the ceiling.
   Done

   Something went wrong?

   • The screws spin without coming out They may be in a stripped hole or a loose clip. Hold the grille steady while you turn, and replace with slightly longer screws when you reinstall.

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5. Seal the gap around the duct bootWith the register off, look at where the metal boot meets the ceiling drywall. There's often a gap of 1/4 inch or more, and you may feel warm air coming through it when the AC is off. Seal it from the room side with duct mastic or a paintable caulk or sealant run all the way around the joint between the boot and the drywall, and let it cure. Foil HVAC tape (UL 181 rated) works on the metal boot seams, but it often won't stay stuck to drywall paper. Seal any open seams on the boot itself too. Reinstall the register, snug but not cranked down.
   Done

   Something went wrong?

   • The gap is too wide for tape Stuff a little foam backer rod into it first, then cover with tape or mastic. For a large gap, the boot may need to be refastened from the attic.

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6. Check the attic duct and insulationIf a vent still drips after sealing, the duct above it may be sweating. From the attic, stepping only on joists or boards, find the duct that feeds that register. Flexible ducts should have an intact outer jacket over their insulation, and metal ducts in an attic should be wrapped in duct insulation with a vapor barrier. Look for torn jackets, crushed or sagging ducts, wet insulation or a loose connection at the boot. Small tears can be taped with foil HVAC tape. Replacing or re-insulating ducts, or reconnecting a boot, is a job for an HVAC contractor, especially in a hot attic.
   Done

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Watch it done

How to fix + seal Wet and leaky ac vents. Prevent mold and mildew with this simple repairBy Jeff & Jeff Home and Garden on YouTube. Their method may vary a little from ours.

Watch on YouTube

If it comes back

If vents sweat again every summer, the house humidity is likely the main cause. A dehumidifier or a dehumidify setting on the system often solves it. See the guide for a dehumidifier that isn't collecting water if yours isn't doing its job.

If water shows up near the indoor unit or furnace instead, the condensate drain is the usual cause. See the guide for a clogged AC condensate drain. Ice on the lines is a different problem; see the guide for a frozen AC.

If a ceiling stain stays after the vent is dry, seal and paint it. See the guide for a water stain on the ceiling.