Why Is My Air Conditioner Vent Leaking Water? 5 Causes and How to Stop the Drip
Water dripping from an AC supply register — the vent in the ceiling, wall, or floor where cooled air enters the room — has one of three sources: condensate from the air handler is backing up into the ductwork, warm humid air is condensing on the cold metal of the duct or the register itself, or water from outside the duct system is entering through a leak. The source of the water determines whether the fix is a 10-minute DIY job, a technician repair, or a roof repair that has nothing to do with the AC.
The fastest diagnostic question is: does the water drip only when the AC is running, or does it also drip when the system is off? Water that drips only during AC operation is condensate — either from the air handler or from the duct itself. Water that drips regardless of whether the AC is running — during a rainstorm, or overnight when the system has been off — is a roof leak, a plumbing leak, or rainwater entering the duct through a disconnected joint. The timing distinguishes an HVAC problem from a building envelope problem.
1. Clogged Condensate Drain: Water Backs Up into the Ducts
The most common cause of water dripping from AC vents is a clogged condensate drain at the air handler. The evaporator coil produces condensate continuously during cooling. That water drips into a drain pan and flows through a PVC drain line to a floor drain or outside. When the drain clogs, water backs up in the pan. If the air handler is installed horizontally — common in attics — the overflowing water runs into the ductwork connected to the supply plenum. The water follows the ducts downhill to the lowest register, where it drips through the grille into the room.
A vent dripping water in a room that is downhill from the air handler — a register on the first floor when the air handler is in the attic, or a register at the low end of a long duct run — is a classic clogged-drain symptom. The water is not condensing at the vent. It traveled there from the air handler through the duct. Clear the condensate drain with a wet-dry vacuum from the outside end of the pipe. Pour a cup of warm water with a few drops of bleach into the drain pan at the air handler to kill algae. If the air handler has a secondary drain pan — a larger metal or plastic pan underneath the unit — check whether it has water in it. A wet secondary pan means the primary drain is clogged and the secondary drain is also clogged or not connected.
2. Frozen Evaporator Coil: Ice Melts into the Ducts
A frozen evaporator coil that thaws produces a large volume of water in a short period. The drain pan overflows, and water runs into the ductwork. The dripping appears at the registers shortly after the AC cycles off or is turned off — not during operation. The water may stop after an hour or two, once the ice has fully melted and the pan has drained. The ice will re-form when the AC runs again, and the cycle repeats.
The root cause of a frozen coil is restricted airflow (dirty filter, closed registers, failing blower motor) or low refrigerant. Replace the air filter. Open all supply registers. Verify that return grilles are unobstructed. If the coil refreezes after these corrections, the refrigerant charge is low. A refrigerant leak requires an EPA-certified technician to repair ($500 to $1,500).
3. Duct Condensation: Cold Duct, Warm Humid Attic
Ductwork running through an unconditioned attic carries 55°F to 60°F air through a space that can reach 130°F on a summer afternoon. If the duct insulation is damaged, compressed, or missing — or if the duct itself is uninsulated metal — the cold duct surface chills the warm, humid attic air on contact, and water condenses on the outside of the duct. The condensation drips onto the ceiling drywall, and eventually water stains appear around the register or water drips through the register opening.
This is the most common cause of water around ceiling registers in attics. The water is not coming from inside the duct. It is condensing on the outside of the duct and dripping onto the back of the ceiling drywall. Inspect the ductwork in the attic. Look for sections where the insulation wrap has come loose, been compressed by stored boxes, or was chewed by rodents. Re-wrap exposed duct sections with R-6 or R-8 duct insulation. Seal the insulation seams with foil tape. A duct that is sweating — covered in condensation droplets — needs insulation. A duct that is properly insulated should have a dry outer surface even on the hottest attic day.
4. Register Condensation: Cold Air Meets Humid Room Air
A supply register that sweats — water droplets forming on the metal grille itself — is experiencing condensation from the room air, not a leak from the duct. The register is cold because 55°F air is passing through it. The room air is warm and humid. The moisture in the room air condenses on the cold metal grille, just like water forms on a cold glass of iced tea. Register sweating is worst in humid climates, on the first floor or basement where the humidity is highest, and on registers that are closest to the air handler (where the air is coldest).
Register condensation is not a mechanical problem. It is a humidity problem. The fix is to reduce the indoor humidity — a whole-house dehumidifier, a portable dehumidifier in the affected room, or running the AC blower continuously (FAN ON instead of AUTO) to keep air moving across the register and prevent moisture from settling. A plastic register grille sweats less than a metal one because plastic is a better thermal insulator. Replacing a metal register with a plastic one costs $10 to $20 and often solves the problem.
5. Roof Leak or Rainwater Entering the Duct
Water dripping from a ceiling register regardless of whether the AC is running — during a rainstorm, or overnight when the system has been off for hours — is not an AC problem. It is a roof leak, a disconnected duct boot, or rainwater entering the duct through the outdoor condenser unit’s duct connection (for package units installed on the roof). The water is following the path of least resistance, which happens to be the duct opening in the ceiling.
Go into the attic during a rainstorm with a flashlight. Look for water running down a rafter, dripping from the underside of the roof deck, or pooling on the ceiling drywall. A roof leak that drips through a register boot — the metal box that connects the duct to the ceiling grille — is a roofing problem, not an HVAC problem. Call a roofer. A disconnected duct boot — the metal box has separated from the ceiling drywall, and rainwater is running into the gap — is an HVAC installation problem. Reconnect the boot and seal it to the drywall with mastic and screws.
FAQ: Common Questions About AC Vent Water Leaks
Only one vent in the house is dripping water. Why just that one?
That vent is the lowest point in the duct system downstream of the air handler. Water from a clogged drain, a frozen coil, or duct condensation flows downhill through the ducts and exits at the lowest register. The other vents are dry because the water has not reached them. The problem is at the air handler or in the ductwork upstream of the dripping vent. The dripping vent is the messenger. The air handler is the problem.
Why do my bathroom AC vents drip more than the others?
Bathrooms have the highest humidity in the house. The cold register grille in a bathroom condenses more moisture from the air than a register in a bedroom with lower humidity. The dripping is register condensation, not a duct leak. Run the bathroom exhaust fan during and after showers to reduce humidity. If the dripping continues even when the bathroom has not been used, the problem is likely duct condensation in the attic above the bathroom, not register condensation.
Water at the Vent Is a Symptom. The Source Is Upstream.
Water dripping from an AC vent has traveled from somewhere else. If it drips only when the AC runs, the source is condensate — from a clogged drain, a frozen coil, duct condensation, or register condensation. If it drips regardless of the AC, the source is a roof leak or rainwater. The timing identifies the category. The location of the dripping vent relative to the air handler identifies the path the water took to get there.
Clear the condensate drain. Replace the filter. Insulate bare ducts in the attic. Replace a sweating metal register with plastic. If those four fixes do not stop the drip, the water is coming from above the ductwork — a roof leak, a plumbing leak, or a disconnected duct boot — and the fix is in the attic, not at the vent.
References: U.S. Department of Energy, Common Air Conditioner Problems, energy.gov/energysaver/common-air-conditioner-problems.