Quick answer
Water dripping from an AC duct or vent usually comes from one of three places: condensation forming on the outside of cold ducts (often called duct sweating), water backing up from a clogged condensate drain line, or a frozen evaporator coil that has melted. The first step is to turn the AC off. Then work out whether the water is on the outside of the duct or coming out through the vent opening, because that points to different causes.
First steps when a vent or duct is dripping
- Turn the AC off at the thermostat so it stops producing more water.
- Place a bucket or towels under the drip. Do not touch light fixtures or outlets that are wet.
- If water is coming through a ceiling, stay clear of any sagging or bulging area. Waterlogged drywall can collapse, and water near wiring is a shock risk. If you see a bulge, dripping from a light fixture or a wet electrical panel, switch off power only if you can do so safely and dry, and call a qualified professional.
- Check the air filter and replace it if dirty.
- Look for ice on the indoor coil or refrigerant line if you can reach the air handler.
Work out where the water is coming from
- Water beads on the outside of the duct or trunk line, usually in an attic or crawl space: likely condensation.
- Water comes out of the vent opening or from the register: likely water inside the duct, from a drain overflow, a frozen coil that melted, or condensation running along a duct.
- Water pools at the air handler or furnace: likely a clogged drain line, full pan or failed pump.
- Drips from a bathroom or kitchen vent: may be a separate problem with moist air in an exhaust duct rather than the AC itself.
Common causes
Duct sweating (condensation)
Cold air inside a duct can chill the duct surface. If the air around the duct is warm and humid, as in many attics and crawl spaces, moisture condenses on the outside, much like a cold glass on a summer day. This can add up to enough water to stain ceilings, soak insulation and encourage mold. Uninsulated or poorly insulated ducts, damaged insulation, and high humidity in the space make it more likely.
Clogged condensate drain line
When the drain line clogs, water backs up in the pan and may overflow into the air handler or ducts. Signs of a clogged drain line include standing water in the pan, a musty odor, water around the indoor unit, and a system that shuts off if it has a safety float switch. See our reasons your AC is leaking water for more on drain lines and pans.
Dirty filter and frozen coil
A dirty filter or other airflow problem can make the evaporator coil freeze. When it thaws, more water arrives than the pan can handle. Low refrigerant can also cause freezing; that needs a licensed technician.
Leaky or uninsulated ductwork
Gaps or poor insulation in ducts can allow warm air to meet cold surfaces. Leaks also waste cooled air. Small, accessible leaks may be sealed with appropriate duct sealant or foil tape, but extensive or hard-to-reach leaks should be looked at by a professional.
High indoor humidity or poor ventilation
Very humid homes and spaces can produce condensation around metal vents and ducts even when the AC is working properly.
Signs of a clogged condensate drain line
- Water pooled around or under the indoor unit
- Standing water in the drain pan
- Musty or moldy smell near the unit
- Water stains on nearby ceilings or walls (once the leak is stopped, see how to fix a ceiling after a leak)
- The AC shutting off unexpectedly, if the unit has an overflow switch
- Visible slime or algae at the drain opening
Fixes you can try vs. call a professional
Reasonable homeowner steps
- Replace a dirty filter.
- Let ice thaw fully with the cooling off.
- Dry the area and remove wet insulation that has become soaked.
- Wrap accessible ducts with insulation designed for HVAC ductwork, following the manufacturer’s directions and vapor barrier guidance.
- Clear a minor clog at the outdoor end of the drain line with a wet/dry vacuum if you are confident about how it is connected.
Call a professional if
- The ceiling is wet, sagging or stained.
- You see mold.
- Dripping continues after the filter change.
- The coil refreezes.
- You suspect refrigerant issues.
- Ducts are in an attic you cannot access safely or are heavily damaged.
Attics are hot, have limited footing and can have exposed wiring and insulation. Do not step between joists onto drywall, and do not go up there if you are unsure. Older insulation and materials in some homes may contain asbestos; if you see old, damaged or crumbling insulation or wrap on ducts in an older home, do not disturb it, and have it tested by a qualified professional first.
Preventing duct condensation
- Insulate ducts running through unconditioned spaces.
- Seal duct joints, which also helps energy efficiency.
- Control humidity in attics and crawl spaces. Crawl spaces may need professional moisture control such as vapor barriers or dehumidification.
- Change filters regularly and keep vents clear.
- Schedule annual maintenance so drain lines and coils are cleaned.
FAQ
What should I do when an AC vent is dripping water?
Turn off the AC, catch the water, check the filter and look for ice or a full pan. If the source is unclear or the ceiling is damaged, call an HVAC professional.
Is water dripping from AC harmful?
The water itself is usually condensation, but it can cause ceiling and insulation damage, encourage mold and create electrical hazards if it reaches wiring. For more detail, read whether an AC leaking water is dangerous.
How much duct leakage is acceptable?
There is no single number that applies to every home. Building codes and testing standards set limits that vary by location and by system. A technician can perform a duct leakage test and tell you what applies locally.