AC leaking water: how to find the cause in 15 minutes

Where the water tells you what is wrong

An air conditioner that is working correctly produces a lot of water. The refrigerant coil inside the air handler runs at about 40 degrees Fahrenheit, which is below the dew point of any humid indoor air, so moisture condenses on it the same way it condenses on a cold glass of iced tea. That condensate drips off the coil into a drain pan and runs out a one-inch PVC line to the outside. On a humid day in Houston or Tampa, a three-ton system makes 15 to 20 gallons a day. Even in Denver, you will see five to eight gallons.

That volume matters. When the drain stops draining, water accumulates fast. A leak you ignore at breakfast can be 10 gallons of soaked drywall by dinner.

The first diagnostic step is not a tool, it is a flashlight. Walk to the air handler. Look at four specific locations and note where the water actually is:

• Pooled on the floor directly under the air handler cabinet. The primary drain is blocked or the drain line is disconnected at a fitting. Roughly 70 percent of cases land here.
• Sitting in the secondary (emergency) drain pan beneath the unit, but the floor is dry. The primary drain backed up and the safety pan caught the overflow. The float switch should have shut the system down. If it did not, the switch is broken or missing, which is its own problem.
• Dripping from a ceiling vent or a ceiling seam below an attic unit. Either the secondary pan is full and overflowing, or condensation is forming on uninsulated supply ducts. These look the same and have different fixes.
• Coming out of the PVC fitting itself, not from the pan or the bottom of the cabinet. The drain line came apart at a glued joint, or the trap was never primed.

Note which one. The rest of the page maps each location to a cause.

The eight causes, by probability

1. Clogged condensate drain line

This single cause accounts for the majority of AC water-leak calls. The drain line is a one-inch PVC pipe running from the bottom of the indoor coil pan to the outside of the house, usually exiting near the condenser unit or above a flowerbed. The line has a P-trap somewhere along the run, and that trap stays wet during cooling season. Wet plus dark plus 60-degree water grows algae, biofilm, and slime. The slime catches dust, the dust catches more slime, and within two or three cooling seasons the trap is choked.

Once the trap closes, the pan fills. If a float switch is wired into the system, the AC shuts off and you find a dry floor with a full pan. If there is no float switch, water keeps coming until it goes over the side and onto whatever is below.

The fix is a shop vacuum. The mistake most homeowner videos make is vacuuming the cleanout at the indoor end. The trade method is to suck from the outside termination, the point where the line exits the house. Pulling toward the inside pushes the clog backward into the coil. Pulling from the outside pulls the clog out into the vacuum where it belongs.

1. Find the drain line outside. White PVC, about an inch in diameter, often near the condenser or above a foundation drip line. There may be a second line a few inches higher, which is the secondary drain. Use the lower one.
2. Wrap a rag around the shop vac hose to seal the connection to the pipe. A loose seal kills the suction.
3. Run the vacuum for 60 to 90 seconds. You will hear the tone change when the clog releases.
4. Pour a cup of distilled white vinegar into the cleanout at the indoor end, where the line meets the coil pan. Vinegar dissolves biofilm without attacking PVC or aluminum.

Do not use bleach. Chlorine bleach degrades PVC primer and softens the glue joints over years, and on backflow it can corrode aluminum evaporator coils. Some equipment manufacturers list bleach as a warranty void. Vinegar is the trade standard.

Cost: $0 if you own a wet/dry vacuum. $75 to $250 if a tech does the same job, which still represents the most common service call in this category.

2. Frozen coil that thawed

If the water shows up only intermittently, especially after the system has been off for a few hours, you may be looking at a thaw flood rather than a drain problem. A frozen evaporator coil holds the condensate as ice. When the compressor finally shuts off (either because you turned it off or because a safety tripped), the ice melts faster than the drain can carry it away, and a single thaw event can produce more water than the pan holds.

Frozen coils come from one of two causes: starved airflow (dirty filter, blocked return, undersized ducts), or low refrigerant charge. The diagnostic for which one is its own page. The AC freezing up walkthrough covers the ice-location decision tree, the gauge readings, and the cost of each fix.

The short version: pull the filter. A matted gray filter that no light passes through is your answer. Swap it, run the system for a day with the new one, and watch the pan. If the water comes back, the airflow restriction is somewhere past the filter (collapsed return, blocked supply, undersized ducts) or the refrigerant charge is low.

The trap for this cause is that homeowners clear the drain line, dry the floor, and consider the problem solved. The next thaw cycle floods again because the underlying ice formation never stopped.

3. Disconnected or improperly sloped drain line

The drain line should fall at least 1/8 of an inch per foot of run, with 1/4 inch per foot preferred. Anything less than that lets water sit in the line, where it grows the biofilm that clogs the trap. Worse, if the line ever runs uphill, condensate cannot drain at all and the pan fills the moment the AC runs.

Disconnections happen at glued joints. Heat cycling expands and contracts the PVC, joints that were not properly primed eventually pull apart, and the water comes out of the fitting itself rather than from the end of the line. Look at every joint between the coil pan and where the line exits the building. Water at a joint, not at the end, is your sign.

If you can see the disconnected joint, the fix is a PVC coupling, a few cents of primer, and a few cents of glue. Total parts under $5. A tech rerouting a poorly installed line with proper slope and a new trap costs $200 to $600.

For new installs or replacements, the condensate drain sizing calculator shows the minimum line diameter and trap depth for any system tonnage.

4. Cracked or rusted drain pan

The primary drain pan sits under the evaporator coil and catches condensate before it reaches the line. On systems 12 years and older, that pan corrodes. Pinhole rust in a galvanized pan or hairline cracks in plastic pans let water through, and the drip shows up at the bottom of the air handler cabinet even though the drain line is clear.

Pan replacement is not a quick repair. The coil sits on top of the pan, so the coil has to come out to get the pan out. Labor is the cost, the part itself is $40 to $120. Total job runs $200 to $600.

Pan rust on a 15-year-old air conditioner is also a strong signal that the rest of the system is aging. If the unit is past 12 years, run the math on whether replacement makes more sense than repair. The AC lifespan guide covers the tipping point.

5. Broken condensate pump

Some installations cannot drain by gravity. Basement air handlers, attic units with poor pitch, and closet-mounted systems often use a small electric pump to lift the condensate to a drain at a higher elevation. The pump has a reservoir, a float switch that triggers the motor when the reservoir fills, and a small-diameter discharge tube.

Pumps fail. The motor burns out, the float switch sticks, or the discharge line clogs. When that happens, the reservoir overflows and water shows up at the pump base. Listen for the pump cycling when the AC runs. Silence means the motor is dead. A constant hum without water moving means the discharge is blocked.

A replacement pump is $40 to $90 at a supply house. Total installed cost from a tech runs $100 to $450 depending on access and whether the discharge plumbing needs reworking. A homeowner with basic plumbing skill can swap one in 45 minutes.

6. Refrigerant undercharge causing the freeze

When a fresh filter does not stop the thaw floods and ice keeps reappearing, the most likely cause is refrigerant charge below spec. A pinhole in the lines, a corroded coil, or a leaking service port drops the suction pressure on the cold side of the system, which pulls the coil surface below 32 degrees Fahrenheit and freezes the condensate to it.

Refrigerant work requires an EPA Section 608 certified technician. Homeowner recharge kits sold at hardware stores cannot legally service modern systems, and on the newer A2L refrigerants (R-454B, R-32) they would not work mechanically anyway. A leak chase plus recharge runs $200 to $600 for an R-410A system with a small leak. The same job on an R-454B system runs $600 to $1,800 because the refrigerant itself costs more and leak detection is more involved.

The decision point: if your AC is more than 10 years old and the leak chase reveals coil corrosion or compressor seal failure, the math usually points at replacement rather than repair.

7. High humidity overwhelming a marginal drain

This one is rare, and it shows up in two specific situations. The first is a Gulf Coast home running 24/7 in 90 percent humidity, where the drain line is borderline-sized and the trap is borderline-clean. The drain works in normal weather, falls behind in extreme weather, and you see water in the secondary pan only during multi-day heat waves.

The second is a new tight-construction home with no mechanical ventilation. Cooking, showers, and breathing add moisture faster than the AC can remove it, and the drain runs continuously to keep up.

The fix is not a drain repair, it is a humidity strategy. A whole-house dehumidifier sized to the actual moisture load takes the burden off the AC and stops the overflow. Sizing math lives in the dehumidifier sizing calculator. Expect $1,200 to $2,500 installed for a whole-house unit, which is steep but appropriate for the cause.

8. Float switch that worked, or did not

The float switch is not a cause. It is a diagnostic divide. Most modern installations include one or two switches: a primary switch in the coil pan, and sometimes a secondary in the emergency drain pan. When water rises, the float lifts, and the switch breaks the 24-volt control circuit to the thermostat, which shuts the AC down.

If you have an active leak and the system is still running, one of three things is true. The switch was never installed. The switch was installed but failed. The switch was wired around by a previous tech (illegal but it happens). Any of those means a code violation if your unit sits in an attic, where the IMC requires a safety switch on the auxiliary pan.

A float switch retrofit is $30 to $100 for the part and $150 to $300 installed. If you have an attic unit and no switch, get one installed before the next storm. The cost of one is roughly 1 percent of the cost of replacing a ruined ceiling.

The diagnostic sequence a tech walks

If the four free checks (location, filter, switch, vacuum) do not solve it, the next steps need access to the coil cabinet and basic test equipment. The order is:

1. Power state and switch check. Is the system off because the float switch did its job, or off because someone turned it off? Trace the control wires from the thermostat to the float switch to the air handler board to confirm.
2. Drain line clear from the outside. Shop vac at the termination for 60 seconds, then vinegar at the indoor cleanout.
3. Pan integrity inspection. Mirror and flashlight under the coil. Look for rust streaks, water marks above the rim, or visible cracks.
4. Filter and coil check. Pull the filter. If clean, check the evaporator coil for ice. If ice is present, shut down, thaw four hours, and re-test airflow.
5. Static pressure across the coil. Magnehelic gauge across the coil. Anything over 0.5 inches water column total external means restricted airflow somewhere, which causes the freeze that causes the flood.
6. Refrigerant gauges. If airflow is fine and the coil keeps freezing, suction and discharge readings under a manifold gauge tell whether the charge is low. EPA 608 territory.

Sometimes it is the duct, not the drain

If water is dripping from a ceiling vent and there is no attic unit directly above, you may be looking at duct condensation, not a drain failure. Cold supply air running through ductwork in a hot attic causes condensation on the outside of the duct when the duct insulation has failed or was never installed properly. The water drips down the duct, follows the path of least resistance, and exits at the vent boot.

The signature is timing. Duct condensation runs continuously during AC operation, looks like clean water (not rusty), and stops when the AC turns off. Drain backup tends to be intermittent and may have a slight oily or musty smell.

The fix is duct insulation rather than drain work. Wrap any exposed supply duct in the attic with R-8 minimum vapor-barrier-faced insulation. On older homes with cloth-wrapped ducts, the wrap may need full replacement. Materials are $1 to $3 per linear foot, labor doubles that. A tech can replace 50 feet of attic duct insulation for $400 to $1,200 depending on access.

What the secondary pan and float switch are supposed to do

Code matters here. The International Mechanical Code section 307.2.3 requires an auxiliary drain pan beneath any cooling coil installed in a location where overflow would damage the building. That covers every attic install and most above-finished-ceiling installs. The pan must be at least 1.5 inches deep, at least 3 inches larger than the equipment footprint on each side, and made of corrosion-resistant material.

The companion requirement is the float switch, which breaks the control circuit when water rises in the auxiliary pan. Most jurisdictions adopt this from the IMC, the IRC, or both. If you have an attic AC and no float switch wired to the auxiliary pan, the install does not meet code. That is a safety call regardless of whether the original installer cut the corner or not.

The math on why this matters: a 3-ton AC producing 15 gallons of condensate per day, leaking onto a ceiling, will saturate the drywall in two to four hours. Replacement ceiling drywall plus paint runs $300 to $1,500. If the leak goes long enough for mold growth, remediation adds $500 to $3,000. A $30 float switch prevents all of it.

What each fix costs

Numbers below are real-world ranges at typical shop rates. The DIY column assumes you already own a wet/dry vac and a screwdriver, nothing exotic.

If the diagnostic points at refrigerant work or pan replacement on a unit past 10 years, the next decision is repair versus replace. The replace versus repair calculator compares lifetime cost. For replacement quotes, the free quote form sends the job to vetted local installers who size by load, not by the old nameplate.

When the leak keeps coming back

Clearing the drain and replacing the filter fixes the immediate event. If the same leak shows up again within a month, the actual cause is upstream:

• Same drain clog within weeks: the trap shape is wrong, the slope is wrong, or biofilm is colonizing faster than vinegar can keep up. A reroute with proper trap and slope solves it.
• Float switch keeps tripping even after a clean drain: the pan itself is cracked at the drain fitting and water is escaping the pipe before reaching the trap. Pan replacement.
• Coil refreezes after every filter swap: something past the filter is restricting return air, or the refrigerant is low. A manometer reading at the coil tells the first, a manifold gauge tells the second.
• Pump runs constantly: the system is producing more condensate than design called for, often because indoor humidity is unusually high. Dehumidifier rather than pump upgrade.

Common questions about an AC water leak

Is it safe to keep running my AC if water is leaking?

No, for two reasons. First, water pooling near the air handler eventually reaches the electrical control box, which is a shock and fire risk. Second, every hour you run a leaking system adds more water to whatever is already wet, which turns a salvageable drywall patch into a mold remediation job. Switch the thermostat off, clear the drain, and only restart when the pan is empty.

Why is water dripping from my ceiling vent and not the unit?

Two possibilities. Either an attic unit is overflowing its secondary pan and the water is migrating down the supply duct to the nearest vent boot, or the supply duct itself is condensing on the outside because the duct insulation has failed. The signature is timing. Steady drip during operation, stops when AC shuts off, clean water = duct condensation. Intermittent drip, may smell musty, often follows ice = pan overflow.

Can I use bleach to clean my AC drain line?

Avoid it. Chlorine bleach degrades PVC glue joints over years, and on backflow it can corrode aluminum evaporator coils. Some equipment manufacturers list bleach use as a warranty void. Use distilled white vinegar instead. Vinegar dissolves the biofilm that causes most clogs without damaging the line or the coil.

Where is the condensate drain on my AC?

The indoor end is at the bottom of the air handler, where a one-inch PVC pipe exits the cabinet. There is usually a tee fitting with a removable cap a few inches downstream, which is the cleanout. The outdoor end is wherever that line exits the building, often near the outdoor condenser, above a flowerbed, or into a drain in basement installs. If you cannot find the outdoor end, follow the indoor line through the wall.

How much condensate is normal for an AC to produce?

Five to twenty gallons per day in cooling season, depending on system size and outdoor humidity. A three-ton system in a humid climate routinely makes 15 gallons. That is normal water draining out the line. It only becomes a problem when it stops draining, which is when it appears somewhere other than the end of the line.

Should I have a float switch on my AC?

If your AC is in an attic, above a finished ceiling, or anywhere overflow would damage the building, the IMC requires a safety switch on the auxiliary pan. Most jurisdictions adopt this code. A retrofit costs $30 to $100 for the part, $150 to $300 installed, and prevents the ceiling-damage scenario that runs into thousands of dollars.

Will homeowner's insurance cover an AC water leak?

Usually yes for a sudden, accidental leak (drain joint pops, pan cracks, pump fails), and usually no for gradual long-term seepage (slow clog over months). Document the incident with photos, file the claim within the policy window, and keep the receipts for any emergency repairs. A working float switch helps the claim because it shows the equipment had a safety in place.