If water is dripping from the indoor air handler unit, pooling on the floor below it, or dripping onto a ceiling (for attic installations), the condensate drain is almost certainly clogged. It’s one of the most common AC service calls — and one of the easiest to fix yourself.
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Why the AC produces water indoors
Your air conditioner doesn’t just cool air — it dehumidifies it. As warm, humid return air passes over the cold evaporator coil, water vapor in the air condenses on the coil fins (the same way a cold glass sweats on a humid day). That water drips into a condensate pan directly below the coil, then flows out through a drain line to a floor drain, utility sink, or condensate pump.
On a humid summer day, a typical central AC system removes 5–20 gallons of water from the air per day. All of that water has to go somewhere. When the drain clogs, it overflows the pan.
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The most common cause: clogged condensate drain
Algae and mold grow readily in the condensate pan and drain line — it’s dark, wet, and warm, which is exactly what they like. Over a cooling season, growth can completely block the drain line. When blocked:
- The pan fills
- Water overflows the pan
- Water drips from the unit, runs down inside walls, or puddles on the floor
This is what you’re looking for — and what you can fix yourself.
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How to clear a clogged condensate drain
What you need: a wet-dry vacuum, or a turkey baster, or compressed air with a cone tip.
Step 1: Turn off the AC
Turn the cooling off at the thermostat. You’re going to be working near electrical components and water — don’t have the system running.
Step 2: Find the drain pan and drain line
The drain pan is below the evaporator coil inside the air handler. On a horizontal air handler (common in attics), it’s under the unit. On a vertical unit, it’s at the bottom.
The drain line exits the pan as a PVC pipe — typically 3/4 inch to 1 inch diameter — and routes to a floor drain, utility sink, or condensate pump.
Find where the drain line exits the building. It’s usually a small PVC pipe that empties somewhere obvious — a utility sink, an outdoor drain near the foundation, or into a condensate pump reservoir.
Step 3: Check for standing water in the pan
If the pan has water in it, that confirms the drain is blocked (or the unit is producing more water than the drain can handle). Use a shop vacuum to suck out the standing water first.
Step 4: Clear the drain
Option A — Wet-dry vac on the drain outlet (easiest, most effective):
- Take the wet-dry vacuum to where the drain line exits (the floor drain end, utility sink end, or outdoor exit).
- Place the vac hose over the end of the drain line and create as good a seal as you can (a damp rag wrapped around helps).
- Run the vac for 2–3 minutes. The suction pulls the clog back through the line toward the outlet.
- Check the pan — it should start draining.
Option B — Flush from the pan end:
- At the air handler, find where the drain line connects to the pan.
- Pour a cup of water mixed with a tablespoon of bleach or white vinegar into the pan.
- The water should flow down the line within 30–60 seconds. If it sits: the line is blocked.
- Use a flexible brush or plumber’s snake to work through the blockage from the pan end of the line.
Option C — Condensate drain access port: Many modern installations have a T-fitting with a cap near the air handler — an access port specifically for flushing. Remove the cap, pour 1/4 cup of vinegar or condensate drain cleaner into the port, wait 30 minutes, then flush with water. Or apply the shop vac to this port.
Step 5: Test and restore
After clearing, pour a small amount of water directly into the drain pan. It should flow freely down the line and out the drain within 30–60 seconds. If it drains: you’re done.
Restore the AC at the thermostat and monitor for 30 minutes. The pan should remain empty or nearly empty during operation.
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Prevent future clogs: monthly tablet maintenance
Algaecide / condensate drain tablets dropped into the pan monthly (during cooling season) significantly reduce growth in the pan and line. They cost $8–15 for a year’s supply and take 10 seconds to install. Many HVAC technicians recommend this as the single best preventive maintenance step for condensate drains.
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Other causes of indoor water leaks
Frozen evaporator coil thawing: if the AC ran with a frozen coil and then the coil thawed, the sudden release of melt water can overwhelm the drain pan. See Frozen Evaporator Coil for how to handle this.
Signs it’s a frozen coil issue: the leak is a one-time event (coil thawed), or it’s accompanied by ice visible on the refrigerant lines.
Failed condensate pump: in basement or crawl space installations, gravity drainage isn’t possible. A condensate pump lifts water up to a drain. When the pump fails, the reservoir overflows. Test by pouring water into the pump reservoir — it should cycle on and pump the water out within a minute. If nothing happens: the pump is failed or the float switch is stuck.
Condensate pump replacement: $30–70 for the unit; 20-minute installation.
Cracked or overflow drain pan: the primary drain pan can crack with age. Secondary overflow pans (below the primary) are a code requirement in some jurisdictions. If the primary pan is cracked, water bypasses it entirely.
Dirty evaporator coil producing excess condensate: a very dirty coil can produce condensate faster than a clear drain line can handle. If the drain is clear but the pan still overflows during very humid weather, the coil may need cleaning.
Improper installation / drain slope: the drain line needs a small slope toward the outlet to gravity-drain. If installed flat or uphill, water pools in the pan. This is an installation issue — the drain line needs to be re-routed.
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Float switch: the safety that stops water damage
Some systems have a float switch in the drain pan or condensate pump reservoir. When water rises above the normal level, the float switch shuts off the AC, preventing an overflow from flooding the ceiling or floor below.
If your AC shuts off and you discover a full drain pan: the float switch did its job. Clear the drain, then reset the float switch (typically just lowering the float back down, or cycling power to reset).
If the float switch repeatedly triggers with a clear drain: the switch may be set too low (triggering at normal water levels) or stuck. A tech can adjust or replace it.
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When to call a technician
- The drain line is clear but water is still overflowing: dirty evaporator coil, incorrect drain slope, or cracked pan
- Condensate pump won’t run after the drain pan is clear
- Water visible inside wall cavities or ceilings: mold risk — address quickly
- Float switch won’t reset after drain is cleared
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FAQ
How much water should my AC produce? On a hot, humid day, a central AC system might remove 10–20 gallons of water from the air. On a mild day with low humidity, much less. You shouldn’t see this water — it should drain quietly through the condensate drain. Seeing water means drainage failed.
Can I use bleach to clean the drain line? Yes — 1/4 cup of undiluted bleach poured into the drain pan or access port kills algae and mold in the line. Flush with water 30 minutes later. Do this once at the start of cooling season and once mid-season. Don’t overdo it — excess bleach can damage certain fittings.
My AC is in the attic and leaking through the ceiling. What do I do? Turn off the AC immediately. Attic condensate leaks hit the ceiling below and can cause significant water damage. The drain line likely blocked, the pan overflowed, and water found a path through the ceiling. If there’s a secondary overflow pan and float switch, the switch should have shut the unit off — if it didn’t, the float switch may need replacement. Call a tech for attic installations — clearing an attic drain safely is more involved than a basement/closet installation.
Why does my drain keep clogging even after I clear it? The drain clears but the underlying growth comes back quickly without treatment. Use monthly condensate drain tablets or add a condensate drain cleaner with algaecide after each clearing. If growth is heavy, have a tech clean the drain pan with a brush and disinfectant during annual service.
Related reading
If the condensate drain checks out clear and water is still appearing, the next most common cause is a dirty evaporator coil restricting airflow enough to cause icing and meltwater overflow. See what an AC capacitor does if the system is also short-cycling, since a failing capacitor can contribute to the exact airflow problems that cause coil icing.
