Clogged Condensate Drain Line: AC and Furnace Fixes

A clogged condensate drain line traps the water your system makes: from the central AC in summer, from a condensing furnace in winter. It backs up into the drain pan under the indoor coil or into the furnace's trap, and you get water around the indoor unit, a ceiling stain under an attic air handler, a musty smell, or a system that shuts itself off when a float switch trips. Carrier's fix: turn the system off, then vacuum the clog out of the line's outdoor end with a wet/dry vac, or pour a cup of distilled vinegar into the indoor access point. If that fails, or the clog is in a furnace's trap, call a technician.

For a furnace that won't start or blows cold air, see furnace won't turn on and furnace blowing cold air.

Check these in order:

1. Power off: thermostat, then the service switch or breaker.
2. Confirm it's the drain, not a frozen coil.
3. Pan and access point: soak up the water, remove the cap.
4. Outside end: a wet/dry vac.
5. Flush: vinegar, not bleach.
6. Condensate pump: power, a full tank, a stuck float.
7. High-efficiency furnace: the trap, a frozen line, a lockout.

What a Condensate Drain Line Does, and Where It Runs

Your AC pulls humid air across a cold evaporator coil indoors. Water condenses, drips into a drain pan, and leaves through the drain line, which Carrier describes as a ¾-inch PVC pipe. It usually ends outside near the outdoor unit, or at a floor drain or pump; Trane notes it may run through an attic or crawl space. A steady drip outside on a hot day, Goodman says, means it's working.

A high-efficiency furnace, 90% AFUE or higher, makes water too: Trane explains that its secondary heat exchanger cools the exhaust until water condenses. PVC exhaust and intake pipes mark one; a metal exhaust marks a standard furnace. Water by a furnace while the AC runs, Trane adds, comes from the cooling coil's pan.

Lennox describes a secondary (overflow) pan at the indoor unit and a secondary line that usually drains above a door or window. Rheem recommends that pan wherever an overflow could cause damage, such as above a living space, and says water from that line signals a problem. Carrier says modern systems often have a safety float switch that shuts the system down when the pan fills.

Signs of a Clogged Condensate Drain Line

• Water around the indoor unit or furnace, or rust on the unit or its pan (Trane, Lennox).
• A ceiling or wall stain under an attic unit; Trane warns an overflow can damage ceilings and drywall.
• Water from a pipe above a window or door: the backup line is working, so the main line has a problem.
• A musty smell from the vents (Carrier), or stickier air, since a blocked drain raises indoor humidity (Lennox).
• The system shuts off and, the Department of Energy notes, can stay off until the drain is cleared.
• No drip outside while the AC runs, which Carrier links to a clog, a frozen coil, or a disconnected pipe.

Rule out a nearby water heater, washer, or humidifier, as Trane suggests. Water that appears after the AC shuts off, Carrier says, is often ice melting off a frozen coil, from a dirty filter or low refrigerant; Trane says to turn the AC off and leave that to a professional.

How to Unclog an AC Drain Line

Trane and Carrier say a technician is safest, and Trane warns DIY work could void a warranty. For a clog caught early, Lennox and Carrier describe these steps.

Step 1: Turn Off the System and the Power

Thermostat to OFF, then the service switch or breaker. Rheem says all power to the furnace and the outdoor unit should be off before maintenance.

Step 2: Dry the Pan and Find the Access Point

Soak up water in the pan and on the floor with towels or a wet/dry vac; the EPA says areas dried within 24 to 48 hours usually won't grow mold. Then find the access point, a capped opening on the line near the indoor unit. Lennox says to remove the cap, look in with a flashlight, and pull out any clog you can reach without pushing it farther in. A cracked or rusted-through pan is a technician call.

Step 3: Vacuum the Line From the Outside End

Carrier says to use a wet/dry vac at the outdoor end of the line; what comes out is often a dark mass of algae, dust, and mold. If you're unsure which line is the condensate drain, Carrier says not to vacuum it; call a technician. Our wet/dry vac guide covers safe models.

Step 4: Flush the Line With Vinegar

Carrier says to pour a cup of distilled vinegar into the indoor access point to dissolve buildup. Lennox says to pour in diluted vinegar, or a cleaner made for PVC pipe, let it sit for 30 minutes or longer, then flush the line with water. Carrier adds that hot water, not boiling, can loosen minor buildup.

Step 5: Test It and Restore Power

Pour clean water into the access point; it should run freely out the outside end (Carrier). Replace the cap and restore power. If water still backs up, a technician can push the clog through with a snake or water pressure (Carrier).

AC Drain Line Vinegar or Bleach? What the Makers Say

The makers point to vinegar. Carrier and Trane both suggest asking your technician to show you how to pour a cup of distilled vinegar down the line to help prevent clogs; Lennox says diluted vinegar for an AC line, and equal parts white vinegar and hot water for a furnace's.

On bleach, they disagree. Lennox mentions a water-and-bleach flush for a furnace drain line, with no ratio. Rheem's coil instructions say not to use caustic household drain cleaners or bleach in the condensate pan or near the indoor coil, because they damage its aluminum fins and tubes, and condensate pump maker Little Giant says chlorine will damage its pumps. Stick with vinegar, and never combine the two: Washington's Department of Health warns that bleach mixed with an acid, vinegar included, gives off chlorine gas.

Vinegar is upkeep, not a cure: a contractor quoted by Carrier notes many clogs aren't organic, and professional cleaning prevents them best.

Condensate Pump Not Working

Where water can't drain by gravity, a small pump lifts it out. Bryant says a pump that fails or loses power lets its tank overflow; on some pumps, Little Giant says, a high-water switch shuts the system off first.

Check that it's plugged in and whether the tank is full (Bryant), then unplug it before you open it. Little Giant's manual says to check that the floats move freely, clean the tank with warm water and mild soap, look for kinks in the outlet tubing, and inspect it every six months; its FAQ allows a 50/50 water-and-vinegar mix once or twice a year. Lennox puts a pump's typical life at three to five years; Little Giant says one that won't start with a full tank and a clean float needs replacing.

Water Leaking From a High-Efficiency Furnace

Lennox notes a conventional furnace makes no water, so water around one in winter comes from a humidifier, the water heater, or plumbing.

The Trap and Condensate Hose

Water leaves the furnace through a trap and hoses, and Rheem calls the trap a likely place for debris. Carrier's instructions leave everything but filter care to trained service personnel; cleaning the trap means shutting off the gas and removing it. Bryant's homeowner checks: kinks, standing water, or loose sections in the drain line you can reach outside the cabinet. Lennox says a clogged line can be flushed with equal parts white vinegar and hot water; with the furnace off, do that only on that outside line.

Frozen Condensate Line in Winter

Carrier and Bryant say a furnace may leak in winter because its condensate line froze, especially a line exposed outdoors. Carrier's instructions for one condensing furnace say the line must end at an indoor drain, and that a trap or line that could freeze needs a trap heater, heat tape, or RV antifreeze: installer work. For ice at the outside end, Bryant says not to use an open flame; call a technician. Carrier also warns that a plugged drain or failed pump may shut the furnace down, so when you travel in winter, keep the heat no lower than 55°F and have someone check the house, or a shutdown can mean frozen pipes.

Furnace Shut Off: Pressure Switch Lockout

On a Rheem condensing furnace, the pressure switch, which checks that exhaust is being drawn through, also senses a blocked condensate drain. On Rheem's upflow 90-plus models, a blocked-drain switch cuts the gas valve, and four trips in one call for heat lock the furnace out, often after long run cycles in very cold weather or after a thermostat setback. Write down the status light's blink code before any reset (see furnace won't turn on). A reset won't clear a blocked drain; if it locks out again, call the technician.

Prevent It

• Annual tune-ups. ENERGY STAR's yearly check-up covers the condensate drain: cooling in spring, heating in fall. Rheem says to check the coil, pan, and line each season.
• Flushing. Lennox says every few months; Carrier says to check it at least once a year, ideally before cooling season, and more often after past clogs or in humid climates. UF/IFAS Extension notes that summer heat and humidity can grow mold and mildew in the line.
• Filters. Check monthly (ENERGY STAR) and replace every one to three months (Carrier); a clogged one can freeze the coil. See our filter guide.
• A clear outside end, with the line sloping downhill all the way (Carrier). A leak sensor by the indoor unit adds an early warning.

When to Call a Pro

• Water near wiring or the control board (Carrier, Lennox).
• A repeated clog, which Lennox says points to a deeper problem.
• A cracked or rusted pan (Carrier, Lennox).
• Mold or a musty odor (Lennox). The EPA says not to run a system you suspect is contaminated with mold, and points to its remediation guidance for patches over about 10 square feet.
• A pump that won't run or keeps overflowing.
• A clogged furnace trap, a frozen line, or repeat lockouts.
• A frozen coil. The EPA requires anyone handling refrigerant to be Section 608-certified.

Frequently Asked Questions

How do I unclog my AC drain line?

Power off, then vacuum the outdoor end with a wet/dry vac or pour a cup of distilled vinegar into the indoor access point, as Carrier describes. Lennox says to let vinegar sit 30 minutes or longer, then flush with water. Still backing up? Call a technician.

Can I put bleach in my AC drain line?

Use vinegar. Rheem says bleach damages the indoor coil, and Little Giant says chlorine damages its condensate pumps. Never mix bleach and vinegar.

Why is water leaking from my furnace?

On a high-efficiency furnace, usually a clogged line, trap, or pump, or a frozen line in winter. With the AC running, it's likely the cooling coil's pan.

The Bottom Line

Power off, soak up the water, vacuum the line from outside, and flush it with vinegar, not bleach. Check the pump. For a repeat clog, a cracked pan, wet wiring, mold, or a furnace trap or frozen line, call a technician.

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