How to Clean an AC Condensate Drain Line (60-Second Fix With a Wet/Dry Vac)
A clogged AC condensate line is the #1 summer AC failure. Sixty seconds with a shop vac at the outdoor exit pulls the clog out. Once-a-month vinegar flush prevents it.
Quick answer: The AC condensate drain line is a PVC pipe that carries water from the indoor coil to outside. Algae and biofilm clog it every summer. The fast fix: take a wet/dry vacuum to the pipe”s outdoor exit, seal the connection with your hand or a rag, and vacuum for 60 seconds. The clog pulls out in one chunk. The prevention: pour 1 cup of distilled white vinegar into the indoor cleanout (a T-fitting near the air handler) once a month from May through September. Total cost: $0 if you have a wet/dry vac, otherwise a one-time $50 purchase that pays back the first time it prevents a service call.
A clogged condensate drain is the number one preventable AC failure, per EnergyStar’s residential maintenance guidance. The symptom most homeowners notice first isn’t water on the floor, it’s the AC stopping entirely. The float switch on the drain pan trips when water can’t drain, and the whole system shuts off as a safety measure. (See our float switch guide for more on that, or the HVAC troubleshooting guide for the broader system context.)
The actual clog is usually a soft plug of algae, biofilm, and dust that built up over a season. A wet/dry vac at the outdoor exit pulls the whole thing out in under a minute. Once you know the trick, you save the $125 to $200 service call this generates every summer for unprepared homeowners.
This article covers the unclog procedure and the monthly prevention that keeps it from happening again.
What the Drain Line Actually Does
Your AC’s indoor coil (the evaporator, sitting on top of the furnace or inside the air handler) is cold. When warm humid air blows across it, water condenses on the coil and drips into a drain pan below. From the pan, a 3/4-inch PVC pipe carries the water by gravity to outside, where it usually exits through the foundation or a basement wall and either drains onto the ground or into a designated drain.
A typical residential AC produces 5 to 20 gallons of condensate water per day during peak summer, more in humid climates. That’s a lot of water moving through a slow-flowing horizontal pipe in a warm dark environment. Perfect conditions for algae growth.
The algae grows as a thin film on the pipe wall, slowly thickens, eventually sloughs off in chunks. A chunk gets stuck at an elbow or at the outdoor exit, traps more material, and within a week the pipe is fully blocked.
How to Tell the Drain Is Clogged
Three symptoms, any of which is a giveaway:
- Water on the floor near the air handler. The drain pan overflows because the pipe isn’t carrying water out fast enough. Most modern systems prevent this with the float switch in symptom #2.
- AC stops working with no fan or compressor running. The float switch on the drain pan tripped, cutting power to the system. The AC won’t restart until the pan drains.
- Visible mold or rust around the air handler base. Sign of chronic slow drainage that’s been happening for weeks or months. The clog is partial but real.
If you see any of these, work through the steps below.
What You’ll Need
- A wet/dry vacuum (shop vac, $40 to $80 if you don’t own one)
- A roll of duct tape or a rag
- A flashlight
- A funnel (kitchen funnel is fine)
- 1 cup of distilled white vinegar
- Optional: a 5/8 to 3/4-inch garden hose adapter for the shop vac (some adapters connect the vac hose directly to the PVC drain line)
About 10 minutes of actual work. The shop vac is the only specialty item, and a $50 vac saves the $125 to $200 service call the first time it prevents a clog.
Step 1: Find the Drain Line, Both Ends
The drain line has two ends: the indoor cleanout (a T-fitting near the air handler) and the outdoor exit (a PVC stub through the wall outside).
Indoor end: Stand at your air handler (the indoor AC unit). Look for a 3/4-inch white or beige PVC pipe leaving the unit horizontally. About 12 to 24 inches downstream, the pipe has a T-fitting with a vertical stub topped by a removable cap. That’s the cleanout.
Outdoor end: Walk outside and look at the foundation or wall closest to the indoor unit. A PVC pipe stub 12 to 18 inches above ground level, often near a hose bib, is the drain exit. It may drip water during AC operation in summer, that’s the giveaway.
If you can’t find the outdoor end, your drain might empty into a designated indoor drain (utility sink, floor drain, condensate pump). Look for where the pipe goes from the indoor cleanout, gravity-drain systems always exit the house somewhere accessible.
Step 2: Vacuum the Outdoor End
This is the fast fix.
- Set up the wet/dry vac outside near the drain exit.
- Place the vac hose over the end of the PVC pipe. Most shop vac hoses are 1-1/4 or 2-1/2 inches, larger than the 3/4-inch drain pipe.
- Seal the connection with your hand, a wet rag, or duct tape wrapped around the gap. The seal needs to be tight enough to create suction, doesn’t have to be perfect.
- Turn on the vac.
- Run for 60 seconds.
- Stop. Pull the hose off.
If the clog was the typical algae plug, you’ll see it in the vac canister: a black or green slime tube, sometimes 3 to 6 inches long. That was the entire problem.
If nothing visible came out, try once more for another 60 seconds. If still nothing, the clog is deeper in the line or completely cemented in, move to Step 3.
Step 3: Flush From the Indoor Cleanout
If the vacuum didn’t fully clear the line, the cleanout end gives you a way to attack from the opposite direction.
- Place a small bowl or shallow tray under the cleanout to catch overflow.
- Remove the cap on the indoor cleanout.
- Pour 1 cup of distilled white vinegar (5% acetic acid, the standard kitchen kind) into the cleanout. Use a funnel if the opening is tight.
- Wait 30 minutes. The vinegar dissolves biofilm and softens algae.
- Repeat the outdoor vacuum from Step 2.
If you still don’t get flow, the clog may be cemented at a fitting and you may need a small drain auger fed in from the outdoor end. A 1/4-inch hand snake works. Push it in gently 6 to 12 inches, twist, pull back. Don’t force it, the PVC fittings can crack.
Step 4: Reset the Float Switch
If the float switch tripped (AC won’t run), the switch resets itself once water drops below its threshold in the pan. After you clear the drain, the pan empties through the now-flowing line and the switch resets. Wait 5 minutes, then turn the AC on.
If the AC still won’t start, the float switch may have failed in the tripped position. See our float switch troubleshooting guide for that case.
The Monthly Prevention
The whole reason this gets to crisis-level clogs is that no one does maintenance until something breaks. The actual prevention is one minute per month during cooling season:
- Remove the cap on the indoor cleanout.
- Pour 1 cup of distilled white vinegar in.
- Replace the cap.
- Done.
That’s it. Vinegar (acetic acid) is mildly acidic, kills algae and biofilm, and breaks down to harmless water and CO2. It’s safe for PVC, doesn’t damage the drain pan or float switch, and costs about $2 per gallon at the grocery store.
Schedule the monthly vinegar pour from May through September (or year-round if you’re in Florida, Texas, or another year-round cooling climate). On a phone reminder, takes the same effort as remembering to change the furnace filter.
You can also drop a “condensate pan tablet” into the drain pan once a quarter as additional prevention. Pan-Treat or Pan Tabs are slow-release algicide tablets ($15 for a year’s supply). They work, but the vinegar pour alone is enough for most systems.
What NOT to Pour Down the Drain
A few common mistakes:
Bleach. Common bad advice on the internet. Bleach kills algae but also degrades PVC over time (slowly oxidizes the plastic) and reacts with any organic material to form chloramine vapor. Vinegar is the right cleaner.
Hot water. PVC fittings are rated to about 140°F. Boiling water from a kettle can soften the fittings and cause joints to loosen. Stick to room-temperature liquids in PVC drains.
Drano or other drain cleaners. These are designed for sink drains, not AC condensate drains. They damage the drain pan, can corrode metal components in the air handler, and are formulated for organic clogs that are very different from algae biofilm.
Common Mistakes to Avoid
Pouring water down the cleanout to “flush” a clog. Water just sits on top of the clog and overflows. Vacuum from the other end first to remove the plug, then flush with vinegar as maintenance, not as the primary clog-clearing method.
Skipping the seal at the vacuum-pipe connection. If the vac hose isn’t sealed tightly to the PVC, you’re just pulling room air, no suction reaches the clog. Press hard, use a rag or duct tape.
Running the vacuum forever. Sixty seconds is enough. If the clog isn’t coming out in a minute, it’s not going to come out in five minutes, switch to the vinegar-then-vacuum approach in Step 3.
Doing this once and considering it done. A drain that clogged once will clog again. The vacuum clears it; the monthly vinegar pour keeps it clear.
Touching the float switch wires. The float switch is a low-voltage 24V circuit, no shock risk, but disconnecting it bypasses the system’s flood protection. If you accidentally pull the wires off while working near the pan, reconnect them before you turn the AC back on.
FAQ
How often should I clean the AC drain line? The monthly vinegar pour from May through September is the maintenance schedule. The vacuum clog-clearing only happens if you skipped maintenance. Stay on schedule and you’ll go years without a real clog.
Why does the drain only clog in summer? Because the AC only produces condensate when it’s actively cooling. In winter, no water flows through the line, no algae growth. The clog is a summer-only issue, and prevention only matters during cooling season.
My AC doesn’t have a cleanout T. How do I do maintenance? Older systems sometimes lack a cleanout. You can pour vinegar into the drain pan directly (lift the access panel on the air handler, the pan is at the bottom of the indoor coil). It’s less convenient but works. For a more permanent fix, an HVAC tech can install a cleanout T for $50 to $150 at a future service visit, worth doing.
Should I install a condensate pump? Only if your system is in a basement below the level of any drain (so gravity won’t work) or if the natural drain path is impractical. A condensate pump runs about $50 to $100 plus installation, lasts 5 to 10 years, and adds its own failure mode (pumps die, drains overflow). If gravity drainage works in your house, stick with it.
My drain line drips constantly even when the AC is off. Is that normal? After the AC shuts off, residual condensate continues to drip from the line for 15 to 30 minutes. After that, it should be dry. Constant dripping (24/7) while the AC is off means water is leaking into the drain pan from somewhere else, possibly a refrigerant line condensation issue, worth a service call.
Can I use this same technique on a window AC unit? Window units have a different drainage system, the condensate usually pools in a small reservoir in the bottom of the unit and either drips out the back through a designated hole or evaporates. There’s no PVC drain line to clean. Regular cleaning of the window unit’s drain hole with a thin wire prevents clogs.
The condensate drain is the most-skipped piece of AC maintenance, and the easiest one to do. Sixty seconds with a shop vac when something’s wrong, sixty seconds with a cup of vinegar each month so something doesn’t go wrong. Whole habit costs about $5 in vinegar per cooling season. For the related “AC turning off on its own” symptom that often traces back to this exact drain issue, see our float switch troubleshooting guide, and for the larger summer AC prep list, the condenser coil cleaning walkthrough covers the outdoor side.