Why does my AC drain line keep clogging?
Updated August 2026
If your AC drain line clogs again a few weeks after you cleared it, the culprit is algae, mold, and biofilm sludge growing inside the warm, damp pipe. Condensate is basically distilled water sitting in a dark tube at room temperature, which is a perfect greenhouse for slime that rebuilds the plug. A one-time clearing treats the symptom. The repeat clogging is biology, and only a regular flush keeps the line open.
Safety first: a plugged condensate line is a slow-motion flood. When the primary drain backs up, the pan overflows onto whatever is below: a ceiling under an attic unit, a finished floor under a closet unit, or the furnace itself. Saturated drywall grows mold within 24–48 hours, so a $200 clog you ignore becomes a $2,000 ceiling repair. Install a float switch that shuts the system off when the pan fills, and never keep running an AC that is dripping through the ceiling.
Causes, most likely first
| Cause | How likely | The fix | Typical cost |
|---|---|---|---|
| Algae and biofilm sludge | Most common | Flush the 3/4-inch PVC line with a wet/dry vac, then pour a cup of white vinegar through it monthly during cooling season to kill the slime before it plugs. | Free DIY, $75–250 pro |
| Insufficient slope | Common | The line must fall at least 1/4 inch per foot toward the exit. A flat or back-pitched section pools water where sludge settles and sets fast. | $150–400 |
| Coil debris washing down | Common | Dust and rust flaking off a dirty evaporator coil collects at the trap. Cleaning the coil upstream keeps grit out of the line. | $150–400 |
| Missing or dry vent tee (airlock) | Occasional | Without a vent the draining water gurgles and airlocks, so it flows slowly and clogs. A tech adds or clears the vent tee. | $75–200 |
| Crushed or undersized trap | Occasional | A collapsed p-trap or a section pinched against a joist restricts flow. It gets rebuilt with a proper cleanout fitting. | $150–350 |
Check these first
- Find the drain line's outdoor exit (usually a white PVC stub near the condenser) and confirm water drips from it when the AC runs. Nothing there means it is blocked.
- Locate the indoor access tee near the air handler and look inside for standing water or green slime.
- Hold a wet/dry vacuum to the outdoor end, seal it with a rag, and run it for a minute to pull the clog out.
- Pour a cup of white vinegar (not bleach on aluminum systems) into the access tee and watch that it flows through freely.
- Check the float switch in the pan or on the line. If it is tripped, the AC will not run until the water clears.
Prevent it next time
The whole point of this page is that clearing the line once is not enough. Flush it with vinegar on a monthly cooling-season schedule and the slime never matures into a plug. Keep the evaporator coil clean so grit stops washing down, and add a float switch as cheap insurance. Our drain-line checklist lays out the routine, and the cost of ignoring a clog shows why a five-minute flush beats a ceiling repair. New to the part? See what a condensate drain line is.
When to call a pro
Call a pro when the line clears but re-clogs within days (a slope or trap problem you cannot see), when water keeps overflowing despite a clear line (the pan or a vent airlock), or when you cannot locate the access to flush it. Clearing a line runs $75–250, re-sloping or adding a cleanout $150–400, and coil cleaning to stop debris $150–400. A repeat clog after a proper flush almost always means the pipe itself needs re-hanging, which is a tech's job.
Most of the causes above are maintenance problems wearing a disguise. OnOtto keeps the boring prevention on schedule so the 2 a.m. version doesn't happen.
Stop chasing symptoms, free for 30 daysFrequently asked questions
How often should I flush my AC condensate drain line?
Once a month during the cooling season is the sweet spot. Pour a cup of distilled white vinegar into the drain access near the air handler, let it sit 30 minutes, and confirm it runs out the far end. That kills the algae before it can build a plug. In humid climates where the AC runs March through October, monthly matters; in a dry, short summer you might stretch it to every 6 to 8 weeks. Bleach also works but is harsh on aluminum coils, so vinegar is the safer default.
Vinegar or bleach for a clogged AC drain line?
Both dissolve the biofilm, but vinegar is the safer routine choice. Many modern coils are aluminum, and chlorine bleach can corrode aluminum and metal drain-pan parts over time, so manufacturers increasingly recommend a quarter cup of white vinegar instead. Bleach is fine on older all-copper systems for an occasional heavy clean, but dilute it and flush with water afterward. Whichever you use, the mechanical clog still needs a wet/dry vacuum at the outlet first; the liquid only prevents regrowth.
Can a clogged drain line make my AC stop cooling?
Yes, on systems with a float safety switch, which most modern installs have. When the drain backs up and the pan fills, the float trips and shuts the whole air conditioner off to stop it flooding your home. So a no-cooling call on a hot day can trace straight back to a plugged drain, not a refrigerant or compressor problem. Clear the line, the pan drains, the float resets, and cooling returns. If there is no float switch, the pan simply overflows instead.
Where does the condensate drain line actually go?
It carries the water your evaporator coil pulls out of the air, by gravity, to a safe exit: outside near the condenser, into a floor drain, or into a condensate pump that lifts it to a sink or standpipe. A typical run is 3/4-inch PVC that must fall at least 1/4 inch per foot the whole way. If yours ends in a pump, that pump becomes another failure point to clean. Knowing the path helps you find where the clog is hiding.