The symptoms usually arrive in a confusing order. The AC runs, then quits even though the house isn't cool yet. Or it cools fine but there's a puddle spreading under the indoor unit, or a water stain blooming on the ceiling below an attic air handler. All three trace back to the same humble part: the condensate drain, which nobody thinks about until it stops draining.
Your indoor coil runs cold, so moisture from the air condenses on it the way it does on a cold glass — a lot of moisture, sometimes several gallons a day in humid weather. It drips into a pan under the coil and flows out through a PVC pipe, usually 3/4 inch, that runs to an outside wall, a floor drain, or a condensate pump. Most lines have a trap, like the one under a sink, and many systems add a float switch that kills the AC if the pan fills — a shutoff is annoying, but it beats a ceiling collapse. Warm, wet, and dark, the line grows a slime of algae and bacteria over the cooling season until the sludge closes it off. That's the clog you're clearing; it's rarely anything more sinister.
Turn the power off first. Set the thermostat to Off and flip the breaker for the air handler. You'll be working around the unit with water nearby, and some steps involve opening a panel. Power off, then start.
Find the outside drain opening and vacuum it. Look for a PVC pipe stub poking out of the exterior wall near the outdoor unit, often a few inches off the ground; on a working system it drips steadily while the AC runs. Press the wet/dry vac hose over the opening to make a seal — a rag wrapped around the joint helps — and let it run for two to three minutes. Most clogs come out as a gray-green slug followed by a rush of backed-up water. Hear that, and you've likely fixed it.
Flush from inside, at the cleanout. Near the indoor coil there's usually a tee in the drain line with a removable cap — that's the cleanout. Take the cap off and pour in a few cups of warm water, then the cup of vinegar, and let it sit for about 30 minutes. Go outside and confirm water is flowing freely out of the drain opening. Replace the cap when you're done; an open cleanout can let the line draw air and drain poorly. A clogged line is also one of the quieter reasons behind an AC freezing up, since a backed-up system often has airflow or maintenance issues traveling with it.
Clear the trap and check the pan. If the flow is still lazy, the trap — the U-shaped dip near the unit — is holding sludge. Work the bottle brush into it from the cleanout, or remove and rinse the trap if yours unscrews. Then look at the drain pan itself. Standing water and jelly-like slime in the pan should be wiped out; a pan that stays full after the line is clear may be cracked or pitched wrong, which is a different repair.
Turn it back on and verify. Restore power, set the thermostat a few degrees below room temperature, and let the system run for 15 to 20 minutes. Check the outside drain: you want a steady drip or thin trickle. Check the pan: water should be moving through, not rising. No drip outside after 20 minutes of cooling on a humid day means the clog is still in there somewhere.
Reach for chemical drain opener. Products made for sinks attack hair and grease with heat and caustics. An AC clog is algae slime in thin PVC with a metal pan at the end of it — drain opener can damage both, and it does a worse job than the vacuum anyway.
Blast it with full-pressure compressed air. A hard shot of air can pop a glued PVC joint apart inside a wall or ceiling, trading a clog for a hidden leak. The vacuum pulls from outside and can't over-pressurize anything, which is why it's the better first move.
Just reset the system when the float switch trips. That switch is a flood alarm, not a nuisance. If your AC keeps shutting off in humid weather and you're wondering why the AC isn't cooling, check the drain pan before you blame the refrigerant — every reset without clearing the line is a bet against your ceiling.
Vacuum from outside, flush with vinegar from inside, verify a steady drip. That's the whole job for the great majority of clogged drain lines, and the monthly cup of vinegar keeps it from coming back. Call a tech if the line runs inside a wall with no outside opening you can find, if the pan is cracked, or if the water came back within a week of a thorough clearing — something beyond slime is going on.
A clogged condensate line is one of the most common reasons. When the drain backs up, water fills the pan and lifts a float switch, which cuts power to the system to stop an overflow. The AC then restarts once the water slowly leaks down — and shuts off again. Clear the line and the cycle stops.
Once a month during the cooling season is the standard advice, and it's cheap insurance: a cup of vinegar through the cleanout each time. Homes in humid climates, and systems whose drain lines run long and flat, clog faster and benefit most from sticking to the schedule.
Vinegar is the safer default. Bleach kills algae too, but it can corrode a metal drain pan and nearby coil parts if it splashes or pools, and it shouldn't be mixed with anything. A cup of plain white vinegar, left to sit about 30 minutes, does the job without the risks.
It's typically billed as a standard service visit, often somewhere in the $75 to $200 range depending on your area and whether it's an emergency call. That's worth knowing, because the DIY version with a wet/dry vac takes about 10 minutes — but if the clog is inside a wall or the pan is damaged, the tech's visit buys diagnosis too.