Someone notices a brown ring on the ceiling under the air handler and calls a roofer. The roof is fine. The stain is from a plastic tube the diameter of a finger, full of slime, that was supposed to carry the water the coil wrings out of the air. When that tube clogs, the water goes into a pan. When the pan fills, it goes through the ceiling, or it trips a float and the system stops cooling on the hottest afternoon of the year. Neither outcome is a mystery. Both are a drain.
What the line is doing
A cooling coil runs below the dew point of the room air. Water condenses on it the way it condenses on a cold glass. That water has to leave. On a central air conditioner or a heat pump in cooling mode, it drips into a primary pan under the coil, then into a trap, then out a condensate line to a floor drain, a sink tailpiece, or a termination outside. A high-efficiency furnace makes condensate too, from the flue gas, and that line is acidic enough that it wants plastic, not a metal pipe someone improvised.
The secondary pan is the backup under an air handler in an attic or a closet over finished space. It is not the design. It is the apology. A float switch in that pan, or in the primary, is supposed to shut the system off before the drywall learns about it. A switch that was never wired, or a pan with no switch at all, is how you get the ring on the ceiling.
Why it clogs
Algae and dust. The line is wet, dark, and fed a slow trickle. A biofilm builds until the trap holds a plug. A trap with no water seal, or a line pitched uphill because someone was in a hurry, does the same job without the biology. A termination buried in mulch, or a line that ends in a sink the homeowner never runs, backs up the same way. Pets and kids have been known to cap an outside end because it dripped on a walk. The system does not know it was capped. It only knows the pan is full.
A dirty filter and a coil caked with the same dust make more water and more debris. Low refrigerant can ice the coil. When that ice melts, it dumps a volume the line was never asked to carry in one hour. That flood is a service problem. The slow clog is a maintenance problem. They look the same on the ceiling.
What you can do without opening the cabinet
Find the line. Attic handlers, closet handlers, and basement furnaces all have one. Follow it to the end. If the end is outside, clear mulch and make sure it can drip. If it ties into a laundry sink, make sure that trap is not a science project.
Once a season in cooling weather, pour a cup of plain white vinegar or a manufacturer-allowed tablet into the access at the trap, if you can reach it without dismantling the unit. Do not pour bleach into a line that shares a drain with a furnace condensate pump unless you know what that pump and any neutralizer can tolerate. Vinegar is the boring choice. It works often enough. A wet-vac on the outside end, with the line disconnected or the trap open so you are not sucking the coil, will pull a plug that vinegar only softens. Shut the system off first. Water and a running blower are a bad pair.
Look at the pan. If it has standing water and the system is off, the primary drain already failed. If there is a float and the system will not run, do not bypass the switch to get cold air. The switch is the only reason the ceiling is still a ceiling.
A condensate pump, the little box that lifts water when you cannot gravity-drain, has a reservoir and its own float. Clean the reservoir. A pump that hums and does not move water is a failed impeller or a stuck float, not a philosophy. Replace the pump. They are not heirlooms.
Climate and the season
Hot and humid. This is the main event. Long cooling seasons grow the biofilm. Check the line in spring before the first heat wave, not during it. An ERV or a dehumidifier adds another drain. Same rules.
Mixed and dry. Shorter season, same clog, easier to forget. A line that sat all winter with a trap full of stale water is a plug waiting for June.
Cold climates with high-efficiency furnaces. The furnace condensate line can freeze at an outside termination. Pitch it, keep the exit close to the house or inside a drain, and do not assume a cooling-season clear-out solved January. A frozen furnace drain shuts the heat down or dumps water in the mechanical room. Different season, same tube.
Attic air handlers in any climate add a second risk. A pan leak up there has a long fall through insulation before you see it. Insulation soaked once holds the shape of the spill and the smell.
Energy, in the small print
A clogged drain does not save energy. A system that short-cycles on a float switch, or a coil sitting in a pan of warm water, does not cool well. A dirty coil from the same neglect makes the blower work harder. Clearing the line is not an efficiency upgrade. It is how the efficiency you already paid for in the equipment gets to happen. A float switch is cheap compared with a ceiling. So is a spring vinegar habit.
What stays with a tech
Anything inside the cabinet, a coil cleaning, a refrigerant question, a float you cannot identify, and a line that was piped uphill or into a vent stack wrong. A tech should confirm the trap depth, the pitch, and that the safety switch actually opens the circuit. An owner who bypasses that switch to finish a dinner party has chosen the stain.
If the ceiling is already wet, stop the water, kill the system, and get the drywall dried before you argue about paint. Mold is a moisture problem with a clock. The drain is the cause. The stain is the receipt.
After it drips where it should
Mark the outside end so nobody buries it. Put a note on the filter calendar: vinegar or a look at the pan when you change the filter in cooling season. A line that drips on a hot afternoon is the system working. A pan with water in it is the system asking for a plumber’s helper or an HVAC tech, depending on whose tube it is.
Where does your condensate actually end — a floor drain, a pump, or a puddle in the mulch — and has the pan ever had water in it? Float switch or no switch belongs in the comments. That is the difference between a shutoff and a stain.