A cooling system can run cleanly all summer and still put water on a hallway ceiling in a single afternoon. The usual culprit is not the compressor or the coil. It is the condensate drain, the small path that carries moisture off the evaporator coil and out of the house. Treat that path as part of the cooling system, because when it fails, the equipment shuts down or spills, and either way you are the one who finds the mess.
Where the water comes from
When warm, humid air crosses the indoor evaporator coil, moisture condenses on the cold fins and drips into a drain pan beneath the coil. On a humid day a working system can move a surprising amount of water. That water has to leave the pan through a drain line, usually PVC, that runs to a floor drain, a utility sink, an exterior termination, or a condensate pump. Any step that stops the flow, slows it, or lets air back into the line causes trouble.
Check the trap first
Most indoor air handlers use a trap, a U-shaped section of pipe that stays full of water and blocks air from moving through the drain. The water seal has to hold against the pressure the fan creates at the pan, so the trap depth matters. A trap that is too shallow lets air push through and stops the water from clearing. The depth spec comes from the unit manual or the installer's drawing, so check there before you change anything. Pour water into the trap by hand to refill it, then look for cracks, loose unions, or a trap that sags below the drain outlet.
Slope the line, then test the float
Drain lines need a steady fall toward the termination point with no low spots where water sits. Guidance commonly cited is about 1/8 inch of fall per foot, but follow the manufacturer's instructions and local code. Long horizontal runs are where trouble hides, especially where a hanger has loosened or the pipe has been pushed up against a joist. Walk the visible length in the attic and watch for dips, kinks, and staining along the pipe.
Next, the float switch. Most drain pans carry a float that rises with water and opens or closes a safety switch to shut off cooling before the pan overflows. To test it with the power off, lift the float by hand and confirm the switch changes state. To test it with the power on and the system set to cooling, carefully pour a small amount of water into the pan until the float rises. The system should stop. For a wire-level check, disconnect the float leads with the power off and read the switch with a multimeter. Note whether the switch is normally open or normally closed, then confirm your readings match the wiring diagram on the unit. If the float trips and the system shuts down, do not bypass it. A tripped float is the equipment telling you the drain is not clearing.
Clear the algae before the clog
Algae and sludge build up in the pan and at the trap, and that is usually what blocks the line. Flush the pan and drain with a cleaner the manufacturer allows, or with a diluted vinegar solution, following the product label. For a stubborn clog at the exterior termination, a wet/dry vacuum sealed around the opening usually works better than a rod. Avoid forcing tools into a line with glued fittings or a fragile trap, since a cracked fitting creates a new leak where none existed.
What monitoring can and cannot see
Remote monitoring is becoming one more way a problem reaches a service company. ACHR News reported on a new Trane and American Standard kit that gives dealers "remote access to system performance data and diagnostics" across 24V systems. A monitor only reports what its sensors can see. Before you count on any kit to catch a drain problem, ask whether a pan or drain sensor is part of the package. If it is not, the manual float check stays on your routine.
A short routine for the attic
Before the first hot week of the season, open the drain access cap and check the trap. Pour water through the pan, then test the float as described above. Check the slope along every visible section of line and look at the ceiling below for staining. Keep a flashlight and a small bucket in the attic, and step only on joists, never on drywall. A ten-minute check in spring costs far less than a wet ceiling in July.