The condensate pump.
A small pump that lifts drain water uphill. When it quits, the pan fills.
Most air conditioners drain by gravity: the pan sits above the termination and water runs downhill. Where that is not possible, a condensate pump does the lifting.
It is a small box with a reservoir, a float, and a pump. Water collects, the float rises, the pump runs and pushes the water up and out through a small flexible line, and it shuts off again.
Where you find one
- An air handler in a basement or a low closet, below the drain exit
- A unit in the middle of a house with no downhill path to an outside wall
- Attic installations where the run to the eave would need to climb
- Mini split indoor heads, which almost always use one
How it fails
The reservoir fouls. Same biology that plugs a drain line, in a tank. Slime builds up around the float and it stops rising, or stops falling.
The float sticks. Then the pump either never runs or never stops.
The motor fails, which is the straightforward version.
The discharge line plugs or kinks. The pump runs, the water goes nowhere.
The symptom in every case is the same: water backs up. In a good installation the pump has its own safety switch that kills the system before the reservoir overflows, which is the same job a float switch does on a gravity drain.
Maintenance
Flush the reservoir annually, which is part of a tune-up, and keep the discharge line clear. A pump that is making more noise than it used to is usually telling you something.
Replacement is a flat price on the list. Where there is no safety switch on the pump, fitting one is worth it, because the failure mode without it finds your ceiling.