Where the water comes from
As humid air crosses the cold indoor coil, moisture condenses and drips into a pan. A high-efficiency furnace also produces water as it squeezes extra heat from its exhaust. Both need a path out of the house.
The parts
- A drain pan under the coil
- A pipe that runs to the outside, a floor drain or a plumbing drain
- A small pump, where the equipment sits below the drain it empties into
- Often a float switch that shuts the system off if water backs up
Why it clogs
The line stays damp and dark, which suits algae and slime. Dust that gets past the filter adds to it. Over a season or two, that buildup can narrow or block the line.
Keeping it clear
Clearing the drain line is a standard part of a cooling tune-up. Between visits, water around the indoor unit or a system that stops cooling on a humid day are the signs worth watching for.