Water on the floor, or an AC that shuts itself off.

An air conditioner is a dehumidifier that happens to cool. Water condenses on the indoor coil, runs into a pan under it, and leaves through a PVC drain line. When that line plugs, the water has to go somewhere, and somewhere is usually your ceiling.

The symptom is often not water

The most common version of this call is not a leak. It is a system that quietly stops working.

Most Valley systems have a float switch sitting in the drain pan or spliced into the line. When the water backs up, the float rises and cuts the control circuit. The system shuts down, the thermostat still has power, nothing is obviously wrong, and you have no idea why the house is getting warm.

That is the switch doing its job. It chose an uncomfortable afternoon over a collapsed ceiling.

Where to look

The outdoor termination. Most homes have a white PVC stub near the outdoor unit or coming through an exterior wall. On a running system in summer it should be dripping. A bone-dry stub on a hot day means the water is not getting out.

The secondary drain. Many installations have a backup line that terminates somewhere deliberately annoying, like over a window or a patio door. Water coming out of that is a warning light, not a leak.

The air handler closet or the attic. Look for staining on the platform, rust streaks on the pan, or a damp patch on the ceiling drywall underneath.

The ceiling under an attic unit. A brown ring that grows is the expensive version of this problem. Shut the system off at the thermostat and deal with the drain before you run it again.

What you can do safely

Put a wet/dry vacuum on the outdoor termination, seal around it with a rag or your hand, and pull for a minute or two. A good share of plugs come out this way, and it costs nothing but the trip to the garage.

Afterwards, find the access tee near the indoor unit, pull the cap, and pour a cup of white vinegar through it. Repeat every couple of months through the cooling season and the problem mostly stops happening. It is also part of a standard tune-up, if you would rather it was somebody else’s job.

What is not worth doing at home is pressurising the line with compressed air, which will find the weakest joint in the attic and open it, or pulling a tripped float switch out of the circuit to get the system running. The switch is the only thing between a plugged drain and the drywall.

What it costs

A drain line clear-out is a flat price, and so is a float / safety switch if the switch itself has failed. Where the unit sits below the drain and the water has to be lifted, a condensate pump is its own flat price. All three are on the price list.

If what is actually wrong is not clear from the symptom, it starts as a diagnostic, and you hear the repair number before anything gets done.

The one that is not the drain

Ice is the other reason water shows up where it should not. A coil that freezes and then thaws dumps far more water at once than the pan was designed to carry, and it will overflow a drain that is perfectly clear. If you have seen ice on the copper lines, the drain is a symptom and something else is the cause: start with why a coil freezes instead.