Water at a ceiling vent is not the start of the problem. It is several steps in. Something upstream overflowed, and that water has been sitting above your ceiling long enough to find its way out.

Turn your system off at the thermostat before you read another line. It keeps pulling moisture out of the air the whole time it runs, and right now that moisture has nowhere good to go.

Here is why water is dripping from your AC vent, and what to do in the next five minutes. Most of the time the cause is a blocked drain line. Sometimes it is not your air conditioner at all.

Houston makes this worse than it needs to be. Most air handlers here sit in the attic, directly above finished ceilings. Our humidity means the system pulls a large volume of water out of the air on any running day, so a blocked line fills fast.

Water drips from an AC vent when moisture the system removed from your air ends up in the ductwork instead of the drain.

Turn the system off at the thermostat first. It keeps producing water while it runs.

The usual causes:

• A clogged condensate drain line, which backs water up until the pan overflows
• A rusted or cracked drain pan letting water through
• A thawing coil after ice built up on it
• Sweating ducts in a hot attic
• A roof leak travelling down the ducts, which is not an AC problem at all

Water at the vent means the overflow already happened. The ceiling above it has been wet for a while.

Five minutes of the right moves now saves a ceiling later.

• Turn the system off at the thermostat. Every minute it runs, it makes more water.
• Put a bucket or towels under the vent. This buys time. It does not fix anything.
• Look at the ceiling around the vent. You are checking for sagging, bulging, or a stain that is spreading.
• If the ceiling sags or bulges, stop and call. Water pooling above drywall gets heavy fast, and that weight has to go somewhere.
• Leave the vent cover on. There is wiring in ceilings, and there is now water in there with it.

With the system off, the drip usually slows or stops within an hour. That tells you the water was coming from your AC rather than somewhere else. It is a useful clue, not a repair.

Do not turn the system back on to test it again. You will restart the same overflow with a wetter ceiling. Wet building materials need to be dried within 24 to 48 hours, so the clock is already running.

With the system off and a bucket down, the next question is where the water is coming from. Timing answers it.

AC-related drips follow the cooling cycle. The system runs, water appears. The system stops, the drip fades within an hour or two. That pattern points at the condensate side.

Rain-timed drips are a different problem entirely. Water gets into the attic through the roof or flashing, lands on your ductwork, runs along it, and exits at the nearest vent boot. The vent is just where it found a hole. Your AC never touched that water.

If the timing points at the roof, you need a roofer and not us. We will tell you that rather than sell you a service call.

Both can be true at the same time. A wet attic raises the humidity up there, and humid attic air against cold duct surfaces produces condensation on the outside of the ducts.

If water is actively coming through and you cannot wait, our emergency AC repair runs around the clock.

The first thing we ask on the phone is when it drips. Your answer decides whether we send a truck or send you to a roofer.

If the timing points at your AC, here is where the water actually escaped.

A clogged condensate drain line. The most common cause by a wide margin. Dust, algae, and growth close the line, water backs up, and the pan fills until it spills over.

A failed drain pan. Older pans rust through or crack. Water skips the drain entirely and goes straight down.

A thawing coil. Ice that built up on the coil melts faster than the pan can drain it away.

Sweating ducts. Poorly insulated ducts in a hot, humid attic collect condensation on the outside. It runs downhill and drips at the boot where the duct meets your ceiling.

A leaking vent boot. Cold air escaping around the grille instead of through it condenses on the ceiling right around the vent.

Those last two are worth separating from the rest. They produce moisture around the vent rather than water coming from inside the duct. If the ceiling is damp in a ring around the grille but nothing is actually running out of it, you are looking at a condensation problem, not an overflow.

That distinction changes the repair. An overflow means clearing a line or replacing a pan. Condensation means insulation and sealing work in the attic.

Any of those causes a leak. None of them should have reached your ceiling.

Your system should have a float switch sitting in the drain pan or on the drain line. It does one job. When water rises past a set point, it cuts power to the system before anything overflows. You get a house that stops cooling, which is annoying, instead of a ceiling full of water, which is expensive.

So if water made it to your vent, one of two things is true. Either there is no float switch on your system, or there is one and it did not work.

That matters beyond today's repair. Clearing the clog fixes the symptom. The same line will clog again, because drain lines always do. Without a working switch, the next clog ends the same way.

We find these missing more often than you would expect. Some were never installed. Some were disconnected during a past repair and never reconnected. Some are sitting in a pan full of debris that keeps the float from moving.

Checking it takes a few minutes. Every Houston home with an attic air handler should have one, and it should be tested rather than assumed. An AC inspection covers the switch along with the pan, the line, and the coil.