What to Do if Your AC Unit Starts Leaking Water into Your Attic

I’ll be honest with you: finding a puddle under your attic AC unit is enough to make any homeowner’s heart sink. Usually, you don’t find the leak until you see a yellow spot on the downstairs ceiling.

As an HVAC tech, I see this a lot during the first few “heat waves” of the year. Your AC is working overtime, which means it is pulling gallons of water out of the air. If that water has nowhere to go, it ends up in your insulation.

If you just discovered a leak, don’t panic. Take these steps immediately to stop the damage.

1. Kill the Power Right Now

The very first thing you need to do is go to your thermostat and turn the AC to OFF.

An air conditioner is basically a giant dehumidifier. As long as it is running, it is creating water. If the drain is clogged, that water is just going to keep spilling into your attic. Shutting it down “stops the bleeding” so you can actually fix the problem.

2. Clear the “Secondary” Drain Pan

If your unit is in the attic, it should be sitting in a big metal tray. We call this the “emergency pan” or “secondary pan.”

• If the pan is dry: The leak might be coming from a cracked pipe or a frozen coil.
• If the pan is full of water: Your main drain line is definitely clogged.

If that pan is full, use an old towel or a small cup to get the water out. If that pan overflows, that is when your ceiling starts to rot. Getting that standing water out is your top priority.

3. Look for the “Float Switch”

Most modern attic units have a little safety device called a float switch. It looks like a small plastic cap with wires coming out of it, sitting in the drain line or the emergency pan.

It works just like the float in your toilet tank. When water rises too high, the float trips a switch and kills the power to your AC so it won’t flood your house. If your AC suddenly won’t turn on at all, check this switch! If there is water around it, it’s doing its job. You just need to clear the water to get the system back on.

4. Attack the Clog

90% of attic leaks are caused by a clogged PVC pipe. Over time, a thick “algae slime” grows inside the pipe and acts like a cork.

• The Vinegar Trick: Find the white PVC pipe coming out of the unit. There should be a small “T” shaped pipe with a cap on it. Pull the cap off and pour a cup of distilled white vinegar down there. Let it sit for 30 minutes. The vinegar kills the algae and can often melt the clog away.
• Check the Exit: Go outside and find where that white pipe ends. Sometimes it’s blocked by a bird’s nest, dirt, or even grass. Clear the end of the pipe and see if the water starts flowing.

5. Check for “Ice”

This one surprises people. If your air filter is really dirty, or if your system is low on Freon, the indoor coils can actually turn into a solid block of ice.

When that ice eventually melts, it’s way more water than the drain can handle, and it overflows everywhere.

• The Test: Open the side panel of your unit. If you see white frost or ice, leave the AC off and turn the Fan to “ON.” This will help melt the ice safely.

My Advice as a Pro

If you’ve cleared the water and the vinegar trick didn’t work, don’t keep running the AC “just for a little bit.” Water damage to a ceiling is way more expensive than an HVAC service call.

Once we get there, we can use high-pressure air to blow that line out in about five minutes, and you won’t have to worry about your living room ceiling ending up on your floor.