In Carolina humidity, an air conditioner pulls a remarkable amount of water out of your air — often 5 to 20 gallons on a muggy day. All of it is supposed to drain away quietly. When it doesn't, the drain pan is where you find out, and if your air handler is in the attic, the next place you find out is a brown stain on the ceiling.
Two pans, two meanings. Most systems have a primary drain pan built in directly under the evaporator coil; it's supposed to be wet whenever the AC runs, and it empties through the main condensate drain line. Units installed in attics or above finished space also sit over a secondary (auxiliary or emergency) pan — a large, shallow metal or plastic tray under the whole unit. That one should be bone dry. Water in the secondary pan means the primary drain has already failed and you're on the backup.
Why the pan fills. The overwhelming cause is a clogged condensate drain line. The line stays wet and dark all summer, which is ideal for algae, mold, and slime; combined with dust from the coil, it eventually forms a plug. Other causes: a drain line installed without enough slope or without a proper trap, a cracked or rusted-through primary pan (common on systems over 12 to 15 years old), a disconnected drain fitting, a failed condensate pump, or a frozen evaporator coil that dumps more water than the pan can handle when it thaws.
What to do right now. Turn the system off at the thermostat so it stops producing water. Remove standing water with a wet/dry vacuum or towels. Then clear the line: find the drain line's outlet outside (usually a PVC pipe near the outdoor unit or foundation) and pull the clog out with a wet/dry vacuum sealed over the pipe end for a minute or two. If your line has a capped cleanout tee near the indoor unit, pouring a cup of distilled vinegar into it afterward helps kill what's growing. We have a full step-by-step guide on unclogging an AC drain line.
The safety switch that saves ceilings. A float switch is a small, inexpensive device installed in the secondary pan or on the drain line that shuts the AC off when water backs up. If your AC has ever mysteriously stopped running and you found water in the pan, the float switch did its job. If your attic unit doesn't have one, adding it is one of the best small investments you can make — a few minutes of installation against a ceiling repair that can run into the thousands.
When to call. If the pan refills after you've cleared the line, if the pan itself is rusted or cracked, if there's ice on the coil or lines, or if water has already reached the ceiling or walls, it's time for a technician. Water and drywall don't negotiate.
Reinhardt Heating and Air clears drain lines, replaces pans, and installs float switches across Lincoln, Gaston, Mecklenburg, and Catawba counties — and drain service is a standard part of our Energy Savings Agreement tune-ups, precisely so you never find out about it from your ceiling.