A ceiling stain or water dripping under the attic AC unit

What to do right now

Turn the cooling off at the thermostat to stop making more water. Put a container under the drip and, if the ceiling is bulging, keep people out from under it. If you can reach the attic safely, look at the secondary drain pan under the unit: standing water there means the primary drain is blocked.

How the drainage is built

The coil sits over a primary drain pan that feeds a condensate line, usually PVC, running to a drain or to the outside. Under the whole unit sits a secondary pan, often with its own drain that ends over a window or an eave where you can see it, or a float switch that shuts the system off when the pan fills.

A drip from a pipe over a window is a warning, not a nuisance. It means the primary drain is blocked and the backup is doing its job.

Common causes

• Algae and sludge clogging the primary drain line
• A disconnected or cracked drain fitting at the coil
• A rusted-through secondary pan on an older system
• A float switch that failed or was bypassed
• Melt water from a frozen coil overwhelming the drain

What a technician checks

The primary line is cleared and flushed, the trap and fittings are checked, and the secondary pan and float switch are inspected. If the coil was icing, the airflow or refrigerant fault behind it gets found too, or the water comes back.

What to have ready when you call

• Where the water is showing, and whether the system still runs
• Whether there is water in the secondary pan, if you looked
• The install year from the label on the attic unit
• Your address, ZIP code and attic access

Common questions

Can I clear the drain myself? Many lines have a cleanout cap near the unit, and some owners flush them. If the water returns within days, the cause is somewhere a flush does not reach.