Why does my attic drip water only when the AC runs?

A line that only leaks while the AC runs is carrying water that has nowhere to go. The drain is blocked, sagging, or disconnected, so the pan overflows onto the ceiling below. A roof leak does the opposite and shows up in wet weather.

What is the secondary drain pan for?

The secondary pan catches water when the primary drain blocks, and its drain runs somewhere you can see it. If that drain empties over a window or another obvious spot, that drip is your early warning. A full secondary pan with no visible drain means the water has nowhere to go but the ceiling.

Can a blocked condensate line stop my AC?

Yes. Most systems have a float switch in the pan, and it shuts the unit off before the pan overflows. That is the system protecting your ceiling, not a failed AC. Clear the line first, and the system restarts on its own.

Will a slow attic leak ruin the ceiling below?

Yes, over time. A condensate leak that runs for weeks soaks the drywall and the insulation above it, and the board sags, stains, and grows mould. Fix the drain first, then replace any board that feels soft.

How do I keep the drain line from blocking again?

Pour a cup of water through the drain twice a season and drop in a pan treatment tablet every few months. Those tablets break down the algae and slime that build up inside the line. If it blocks every season, ask for the line to be re-run with a steeper fall.

Attic AC condensate drain is leaking water

Reviewed by Josh Arneson, Handyman and Property ManagerPublished November 5, 2026

Water showing up in the attic around an air handler is almost always a clogged or disconnected condensate drain, not a roof leak. A roof leak tracks the weather, so it only shows up when it rains. A condensate leak shows up whenever the AC runs, even on a dry day. Clear the blocked line and the drip stops. If the pan or the cabinet has rusted through, or the ceiling below has gone soft, that part gets replaced.

Tell a condensate leak from a roof leak

Turn the thermostat off so the system stops making water, and leave it off while you look.
Get into the attic and find the air handler. Look at the plastic pan under the unit and the white PVC line leaving it.
Check the pan. Standing water in the pan means the drain line is blocked, and the pan is overflowing onto the ceiling below.
Follow the PVC line to where it leaves the attic. Look for a low spot, a sag, or a joint that has pulled apart.
Watch it for a day. Water that only appears during or after rain is a roof leak. Water that appears every time the AC runs is the condensate line.
Pour a cup of water into the pan with the system off. If it sits there, the line or its trap is blocked.

If the water only appears in wet weather, stop here and treat it as a roof leak. If the pan fills every time the AC runs, or the line is wet at a joint, you have a condensate fault. Hire an HVAC tech to clear and re-slope the line, because the air handler sits next to refrigerant lines and line voltage.

Repair or replace

Situation	What to do
Pan holds standing water and the line is blocked	Repair. Clear the trap and the line, then pour water through to confirm it drains.
Line has a low spot or no fall	Repair. Re-support the line so it runs downhill to the outlet.
Line cracked or pulled apart at a fitting	Repair. Cut out the bad section and glue in new PVC.
Float switch tripped and the system shut off	Repair. Clear the line first, then the switch resets and the system runs again.
Secondary pan full because the line misses it	Repair. Re-route the secondary drain to a spot you can see.
Pan or cabinet rusted through	Replace the pan. Replace the air handler only when the cabinet itself has rotted.
Ceiling below stained and soft	Repair. Fix the drain first, then replace any board that sags or feels soft.

What it costs

Runhome contractor estimates for clearing the clogged attic AC condensate line and repairing the water damage below ran from $345 to $860 as of October 2026, including the trip and assessment.

Option	Range
Clear the condensate line, renew fittings, and repair the ceiling below	$345 to $860

Can I do it myself?

Clearing a condensate line is a homeowner job when the line is easy to reach. Switch the system off at the thermostat and at the air handler's service switch, then pour a cup of water into the pan and watch it flow. If nothing flows, take the trap apart, flush it with water, and clear the line with a shop vac run from the outdoor end. Put the trap back and pour water through again to confirm.

Hire an HVAC tech for anything inside the cabinet, for any wiring, and for the refrigerant lines, because those are live and pressurised. Hire someone too if the air handler sits above a finished ceiling and you cannot reach the line safely, or if the drain has to be re-run across the attic.

What a contractor will ask you

Have these ready and you skip the back-and-forth.

Where the water shows up: at the air handler cabinet, along the condensate line, or at the ductwork.
Whether the wet spot is damp right now or old staining.
Whether it appears every time the AC runs or only when it rains.
A photo of the air handler, its pan, and the condensate line leaving the attic.
The age of the air handler, and whether the line has blocked before.
How the line drains: to a floor drain, outside, or into a secondary pan.

Common questions

Why does my attic drip water only when the AC runs?: A line that only leaks while the AC runs is carrying water that has nowhere to go. The drain is blocked, sagging, or disconnected, so the pan overflows onto the ceiling below. A roof leak does the opposite and shows up in wet weather.
What is the secondary drain pan for?: The secondary pan catches water when the primary drain blocks, and its drain runs somewhere you can see it. If that drain empties over a window or another obvious spot, that drip is your early warning. A full secondary pan with no visible drain means the water has nowhere to go but the ceiling.
Can a blocked condensate line stop my AC?: Yes. Most systems have a float switch in the pan, and it shuts the unit off before the pan overflows. That is the system protecting your ceiling, not a failed AC. Clear the line first, and the system restarts on its own.
Will a slow attic leak ruin the ceiling below?: Yes, over time. A condensate leak that runs for weeks soaks the drywall and the insulation above it, and the board sags, stains, and grows mould. Fix the drain first, then replace any board that feels soft.
How do I keep the drain line from blocking again?: Pour a cup of water through the drain twice a season and drop in a pan treatment tablet every few months. Those tablets break down the algae and slime that build up inside the line. If it blocks every season, ask for the line to be re-run with a steeper fall.