Your air conditioner is also a dehumidifier. On a muggy Huntsville day it can pull several gallons of water out of the air. That water drips off the indoor coil into a drain pan, then flows out a plastic drain line, usually to the outside. When water shows up where it should not, something in that path is blocked, broken or overflowing.
Where the water usually comes from
If your indoor unit is in a closet or garage, the AC coil often sits on top of the furnace. That is why people search “furnace leaking water when the AC is on.” The furnace is usually fine. The water is coming from the AC coil above it. If your indoor unit is in the attic, a leak can show up as a stain on the ceiling below it.
The most common causes
- A clogged condensate drain line. Algae and sludge build up inside the line until water backs up. This is the most common cause by far.
- A full secondary pan or tripped float switch. Attic units usually sit in a second, backup pan. Water in that pan means the main drain is blocked. Many systems have a float switch that shuts the AC off when water rises, so a leak and “my AC stopped cooling” often happen together.
- A rusted or cracked drain pan. Common on older systems.
- A frozen coil that is melting. See why an AC freezes up.
- A failed condensate pump. Units that cannot drain by gravity use a small pump. When it fails, the water has nowhere to go.
- A loose or poorly sloped drain line. Lines need a steady downhill run to drain.
- A dirty filter. It can lead to a frozen coil, then an overflow when the ice melts.
What you can safely check
- Set the thermostat to Off so the coil stops making water.
- Look in the secondary pan (attic units) and around the indoor unit. Soak up standing water.
- Find the drain line outside near the foundation. When the AC is running, it should drip steadily. No drip on a humid day usually means a clog.
- Clear the line from the outside end with a wet/dry vacuum for a minute or two.
- If there is a capped T-fitting (cleanout) on the line near the indoor unit, you can pour a cup of water down it to check that it drains. Some homeowners use white vinegar; check your owner’s manual first.
- Replace the filter, then restart the system and watch for leaks.
Can I still run my AC if it is leaking water?
Not until you know why. Water can stain ceilings, ruin drywall and flooring, and grow mold, especially in an attic. If the float switch is working, it will keep shutting the system off anyway. Clear the drain or call us before you run it for long.
Is an AC leak an emergency?
A small drip you can catch in a pan should be fixed soon, but it is not an emergency. Water coming through a ceiling, a sagging ceiling, or water near outlets or wiring is urgent. Turn the system off at the thermostat and the breaker, move belongings away, and call.
How to prevent it
A yearly AC tune-up should include flushing the drain line, checking the pan for rust, and testing the float switch. That quick check prevents many summer leaks.
Call North Alabama Heating & A/C at (256) 534-2941. We have served Huntsville, Madison and North Alabama since 1977, Monday to Saturday, 7 AM to 7 PM. See our AC repair service.
Quick answers
Why is my AC leaking water inside?
Most often the condensate drain line is clogged with algae, so water backs up out of the drain pan. Other causes include a cracked pan, a failed condensate pump, a frozen coil that is melting, or a dirty filter.
Why is my furnace leaking water when the AC is on?
In most homes the AC coil sits on top of the furnace. When the AC drain clogs, water overflows the coil pan and runs down the furnace. The fix is usually clearing the AC drain line, not repairing the furnace.
Can I still run my AC if it’s leaking water?
It is best not to until the cause is fixed. Leaking water can damage ceilings and floors and lead to mold. Turn the system off, clear the drain line if you can, and call a technician if it keeps leaking.
