HVAC & Climate

Why Is My AC Leaking Water Inside?

The short answer: A clogged condensate drain line is the most common cause — especially in humid climates where algae builds up fast. Turn off your AC immediately to prevent water damage, then check the drain line. You can usually clear it with a wet/dry vac or vinegar flush. If the drain line is clear, you may have a frozen evaporator coil or a cracked drain pan.

Written by Solomon, former HVAC/plumbing field technician with thousands of residential service calls in NW Florida

First: Turn Off the AC

Before diagnosing anything, turn the system off. Water running through your ceiling or down your walls causes damage fast — mold can start growing in 24-48 hours in humid environments. Switch the thermostat to OFF, then go find the source.

The #1 Cause: Clogged Condensate Drain Line

Your AC removes moisture from the air — in a humid climate, a residential system can pull 5-20 gallons of water per day. That water drips off the evaporator coil into a drain pan, then flows out through a PVC drain line (usually a 3/4" white pipe).

Over time, algae, mold, and sludge build up inside that pipe and clog it. When it clogs, water backs up in the drain pan and eventually overflows — into your ceiling, down your walls, or onto your floor.

How to Clear the Drain Line

Find the drain line. Follow the PVC pipe from your air handler. It usually exits the house near the outdoor unit or into a plumbing drain.
Try a wet/dry vacuum. Attach the hose to the outdoor end of the drain line, seal it with a rag, and run the vacuum for 30-60 seconds. This sucks the clog out. This works about 80% of the time.
Vinegar flush. Find the cleanout port on the drain line near the air handler (a capped T-fitting). Remove the cap, pour 1 cup of distilled white vinegar in, and let it sit for 30 minutes. Then flush with water.
Use a drain line brush or nitrogen flush. If the clog is stubborn, you may need a technician to blow it out with nitrogen or run a small brush through the line.

Field Tip: In NW Florida, I'd clear clogged drain lines almost daily during summer. The #1 preventive measure is pouring 1/4 cup of vinegar down the cleanout port monthly during cooling season. It kills the algae before it builds up.

Cause #2: Frozen Evaporator Coil

If you see ice on the indoor coil or the copper refrigerant lines, the coil has frozen over. When it thaws, all that ice becomes water — more water than the drain pan can handle at once.

A frozen coil is caused by:

Dirty air filter (by far the most common)
Low refrigerant (indicates a leak)
Blower motor failure (not moving enough air)

What to do: Turn the system to FAN ONLY and let the ice thaw completely (2-4 hours). Check and replace the air filter. If the coil freezes again after that, you need a technician — it's likely a refrigerant leak.

Cause #3: Cracked or Rusted Drain Pan

The drain pan sits directly under the evaporator coil. In older systems, these pans can crack, rust through, or develop leaks at the seams. If water is dripping from the bottom of the air handler rather than overflowing from the top of the pan, inspect the pan itself.

The fix: A secondary drain pan (the one below the air handler) can be replaced for $50-$100 DIY. The primary pan (inside the air handler) usually requires a technician.

Cause #4: Disconnected Drain Line

If you recently had work done on your system, or if your air handler is in a tight attic space, the drain line connection can come loose. Check that the PVC fittings are connected and sealed where the drain line meets the drain pan.

Preventing Future Leaks

Task	Frequency	Time
Pour vinegar in drain line	Monthly (summer)	2 minutes
Change air filter	Monthly	2 minutes
Check drain pan for cracks	Annually	5 minutes
Have AC maintenance done	Annually (spring)	Professional

Safety Warning: If your air handler is in the attic and the drain line clogs, water will run through your ceiling. Many codes require a secondary drain pan and/or a float switch that shuts the system off when the primary pan overflows. If you don't have a float switch, ask your technician to install one — it's a $50-$100 investment that can prevent thousands in water damage.

When to Call a Professional

The drain line is clear but water is still leaking
The evaporator coil keeps freezing after you've replaced the filter
Water stains on your ceiling suggest this has been happening for a while (check for mold)
The drain pan is cracked or rusted and needs replacement inside the unit

Expect to pay: $75-$200 for a drain line clearing, $150-$400 for a float switch installation, $200-$500 for a drain pan replacement.

Florida Factor: Condensate drain issues are the single most common AC repair call in Florida. The humidity here means your system works overtime removing moisture. Invest in preventive drain maintenance — it's the cheapest insurance against water damage you can get.

This answer covers the basics, but every home is different. Kept's AI Advisor knows your systems — their age, your climate, your maintenance history — and can give you guidance specific to your situation.

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