The short answer: it's almost always the drain or the coil
If your air conditioner is leaking water inside the house, three causes account for the overwhelming majority of calls we run in Stillwater, Perkins, and the surrounding cities: a clogged condensate drain line, a frozen evaporator coil, or a failed drain pan. Each one looks similar from the floor — water dripping, staining a ceiling, or pooling near the indoor unit — but they're caused by different problems and they need different fixes.
The good news: an indoor AC water leak is rarely a system-killer if you respond quickly. Shut the thermostat off, dry what you can, and read the next 1,000 words so you know what's happening before our tech rolls up.
What is your AC actually leaking?
An air conditioner doesn't just cool air — it dehumidifies it. When warm indoor air passes across the cold evaporator coil, moisture in the air condenses on the coil's surface (the same way water beads on a glass of ice tea on a humid July afternoon). Under normal operation, a residential system can produce 10 to 20 gallons of condensate water per day in peak Oklahoma humidity. That water is supposed to:
- Drip off the coil into the primary drain pan directly beneath it.
- Flow down a PVC drain line, by gravity, out of the house — usually exiting near the outdoor condenser or at a floor drain.
- (Belt and suspenders) If anything goes wrong with steps 1 or 2, a secondary drain pan beneath the air handler catches the overflow and trips a safety float switch that shuts the system off.
An indoor leak means at least one of those three layers has failed.
When each cause is most likely (the real-world failure modes)
Cause #1: A clogged condensate drain line (~60% of our leak calls)
Algae, dust, insulation fibers, and biofilm slowly build up inside the PVC drain line. Eventually it plugs solid and the primary drain pan overflows the moment water can't escape. Tell-tale signs:
- Leak shows up on humid days, especially after the system has been running for an hour or more.
- Water is mostly clear but may have a faint musty smell.
- Outdoor end of the PVC drain line is dry (no condensate dripping out where it should).
- Your thermostat or system control may show a "drain pan full" or similar alert if the float switch has tripped.
Cause #2: A frozen evaporator coil (~25% of our leak calls)
When the coil freezes solid, the system can't dehumidify properly while it's iced, and then dumps a flood of water when it finally thaws. The coil freezes for a reason — there's always an underlying cause:
- Dirty filter — restricted airflow drops coil temperature below freezing.
- Low refrigerant charge — usually means a small leak somewhere in the line set.
- Blower motor running slow — failing capacitor or a control-board issue.
- Closed or blocked supply registers in too many rooms.
If you see actual ice on the copper refrigerant line outside, or frost on the indoor coil, this is your cause. Switch the system off and let it thaw before doing anything else.
Cause #3: A failed drain pan, condensate pump, or trap (~15%)
Drain pans rust through. Condensate pumps fail (especially in finished basements and attic installs). P-traps dry out and let sewer-side air pressure push water back into the cabinet. These are physical-component failures — the leak shows up suddenly even though nothing has changed about your usage or filter.
"On any leak call, the first thing my techs check is whether the float switch has actually done its job. If it tripped and shut the system off, you have a drain problem. If the system was happily running while the leak got worse, the safety chain didn't engage — that's a separate conversation we need to have about the install." — Bryan Sitton, Owner, Sitton Mechanical
Where the DIY internet gets this wrong
A lot of advice online tells you to "pour a cup of bleach down the drain line" as a maintenance step. We don't do this on any system we service — and we tell our customers not to either. Three reasons:
- Modern high-efficiency air handlers use plastic and rubber drain-line fittings that bleach degrades over time. We've replaced more than one cracked drain elbow that was bleached weekly for years.
- Bleach kills biofilm but leaves the dead material in the line — it doesn't actually clear a clog.
- If your line has a P-trap (most do), bleach pools in the trap and never reaches the clog you're trying to clear.
The professional approach: a wet/dry vacuum at the outdoor end to mechanically clear the line, followed by an enzyme-based condensate treatment (NOT bleach) to break up biofilm. We include that on every Comfort Plan maintenance visit.
What a Sitton tech actually does on an AC leak call
If you call us about an indoor AC leak, here's the visit, in order:
- Confirm system off and coil temperature — we won't open the cabinet on a frozen coil; we wait for it to thaw safely.
- Test the float switch and inspect the secondary drain pan — if the safety chain failed, that's a code/install issue we have to fix before reopening the system.
- Vacuum-clear the drain line from both ends, then flow-test with a measured volume of clean water.
- Inspect the coil, blower wheel, and filter for the upstream cause if the coil froze.
- Check refrigerant charge against manufacturer spec using digital gauges and a temperature/pressure analysis. We never "top off" refrigerant without finding the leak first.
- Recommend the cheapest correct fix. Sometimes that's a $0 customer-side filter change. Sometimes it's a coil replacement. We tell you which.
Every Sitton service van is EPA 608 certified for refrigerant handling and stocks the most-common drain components, capacitors, and float switches so we can usually fix the leak on the first visit.
Your next step
If your AC is actively leaking right now: switch the thermostat OFF, place a towel under the indoor unit, and call (405) 780-3034. We schedule same-day appointments across Stillwater, Perkins, Cushing, and Perry. If your leak has resolved on its own but you want to know why it happened and how to prevent it from happening again next summer, request a Sitton Comfort Plan tune-up — we'll find the root cause before next July's first 95°F day.