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AC leaking water? First-hour steps, then the causes in order
Water where it shouldn't be is the one AC problem with a clock on it — the fix is usually a clogged drain, but the damage runs by the hour. Stop the water first, then find out why it came.
Guide by Zach Barbre · Denver HVAC expert
Updated October 2026 · Written from first-hand field experience in the north Denver metro
The first hour: limit the damage
- Turn the cooling OFF at the thermostat. A running AC manufactures water constantly; stopping it stops the supply. (Leave the fan setting alone for now.)
- Protect the house. Towels or a pan under the drip, move what's under it, and if water is coming through a ceiling, place a bucket under the stain — a ceiling holding water gets heavy fast.
- Find where it's coming from. Around the indoor unit/furnace base, from a white PVC drain line, or from the pan under the coil — the source narrows the cause list below.
- Check for ice. If the copper lines or coil are frosted, the "leak" may be a frozen coil thawing — and the real problem is airflow or refrigerant, in our AC blowing warm air guide. Let it thaw with the cooling off.
One urgency note: dry things out promptly. Mold can start growing in a wet wall or ceiling cavity within a day or two in summer conditions, and the EPA's guidance after any water event is to dry within 24–48 hours. The HVAC fix matters; the drying matters just as much.
The causes, most common first
1. A clogged condensate drain
The runaway leader. Your AC pulls gallons of water out of the air all summer, and the drain line that carries it away grows algae and sludge until it plugs. Water backs up into the drain pan and overflows wherever it can. The homeowner fix: find where the white PVC drain line exits the house, put a wet/dry vacuum on the end, seal it with a rag, and run it for a few minutes — most clogs pull through with a satisfying gurgle. Follow with a cup of distilled white vinegar poured into the drain's access tee near the indoor unit; doing that every couple of months keeps the line clear.
2. A frozen coil, thawing
When a coil freezes (dirty filter, low refrigerant, weak blower), the ice eventually melts faster than the drain can handle it — or melts off surfaces that were never meant to be wet. If you found ice in step 4 above, the leak is a symptom: thaw fully, replace the filter, and watch whether it freezes again. Refreezing means gauges, not towels.
3. A cracked or rusted drain pan
The pan under the indoor coil lives wet for decades; metal pans eventually rust through, and plastic ones crack with age and heat cycles. Drips that come from directly under the unit, with a clear drain line, point here. Pan replacement is pro work — access usually means opening the coil cabinet — and it's quoted after inspection rather than off a table.
4. A failed condensate pump
Basement installs that can't gravity-drain use a small pump beside the furnace. If its reservoir is full and the pump is silent (or humming without moving water), the overflow is the pump's resignation letter. Check it's plugged in and the float inside moves freely; a replacement pump is ordinary tech work. Some pumps wire into the system so a failure shuts the AC down before it floods — if yours didn't, ask about it.
5. A disconnected or badly trapped drain line
Lines get knocked loose by storage bins and holiday decorations in mechanical rooms, and a missing or incorrect trap can let air pressure hold water in the pan until it spills. Look along the whole run for separated joints or a line that's been stepped flat.
6. And the float switch that should have saved you
Most modern installs have a safety float switch that kills the AC when the drain backs up — precisely so this leak never happens. If you got the flood anyway, either the system predates one, or the switch failed or was never wired. Adding a float switch during the repair visit is inexpensive insurance against the sequel; ask for it by name.
What the fix costs
| Repair | Typical range | Notes |
|---|---|---|
| Service / diagnostic visit | $75–$150 | Often applied to the repair if you approve it on the spot |
| Drain line clearing | $75–$200 | When the clog won't pull through with a homeowner's wet vac |
| Blower motor | $400–$900 | If weak airflow is what froze the coil in the first place |
| R-410A refrigerant recharge | $200–$600 | Only with a leak check — low charge is the other freeze cause |
Stop and call instead if…
- Water is coming through a ceiling from an attic air handler and the pan is overflowing — that's active water damage by the hour.
- The coil refreezes after a full thaw and a clean filter — the cause is refrigerant or airflow at a level that needs gauges.
- The drain pan itself is rusted through or cracked — running the system "a little" just schedules the ceiling repair.
Questions homeowners ask about this
How much water should an AC produce?
A lot, on a humid day — a central AC can pull several gallons of moisture out of the air daily, all of it meant to leave through the condensate drain. You should never see it. Visible water anywhere other than the end of the drain line outside means the path is blocked, broken, or overwhelmed.
Can I pour bleach down the AC drain line?
Vinegar is the better habit: a cup of distilled white vinegar into the drain access every couple of months suppresses the algae growth that causes clogs, without attacking the pipe or the coil the way bleach can. If the line is already plugged, vacuum it from the outside end first — chemicals don't move a physical clog.
Why is the water only there on the hottest days?
Peak humidity days mean peak condensate, and a partially clogged drain can keep up at half flow but not at full. The hottest-day leak is a drain that's 80 percent blocked announcing the last 20 percent. Clear it now, while it's a nuisance instead of a ceiling.
My AC leaked once and stopped. Do I still need to do anything?
Yes — find out why before the sequel. A one-time overflow usually means the drain clogged and partially cleared, the coil froze and thawed, or the pump is intermittent. All three return, usually on a hotter day. The wet-vac pull and a filter check take twenty minutes and answer most of it.
Rather have a pro just handle it?
We connect Denver-metro homeowners with licensed local installers — free second opinions, written quotes, no pressure. Tell them what you found in the checks above and you'll skip half the diagnosis.
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