Why Is My AC Leaking Water?
A puddle of water shows up under the indoor unit or near the AC, and worry sets in right away. The good news: the most common cause is also the simplest to fix.
Why an AC normally produces water
An air conditioner removes humidity from the air by condensing it on its cold coil, exactly like the fog that forms on a glass of ice water. This water normally drains through a tube to the outside or a floor drain — the problem happens when that drainage path gets blocked.
The most common causes
| Cause | Distinctive sign |
|---|---|
| Clogged condensate drain (algae, dirt) | The most common cause, especially after years without maintenance |
| Cracked or overflowing drain pan | Leak visible right at the unit's base |
| Very dirty filter causing coil freeze that later melts | Water shows up after the system stops, not during |
| Poorly leveled installation | Persistent leak despite a clear drain |
What you can check yourself
Locate the drainage tube, usually a PVC pipe near the indoor unit or exiting outside, and check whether it's visibly blocked or water seems to back up rather than flow freely. A very dirty filter can also cause coil freeze that, when it melts, produces more water than the drain can normally handle — replacing a clogged filter sometimes fixes the problem on its own.
How to clear a condensate drain yourself
For many systems, pouring a water-and-vinegar mix into the drain's access opening, if your system has one, can dissolve the algae and dirt buildup that most often causes the blockage. This simple step resolves a good share of cases without specialized tools.
When to call a technician
If the drain seems clear but the leak persists, or you notice frost on the coil before water appears, the problem goes beyond a simple blockage and warrants a professional diagnosis — possibly low refrigerant or a damaged drain pan needing replacement.
Frequently asked questions
Can I keep using the AC while it's leaking?+
It's better to shut it off until the problem is resolved to avoid further water damage, particularly if the unit is indoors.
How often should I clean the condensate drain?+
Once a year, ideally in spring before cooling season, prevents most blockages.
Can the leaking water damage my flooring?+
Yes, a prolonged leak can damage the flooring or ceiling below — better to fix the problem quickly rather than wait.
Is vinegar safe for all systems?+
Generally yes for basic maintenance, but check your unit's manual or ask a technician if unsure.
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More to know before your project.
By Marc Tremblay — Certified HVAC technician, 12 years of experience in Quebec
Updated on July 22, 2026