A clogged condensate drain line is the single most common cause of a central air conditioner leaking water inside the house — dust, dirt, sludge, or mold buildup blocks the line, and the water that should drain outside backs up indoors instead. A dirty air filter causing a frozen evaporator coil, a damaged drain pan, and a failed condensate pump are the next most common causes.
Check the condensate drain line first
- Locate the drain line
Usually a PVC pipe running from the indoor air handler to an exterior drain point, often near the outdoor condensing unit.
- Check for visible blockage at the accessible end
Algae, mold, and sludge commonly build up inside the line over time, especially in humid climates.
- Clear the line if a shutoff/cleanout access point exists
Many systems have a dedicated access point for exactly this kind of clearing — check your system’s documentation before attempting to clear it yourself.
This single blockage is behind the majority of indoor AC leaks, and clearing it resolves the issue without needing any other component replaced.
A dirty filter can cause a frozen coil
A heavily clogged air filter restricts airflow across the evaporator coil. Reduced airflow lets the coil get too cold and ice over — when the system later cycles off, or when warm air reaches it again, that ice melts all at once, often producing more water than the drain system is designed to handle in a short period, which shows up as a leak. Check and replace the filter on a regular schedule to prevent this from recurring.
A damaged or rusted drain pan
| What you notice | Likely cause |
|---|---|
| Water appears right under the indoor air handler | Possible drain pan damage or rust-through |
| Water appears along the drain line’s path | Points more toward a line blockage or disconnection |
| Leak coincides with the system just finishing a cycle | Consistent with a frozen coil that has just melted |
The drain pan collects condensation before it flows into the drain line. On older systems, this pan can rust through or crack, letting water escape directly rather than reaching the drain line at all. This generally needs a technician to access and replace.
A failed condensate pump
Systems where gravity alone cannot carry condensation to an exterior drain point (common in basements or certain installations) rely on a condensate pump to actively move the water out. If this pump fails, water has nowhere to go and simply collects in the tray until it overflows — check that the pump is running and listen for it actually cycling during operation.
A disconnected drain line
Over time, a drain line connection can work loose or come apart entirely, particularly if it was not solvent-welded or securely clamped during installation. Water then spills wherever the disconnection is rather than reaching the intended exterior drain point — trace the line’s full path for any obvious gap or separation.
Improper installation
If leaking started shortly after a new installation, a poorly configured condensate drain line — wrong slope, undersized piping, or a missing trap where one was needed — is worth suspecting before assuming a component has already failed. This is an installation issue to raise with whoever performed the work rather than a wear-and-tear repair.
When to call an HVAC technician
Low refrigerant causing a frozen coil is a less common cause of this specific symptom but is worth having a technician check if the filter is confirmed clean and the drain line is confirmed clear — refrigerant work requires proper certification and equipment, not a DIY fix. A damaged drain pan or a failed condensate pump are both parts-replacement jobs best handled professionally as well.
Stop and call a qualified appliance technician or electrician if you find scorched or melted wiring, smell burning, see water that has reached electrical components, or if the fault returns immediately after a repair. Never bypass a safety device — thermal fuses, door interlocks and flood switches fail for a reason, and defeating one converts a broken appliance into a fire or flood risk.
