AC and HVAC Water Leaks: Common Causes and When to Call a Professional

Air conditioning and HVAC systems are one of the more overlooked sources of indoor water damage — often because the leak starts small, in a location (an attic, closet, or ceiling void) that isn't checked regularly.

Cause 1: Clogged Condensate Drain Line. As an AC unit cools air, it produces condensation that is supposed to drain away through a condensate line. Over time, algae, dust, and debris can clog this line, causing water to back up and overflow — often through a secondary drain pan that eventually overflows too.

Cause 2: Damaged or Rusted Drain Pan. The pan beneath an air handler or evaporator coil that collects condensation can crack or rust through over time, allowing water to leak directly onto the surrounding structure rather than draining properly.

Cause 3: Frozen Evaporator Coil. A dirty air filter or low refrigerant can cause the evaporator coil to freeze. When the system is turned off (or the ice naturally melts), that frozen buildup turns into a larger-than-normal amount of water all at once — sometimes more than the drain pan can handle.

Cause 4: Broken Condensate Pump. In systems where the condensate can't drain by gravity alone (such as a basement furnace/AC coil), a small pump moves the water out. If that pump fails, water backs up at the source.

Why This Type of Leak Often Goes Unnoticed Longer. Because HVAC equipment is often in attics, closets, or basements that aren't part of daily living space, a slow leak here can continue for weeks before staining appears on a ceiling below — by which point insulation, drywall, and framing may already be significantly affected. This is a common source of the hidden water damage many homeowners eventually discover.

What to Do If You Notice HVAC Water Damage. Turn off the system to stop it from continuing to run and produce more condensation, and check the area beneath and around the unit for standing water or staining. If water has already reached drywall, insulation, or flooring, professional water extraction and drying is typically needed even after the mechanical issue itself is fixed by an HVAC technician.

A yearly HVAC maintenance check — including clearing the condensate line — is one of the simplest ways to prevent this type of water damage before it starts.