Most Likely Causes (Ranked)
Your AC removes 5-20 gallons of water per day from the air. This water drains through a PVC condensate line to the outside or a floor drain. Algae, mold, and dust naturally accumulate in this warm, moist line. A clog causes water to back up into the drain pan, overflow, and leak into your home. This is the #1 AC leak cause — responsible for 80%+ of indoor AC water leaks.
The primary drain pan under the evaporator coil catches condensate and directs it to the drain line. Over 10-15 years, metal pans rust through and plastic pans crack. The secondary (safety) pan catches overflow but has its own drain — if that's also clogged, water overflows into the home.
When the evaporator coil freezes (from low refrigerant, restricted airflow, or a dirty filter), ice builds up on the coil. When the system cycles off or defrosts, the massive ice volume melts faster than the drain can handle, overwhelming the drain pan. A fully frozen coil can produce 5-10 gallons of water when it thaws.
The PVC drain line can crack from impact, separate at cemented joints, or get disconnected during maintenance. Attic-mounted units are especially vulnerable because the drain line runs through the attic to an exterior wall. A disconnected line dumps water directly into the attic or ceiling cavity.
Low refrigerant charge causes the evaporator coil to run extremely cold, producing excessive condensation and eventually freezing. Before it freezes solid, it generates more condensate than normal operation. This excess overwhelms a partially clogged drain.
How to Fix It
Clear the Condensate Drain Line
$0 DIY (20 minutes)- 1Locate the drain line access — usually a PVC cleanout tee near the indoor air handler.
- 2Pour 1 cup of distilled white vinegar or 1/2 cup bleach down the drain line access. Wait 30 minutes.
- 3Use a wet/dry vacuum on the outside drain termination to suck out the clog. Put the vac hose directly on the drain pipe end and seal with a wet rag.
- 4Alternatively, use a drain gun ($10-15) to blast compressed air or CO2 through the line.
- 5Verify flow by pouring water into the drain pan and confirming it exits the outdoor drain termination.
- 6Install a drain line cleanout tee if one doesn't exist — it makes future maintenance easy.
Replace the Drain Pan
$20-50 DIY / $200-400 Pro- 1Turn off the AC system. Remove the access panel to the evaporator coil.
- 2The drain pan is under the coil. Some are removable (slide out), others are welded to the coil assembly.
- 3For removable pans: slide out, install replacement matching your unit's dimensions.
- 4For integrated pans: you can seal cracks with waterproof epoxy as a temporary fix, or an HVAC tech can replace the entire coil assembly if the pan is severely corroded.
Thaw a Frozen Coil
$0 (but find root cause)- 1Turn off the AC but leave the fan on 'ON' (not 'AUTO'). This circulates warm air over the coil to thaw it.
- 2Place towels or a shop vac at the drain pan to catch the melt.
- 3Thawing takes 2-24 hours depending on ice severity.
- 4FIND THE ROOT CAUSE: dirty filter (replace it), low refrigerant (call HVAC tech), restricted airflow (check ducts and vents).
- 5A frozen coil is a symptom — if you don't fix the cause, it will freeze again.
When to Call a Professional
An attic-mounted AC with a clogged drain can dump water into the ceiling for hours before you notice — the ceiling can collapse. Install a float switch ($15) that shuts off the AC if the drain pan fills.
Never run the AC with a dirty filter — restricted airflow causes the coil to freeze, which causes flooding when it thaws.
Mold grows in 24-48 hours in a wet drain pan. If the pan has standing water, clean with bleach before reassembling.
If the condensate drain connects to a floor drain, ensure the floor drain has a trap — otherwise sewer gas enters the home through the AC drain.
How to Prevent It
Pour 1 cup of vinegar down the condensate drain line every 3 months during cooling season.
Change the AC filter every 30-90 days depending on type.
Install a condensate drain line float switch ($15-30) — it shuts off the AC if the drain backs up, preventing water damage.
Have the HVAC system serviced annually — technicians clear the drain as part of standard maintenance.
Keep the drain pan clean — wipe with a diluted bleach solution annually.
Frequently Asked Questions
90% of the time: clogged condensate drain line. The AC removes moisture from the air (5-20 gallons/day), and that water has to go somewhere. If the drain is clogged, it goes into your home.
5-20 gallons per day depending on humidity and AC runtime. In humid climates, closer to 20 gallons. All of this should drain outside through the condensate line.
Not immediately, but turn off the cooling and let it thaw. Find the cause (dirty filter, low refrigerant, blocked vents). Running a frozen AC damages the compressor ($1,500-3,000 to replace).
$100-200 as a standalone service. It's included in most annual maintenance contracts ($150-250/year for two visits).