Answers / Problems & Symptoms

AC Drain Pan Overflow or Slime

Urgency: Low Last updated September 24, 2026 · Monitor My Air

Talk to an expert, free (757) 799-8535

Most likely causes, in order

1. Clogged condensate drain line (most common)
2. Improper drain slope or installation
3. Biofilm buildup in pan or line
4. Excessive condensate production
5. Secondary drain pan issues

What's going on in simple terms

Your AC pulls moisture out of the air, that's a big part of what makes your home feel comfortable. All that moisture has to go somewhere. It drips off the cold coil into a shallow pan underneath, then flows out through a drain pipe (usually a small white PVC pipe). When that drain gets clogged with slime, algae, or gunk, water backs up in the pan. If it overflows, you get water damage. If it sits there, you get a smelly, slimy mess that grows mold and bacteria.

Why this matters to you

A clogged drain pan is one of the most common causes of water damage in homes with central AC, especially when the air handler is in the attic. Overflow from above can soak ceilings, insulation, and drywall before you even notice. Even without overflow, the slime and standing water create the musty smell that comes out of your vents. Many modern systems have a safety float switch that shuts the AC off when the pan fills up, so if your AC keeps randomly turning off on hot days, this could be why.

When to bring in a pro

• If the drain line is buried in a wall or slab and you can't access it, an HVAC technician can use a nitrogen flush or specialized tools to clear it ($100–250).
• If the drain pan itself is cracked or corroded (common on older units), it needs replacement. A new pan plus labor typically runs $150–400.
• If you've had overflow that caused water damage to drywall, insulation, or ceiling materials, those wet materials need to be dried or removed. If they've been wet for more than 48 hours, mold has likely started. Have a mold professional assess ($200–500 for inspection) before just patching things up.
• If the clog keeps coming back every few weeks despite flushing, the drain line may be improperly sloped (installed flat or slightly uphill). Correcting the slope is a job for an HVAC contractor ($150–400).

Key point to remember

Your AC's drain system is gravity-powered, water flows downhill through a pipe, and that's it. There's no pump (in most systems). If things get slimy or the pipe gets blocked, water sits there and problems start. A 5-minute vinegar flush every couple of months is one of the cheapest, easiest things you can do to prevent expensive water damage and bad smells.

Related answers

• Musty Smell When AC Turns On
• Mold on AC Evaporator Coil

Education, not medical advice or a professional inspection of your property.