Mini-Split Leaking Water Inside? Drain Line Fix Guide

If your mini-split is leaking water from the indoor unit, the cause is almost always a clogged condensate drain line. Algae and biofilm build up inside the line, water backs up into the drain pan, and the pan eventually overflows. The fix is to clear the line. You can sometimes do this yourself with a wet/dry vacuum.

Why drain lines clog

The indoor unit produces condensation during cooling. That water drains through a small (5/8 inch) plastic tube to outside. The line slopes downward by gravity. Over time, three things accumulate:

1. Algae growth. The line is dark, damp, and warm enough year-round to grow algae.
2. Biofilm. Same bacterial community that grows on the coil and blower wheel.
3. Mineral deposits. Vermont's hard water leaves residue at fittings.

Together, these reduce the line diameter until water cannot drain fast enough during peak humidity. Backup and leak.

The wet/dry vac fix

1. Find the outdoor end of the drain line. Usually a small white plastic tube exiting your home's exterior near the outdoor unit.
2. Attach a wet/dry vacuum to the line with a snug seal (wrap a rag around it if needed).
3. Run the vac for 60-90 seconds at full power.
4. Detach. Check the indoor unit. If the leak has stopped, you are done.

This works for about 60% of drain leaks we see. Quick, free, and worth trying before calling.

When DIY does not work

If the leak persists:

• The line may be clogged with hard buildup that needs mechanical clearing
• The drain pan itself may be cracked or misaligned
• The indoor coil may be iced over (separate issue, see frozen coil)
• A pump-equipped unit (Mitsubishi P5 territory) may have a failed condensate pump

At that point, a cleaning visit will clear the line as part of standard service and replace the drain pan gasket if needed.

Vermont winter consideration

Drain lines that exit outside through an unheated wall cavity or run along an outdoor surface freeze in Vermont winter. The line plugs with ice, water backs up, the system trips. Heat tape ($20 at any hardware store) wrapped around the exposed run prevents this. We install it during a cleaning visit if you ask.

When to call us

• DIY wet/dry vac fix did not work
• Water is leaking from multiple indoor units at once
• You see a continuous drip rather than an occasional drop
• The leak started after a cold snap (probable line freeze)

Book a cleaning

Book in 60 seconds. A standard cleaning ($199) includes drain line clearing as a baseline step.

Related

• Leaking water diagnostic
• Dripping water inside
• P5 / E6 error codes