Why is my mini-split leaking water from the indoor unit?
Updated August 2026
Water dripping or streaming from the indoor head of a ductless mini-split is a clogged condensate drain line letting the pan overflow in the large majority of cases. The line drains by gravity, so algae, dust, and biofilm build a plug that backs water up until it spills out the bottom of the unit and runs down the wall. Clear the line first: the other causes below are worth checking only after the drain flows freely again.
Safety first: water running down drywall or into a floor cavity does real damage fast: a saturated wall can start growing mold within 24–48 hours, and a stained ceiling below an upstairs head is an expensive repair. Kill the unit at the breaker or remote, put a towel and bucket under the head, and dry the wall before you chase the cause. If the leak sits above an outlet or light fixture, leave that circuit off until everything is fully dry.
Causes, most likely first
| Cause | How likely | The fix | Typical cost |
|---|---|---|---|
| Clogged condensate drain line | Most common | Flush the line with a wet/dry vac at the outdoor end, or pour a cup of warm water and white vinegar through the indoor access to clear the algae plug that gravity drainage invites. | Free DIY, $75–250 pro |
| Improper drain slope from install | Common | The line needs a continuous downhill fall of about 1/4 inch per foot. A sagging or back-pitched run holds water and overflows, so it has to be re-hung correctly. | $150–400 |
| Dirty filters freezing the coil | Common | Clogged mesh filters starve airflow, the coil ices, then dumps a sheet of water when it thaws. Rinse the filters and let the coil dry before restarting. | Free |
| Cracked or dislodged drain pan | Occasional | A brittle or bumped plastic pan drips behind the head even with a clear line. A tech reseats or replaces it. | $150–350 |
| Failed condensate pump (if fitted) | Rare | Heads mounted below the drain exit rely on a small lift pump. A stuck float or dead motor backs water up into the unit. | $50–200 |
Check these first
- Turn the mini-split off and wipe the indoor unit dry so you can see where the water actually originates: the bottom seam, the pipe chase, or the filters.
- Pop the front cover and pull the mesh filters. If they are furred with dust, rinse them under the tap and let the coil thaw and dry before restarting.
- Find where the drain line exits the wall outside and check that water dribbles out when the unit runs. No flow there means the plug is between the head and the exit.
- Look along the line's path for a sag or an uphill kink. The whole run should fall gently toward the outside.
- If the head sits below the drain exit and uses a condensate pump, listen for the pump cycling and check its reservoir for gunk.
Prevent it next time
Mini-split leaks are a cleaning problem far more than a mechanical one. Rinse the mesh filters on a monthly-to-quarterly schedule so the coil never ices, and flush the drain on the recommended interval to keep the algae plug from forming. A yearly deep clean clears the coil and blower that feed sludge into the line. Those three habits cost a few dollars a year and head off nearly every indoor leak.
When to call a pro
Call an installer when the line is clear but the head still drips (that points to slope or a cracked pan behind the unit, which means unmounting it), when the coil ices repeatedly despite clean filters (a low-refrigerant sign), or when water has already reached an outlet or the floor. A drain clearing runs $75–250, re-hanging a mis-sloped line $150–400, and a pan or pump replacement $50–350. If the unit is past 12–15 years and needs pan work, price a replacement first.
Most of the causes above are maintenance problems wearing a disguise. OnOtto keeps the boring prevention on schedule so the 2 a.m. version doesn't happen.
Stop chasing symptoms, free for 30 daysFrequently asked questions
Can I keep running my mini-split while it leaks?
Not for long. Every hour it runs, it pushes more water into the wall, and drywall that stays wet 24–48 hours starts to grow mold. Shut the head off at the remote or breaker, set a bucket and towel underneath, and dry the surface. You can flush the drain and rinse the filters the same day, then restart and watch whether the drip returns. If it does, leave it off and call a tech rather than soaking the wall further.
How do I flush a mini-split drain line myself?
Find where the clear or white line exits the wall outside. The easiest method is to hold a wet/dry vacuum to the end, seal around it with a rag, and pull the clog out in a few seconds. From inside, pour a cup of warm water mixed with a little white vinegar through the drain access to break up algae. Avoid bleach in aluminum-coil systems, since vinegar is gentler. Repeat monthly through the cooling season and the line rarely clogs.
Why does my mini-split leak only on humid days?
Humidity is the reason a marginal drain finally overflows. The wetter the air, the more condensate the coil pulls off, sometimes several quarts an hour on a muggy afternoon. A line that is half-blocked or slightly back-pitched can keep up on a dry day but not when the volume doubles, so it spills. The fix is the same as any clog: clear the line and confirm it falls downhill the whole way. Persistent humid-day leaks with a clean line usually mean the slope is wrong.