# How does a mini split condensate drain work?

> How a mini split condensate drain works: gravity lines, pumps and float switches, traps, outdoor drains that freeze in a Portland winter, and why heads leak.

URL: https://firlandsportlandminisplits.com/guide/mini-split-condensate-drain/

Learn

Where the water goes, gravity or pump, and the common reasons a head starts to drip.

Last Updated: November 12, 2026

The short answer

When a mini split cools, water condenses on the indoor coil and drips into a pan inside the head. A small drain line carries it outside, either by gravity, falling steadily all the way, or with a condensate pump when the head sits where water can't fall to the outside. Most leaks start with a blocked, flat or kinked drain.

Key takeaways

- Every head makes water when it cools, and the outdoor unit makes water when it heats.
- Gravity drains need a steady fall the whole way out.
- A head on an inside wall, a cassette or a ducted unit usually needs a pump.
- A float switch shuts the head off before a blocked drain overflows.

Water on the wall under a head is one of the most common mini split problems, and nearly every case traces back to the drain. At [Firlands Portland Mini Splits](https://firlandsportlandminisplits.com/) we plan every drain at the first visit and test each one with water before we leave. Here’s how drains work, and why they fail.

## Where the water comes from

A mini split head cools the room by running air over a cold coil. Moisture in that air condenses on the coil, the same way a cold glass sweats, and drips into a pan under it.

The pan has an outlet, and the drain line takes the water from there to the outside. In heat mode the process flips: the outdoor coil gets cold, frosts up, and the unit melts it off in a defrost cycle. That water drains from the base of the outdoor unit, which matters in a freeze.

## Gravity or pump

A gravity drain is the simplest and most reliable. A pump is the answer when water can’t fall to the outside on its own.

|  | Gravity drain | Condensate pump |
| --- | --- | --- |
| When it fits | Head on an outside wall, drain can fall the whole way | Head on an inside wall, cassettes, most ducted units, basements |
| How it works | Water runs downhill to the outside | A small pump lifts water to a point where it can drain |
| What can fail | Blockage, a flat or uphill spot, a kink | Blockage, a failed pump, a stuck float, a tripped breaker |
| Noise | None | A faint hum when it runs |
| Added cost | None | Typically $150 to $500 at install |

### Getting gravity right

The line needs a steady downward slope from the head all the way out; the mechanical code sets at least 1/8 inch of fall per foot. On an outside wall, it runs down in the line-hide beside the refrigerant lines. A single flat spot holds water, grows slime and backs up.

### When a pump is the answer

A head on an inside wall, a [ceiling cassette](https://firlandsportlandminisplits.com/ceiling-cassette-mini-split-installation/) or a [ducted air handler](https://firlandsportlandminisplits.com/ducted-mini-split-installation/) usually can’t drain by gravity. A small pump, mounted in the line-hide, in the head or beside the air handler, lifts the water to a point where it can fall away.

## Float switches and traps

A float switch is the backstop. If water rises in the pan or the pump’s reservoir, the switch shuts the head off before the water spills.

### Float switches

Most pumps include one, wired to stop the head when the pump can’t keep up. On a cassette or ducted unit above a finished ceiling, we treat a working float switch as essential. A head that stops cooling for no clear reason, with a full pan, is often the switch doing its job.

### Traps

Most wall heads drain straight out with no trap. A trap matters when the drain ties into a plumbing line, where it stops sewer gas coming back up. Some ducted air handlers also need one, because their fan pulls on the drain; the installation manual says which.

## Outdoor drains in a Portland freeze

The outdoor end of the drain, and the outdoor unit’s own defrost water, both have to cope with freezing nights. Portland’s airport averages about 32 nights a year at or below freezing; Hillsboro averages about 64.

### What we do about it

- **End the drain where it can’t freeze shut.** We stop the line above a gravel strip or a downspout, not buried in soil or a puddle.
- **Keep it off paths.** Water from a drain or a defrosting unit freezes on concrete. We route it away from steps and walkways.
- **Lift the outdoor unit.** A stand or bracket lets defrost water fall clear instead of freezing under the base ([placement guide](https://firlandsportlandminisplits.com/guide/mini-split-placement/)).
- **Heat cable where needed.** On a long outdoor run in a cold, shaded spot, a small heat cable can keep the line open.

## Why a head starts to leak

When a head drips, the cause is usually one of a handful of things. Most are cheap to fix if caught early.

1. **Slime in the line.** Algae and dust grow in the pan and the line, especially after a smoky or dusty summer ([mini splits and wildfire smoke](https://firlandsportlandminisplits.com/guide/do-mini-splits-filter-wildfire-smoke/)).
2. **A flat or uphill section.** Often from a line pushed or bent after install, or one never given enough fall.
3. **A kinked or crushed line.** Behind the head, in the line-hide, or under a deck.
4. **A failed pump.** Pumps wear out; a noisy or silent pump is a sign.
5. **A head that isn’t level.** The pan tips away from its outlet.
6. **A frozen coil.** A dirty filter or low refrigerant can freeze the coil; when it melts, it overflows the pan.
7. **A blocked outdoor end.** Ice, mud, leaves or an insect nest at the outlet.

### What you can check first

Turn the head off. Check whether the filter is clogged, and look at the outside end of the drain for a blockage. Don’t push anything up the line from the head end; that can crack the pan’s outlet.

## Fixing and preventing drain problems

Clearing a drain is one of the most common repairs, and a yearly cleaning catches most problems before they drip.

Firlands’ typical range for most mini split repairs, drain clearing included, is $150 to $1,000; a simple drain clearing sits toward the low end. A maintenance visit typically runs $150 to $250 for one outdoor unit and one head, and includes flushing the pan and line. See [mini split repair](https://firlandsportlandminisplits.com/mini-split-repair/) and [cleaning and maintenance](https://firlandsportlandminisplits.com/mini-split-cleaning-maintenance/).

### On a new install

Every drain is tested with water before we leave, gravity or pump. We pour water into each pan and watch it come out the far end.

A head in a [basement](https://firlandsportlandminisplits.com/basement-mini-split-installation/) almost always needs a pump, because there’s nowhere lower for the water to go. The written estimate says which drains are gravity and which are pumped. The [installation process guide](https://firlandsportlandminisplits.com/guide/mini-split-installation-process/) shows where that test falls.

## Questions about this guide

### Why is water dripping from my mini split head?

Most often the drain line is blocked with slime or debris, the line has a flat or uphill spot, or the pump has failed. A dirty filter or low refrigerant can also freeze the coil, and the ice melts faster than the pan can drain. Switch the head off and call us.

### How much water does a mini split make?

It depends on how humid it is and how hard the head works. In Portland's dry summers a head makes far less than in a humid climate, but a working head still makes a steady trickle whenever it cools, and more on a muggy August day. That's why every drain needs a clear path out.

### Can the drain go into a sink or a sewer line?

It can tie into a plumbing drain, but only through a proper trap or an air gap so sewer gas can't come back up the line, and where the plumbing code allows it. Most Portland installs drain outside to the ground or a downspout instead.

## Related service

- [Service**Repair** Mini split repair in Portland for leaks, iced coils and systems that stop heating or cooling: a test first, then the repair.](https://firlandsportlandminisplits.com/mini-split-repair/)

## More guides

- [Learn**Where should a mini split go?** Head placement inside, the outdoor unit outside, and the Portland rules and weather that decide both.](https://firlandsportlandminisplits.com/guide/mini-split-placement/)
- [Decision Guide**Should you get a ceiling cassette or a wall-mounted mini split?** The four kinds of indoor unit, what each needs from the house, and which rooms suit which.](https://firlandsportlandminisplits.com/guide/mini-split-cassette-vs-wall-mount/)
- [Learn**Do mini splits filter wildfire smoke?** What a ductless head does with smoky air, what it can't catch, and how to keep one room clean in a Portland smoke week.](https://firlandsportlandminisplits.com/guide/do-mini-splits-filter-wildfire-smoke/)

## Get an estimate for your house

(971) 458-4515