Why is my mini-split leaking water inside?

A mini-split leaks water inside when the water it pulls out of the air in cooling mode has nowhere to go. The usual causes are a clogged drain line, a frozen coil that is now melting, a damaged drain pan, an indoor unit hung without the right slope, or low refrigerant. Filters are the one cause an owner can rule out; the rest is a call for a licensed HVAC technician.

Where the water comes from

In cooling mode, the refrigerant in the indoor coil absorbs heat and water condenses on the coil.^[2] That water runs into a drain pan, then into the condensate drain line and outside.^[1] When any part of that path fails, the water comes out of the indoor unit instead, down the wall or onto the floor.

The mode matters. A mini-split should not leak water during heat mode,^[1] because it produces condensation during the cooling cycle, but not during heat mode.^[1] Water from the indoor unit in winter is a sign to have it checked. Water outside is a different matter: outdoor units release water when they defrost in the winter,^[7] and that is normal.

The common causes

Dirty filters and a frozen coil. If filters become severely clogged, they restrict airflow and can freeze the evaporator coil, and a frozen coil eventually drips.^[1] You may see frost on the coil behind the front panel, weak airflow, or water that starts after the unit has run for a while. Clogged filters can also cause water leakage, mold and mildew, a musty smell and poor heating or cooling.^[4] This is the one cause you can check yourself: clean the filters as your owner's manual describes, let them dry, and see whether the leak stops.

A clogged condensate line. When the drain line clogs, water backs up into the unit and drips out onto the wall or floor.^[1] Poor drainage from a clogged condensate line is one cause of mold growth in the unit.^[3] A musty smell from the indoor unit can come with it.

A cracked or rusted drain pan. In older mini-splits the drain pan can rust or crack,^[1] and the fix is that an HVAC professional services the unit and replaces the pan.^[1]

Low refrigerant. Low refrigerant can freeze the evaporator coil,^[1] which then leaks as it thaws. Trane points out that your system should not be low on refrigerant unless there is a leak or it was charged incorrectly when it was installed.^[5] Other signs are weak cooling or heating and a hiss: a whistling or hissing sound inside or outside near the unit is most likely a refrigerant leak.^[5]

Installation problems. An installer who does not mount the indoor unit correctly or give it the slope it needs to drain can leave it tilted forward, so water overflows and drips or comes out through the air vents.^[1] Trane notes that this is more common in DIY installations.^[1] If a new system leaks from the start, call the installer first.^[1]

A failed condensate pump. Condensate pumps are most common on ceiling cassettes, which do not drain by gravity the way wall-mount and floor-mount units do.^[3] If the pump stops, the pan overflows.

What to do right now

Turn the unit off at the remote or wall controller, and put towels or a bucket under it. Running the system with a frozen coil can damage it,^[1] and a blocked drain only fills further while it runs. Move furniture and electronics away from the wall, and note when the leak started and which mode the unit was in.

Leave the drain line and the inside of the unit to a technician. Trane advises that if the unit is still under warranty, have an HVAC professional take it apart and clear the condensate line and drain pan, because some DIY maintenance and repairs can void the warranty.^[1] Do not pour bleach or other harsh chemicals into the line: Carrier warns that harsh chemicals can damage system components,^[6] and that a professional technician is best equipped to clear clogs safely.^[6]

Do not wait it out. ENERGY STAR warns: "A plugged drain can cause water damage in the house and affect indoor humidity levels".^[8] Moisture that does not drain, combined with dust and pet hair, gives mold a place to grow.^[3]

What the technician does

A technician checks the filters and coil for ice, tests the drain path from the pan to the outside, looks at the pan and any condensate pump, and checks that the unit is level and sloped to drain. If the coil froze without a filter or airflow problem, they check the refrigerant charge and look for a leak. Federal rules apply there: technicians who maintain, service, repair or dispose of equipment that could release ozone-depleting refrigerants must be certified^[9] under EPA's Section 608 program, and only a trained HVAC professional can detect and fix a refrigerant leak and safely perform a refrigerant recharge,^[5] in Trane's words.

When to call a licensed pro

Call once the filters are clean and the leak continues, straight away if the coil is iced, you hear hissing, the system is new, or water is reaching the ceiling below or the inside of a wall. In Virginia, no individual may work, or offer to work, as a tradesman unless licensed, unless working under the supervision of a licensed tradesman,^[11] and the Board for Contractors requires a master tradesman licence as a condition of licensure for HVAC contractors.^[10] Check the licence before you book, and if the system is under warranty, call the installer first.

To keep it from happening again, read how often a mini-split should be cleaned, and for ducted systems see how a clogged AC condensate drain is cleared.

Loclis lists DPOR-licensed HVAC businesses with their licence number and class. Browse mini-split services, find HVAC businesses in Blacksburg, or read more heating and cooling answers.

To get it fixed, describe the leak once and licensed local HVAC businesses quote the repair.