Split System Air Conditioner Leaking Water Inside — What Is Actually Causing It

Water dripping from the indoor unit of your split system air conditioner is alarming but not unusual. The indoor unit produces condensation as part of normal operation. That condensation is supposed to drain away through a pipe that exits through the wall to outside. When it does not drain properly, water backs up and finds another way out, usually through the bottom of the indoor unit and onto whatever is below it.

Here is a clear breakdown of the causes, from simplest to most complex.

Blocked Condensate Drain Line

This is the most common cause by a significant margin. The condensate drain is a small diameter pipe, typically 20 to 25mm, that carries the moisture collected from the indoor coil to the outside. Over time, algae, mould, dust, and debris accumulate inside this pipe and reduce the flow. Eventually it blocks completely, the condensate tray overflows, and water comes out the front or bottom of the unit.

The fix is to clear the drain line. This can sometimes be done by pouring a diluted bleach and water solution down the drain access point at the indoor unit, or by using a wet/dry vacuum to suck the blockage out from the external end. If the blockage is significant or the pipe layout prevents gravity drain entirely, a technician will need to address it properly.

Our Sydney air conditioning team and Melbourne air conditioning team both attend to blocked condensate drains as a routine service call.

Dirty or Blocked Air Filter

When the return air filter is heavily blocked, airflow over the evaporator coil drops. The coil gets colder than it should because less heat is being transferred to it. In extreme cases, the coil actually freezes over. When the system eventually defrosts, either naturally when you turn it off or when the ice accumulation blocks the coil entirely, the sudden large volume of meltwater overwhelms the condensate tray and overflows.

You will often notice this fault as the unit producing little cooling output before the leaking starts, and the coil may be visibly iced over if you open the front panel.

Clean the filter. The filter should be cleaned every four to six weeks during heavy use periods in Sydney and Melbourne. If the coil has iced over, turn the system to fan-only mode to let it defrost before cleaning the filter and restarting in cooling mode.

Incorrectly Installed or Tilted Indoor Unit

The indoor unit needs to be mounted with a slight tilt toward the drain outlet so condensate flows in the correct direction. If the unit was installed incorrectly, or if the mounting bracket has shifted over time due to wall movement, the condensate tray may be sloping the wrong way and directing water toward the front of the unit rather than the drain outlet.

This is a physical installation issue that requires the unit to be remounted correctly. It is not a fault with the air conditioner itself, but it does need to be rectified by a licensed refrigeration mechanic or air conditioning installer.

Cracked or Dislodged Condensate Tray

The condensate tray is a shallow plastic tray inside the indoor unit that collects condensation before it drains. Over years of thermal cycling, the plastic can crack. The crack may be hairline and invisible from the outside but sufficient to allow water to escape before it reaches the drain outlet.

If your unit has been inspected and the drain line is clear but water is still leaking, have the condensate tray inspected directly. Replacement trays are available for most common brands sold in Australia.

Low Refrigerant Causing Coil Freeze

Low refrigerant causes the evaporator coil to run significantly colder than it should, potentially below zero degrees Celsius even in warm conditions. A coil running at sub-zero temperatures will freeze over. The resulting ice formation and subsequent melt produces more water than the drainage system is designed to handle.

Low refrigerant is not a consumable, it means there is a leak in the refrigerant circuit. This must be located, repaired, and the system recharged by a licensed refrigeration mechanic.

Frequently Asked Questions

Q1: Is water dripping from my split system dangerous?
The water itself is not dangerous, but the underlying cause may warrant attention. A blocked drain or icy coil is a maintenance issue. Water dripping onto electrical wiring, switchboards, or other electrical installations in the area below is a genuine hazard. If water from the unit is near any electrical components, turn the system off and call a technician before using it again.

Q2: How do I clean the condensate drain on my split system myself?
Locate the drain line exit point on the outdoor wall of your property. Use a small wet/dry vacuum placed over the end of the drain pipe to suck debris out. Alternatively, remove the indoor unit’s front panel to access the condensate tray and pour a 50/50 bleach and water solution slowly into the tray to kill algae and flush the drain line. Run the unit in fan-only mode afterwards to flush any remaining solution through.

Q3: How often should a split system be serviced to prevent leaks?
Annual servicing is recommended for most residential split systems in Sydney and Melbourne. A service visit includes cleaning the filter, coil, and drain line, checking refrigerant level and pressures, and inspecting all electrical connections. Units that operate in dusty environments or that are used heavily year-round may benefit from a six-monthly service.

Q4: Why does my split system only leak in summer and not in winter?
In cooling mode, the indoor coil is cold and produces significant condensation, especially on humid summer days. In heating mode, the indoor coil is warm and produces no condensation. The drain system is only under load in summer. This is why leaks appear seasonally and can give the impression that the system is fine outside of summer.

Q5: My split system is dripping onto the wall below. Will this cause damage?
Yes, over time. Persistent water running down an interior wall can penetrate the plasterboard, cause mould growth, and eventually damage the internal wall structure. If water has been leaking down the wall for more than a week or two, have the wall inspected for moisture penetration once the unit is repaired. A prolonged leak behind plasterboard creates ideal mould conditions.