Aircon Indoor Unit Leaking Water – Causes and Fixes
I see this fault constantly on wall-mounted split systems, especially in hot and humid climates.
The indoor unit is cooling, but after a while water starts dripping from the front of the aircon, running down the wall, or leaking from one side of the unit.
What are the most common causes?
1. Blocked condensate drain
This is probably the first thing to check.
As the evaporator cools the room air, moisture condenses on the coil and runs into the drain tray.
If the drain pipe becomes blocked with:
Dust
Slime
Algae
Mould
Insects
Dirt
the water level rises until it eventually overflows from the indoor unit.
A partially blocked drain can also cause intermittent leaking rather than a constant leak.
2. Dirty evaporator coil
A heavily contaminated evaporator can affect both airflow and condensate drainage.
Dust and dirt can cause water to track incorrectly across the coil rather than draining cleanly into the tray.
A dirty coil can also reduce airflow enough to cause the evaporator temperature to fall too low.
3. Dirty air filters
Blocked filters reduce airflow across the evaporator.
Low airflow can cause the evaporator to become excessively cold and, in some cases, ice up.
When the ice eventually melts, the drain tray may not be able to handle the sudden amount of water and the indoor unit can start leaking.
If the aircon is leaking and airflow feels weak, checking the filters is an easy first step.
4. Evaporator coil freezing
If there is visible ice on the coil or refrigerant pipework, the water leak may actually be a secondary problem.
Possible causes of evaporator icing include:
Dirty filters
Dirty evaporator coil
Low indoor fan speed
Faulty indoor fan motor
Restricted airflow
Refrigerant shortage
Refrigerant control problem
Sensor fault
When the ice melts, the water can overflow the drain tray.
If the coil is freezing, it is important to find out why rather than simply clearing the drain.
5. Drain pipe has incorrect fall
The condensate drain normally relies on gravity.
If the drain pipe:
Runs uphill
Has a sag in it
Has insufficient fall
Is kinked
Has been incorrectly installed
water may sit inside the pipe instead of draining away.
This can eventually cause the tray to overflow.
A drain can look perfectly clean but still fail because of poor pipework.
6. Indoor unit is not level
The indoor unit needs to be mounted correctly so condensate flows towards the drain outlet.
If the unit is tilted away from the drain connection, water can build up at the opposite end of the tray and leak from the casing.
This is worth checking if the aircon has leaked since installation.
7. Cracked or damaged drain tray
Older units can develop cracks in the condensate tray.
This can be harder to spot because the drain itself may be perfectly clear.
Water may leak from underneath the coil or appear to come from an unusual part of the indoor unit.
8. Drain hose disconnected
Sometimes the drain hose has simply come loose from the indoor unit.
This can happen after servicing, cleaning or movement of the pipework.
If the leak suddenly starts immediately after maintenance, the drain connection is worth checking.
9. High humidity
In tropical climates, an air conditioner can remove a significant amount of moisture from the air.
Higher humidity means more condensate.
This does not normally cause a properly installed system to leak, but it can make a partially blocked drain or poor installation show up much faster.
10. Sweating on pipework or insulation
Not every water leak is coming from the condensate tray.
Cold refrigerant pipework can create condensation if the insulation is:
Damaged
Missing
Split
Poorly sealed
This water can drip down the wall and look like a drain problem.
Check where the water is actually coming from before assuming the drain is blocked.
Quick troubleshooting order
If I was checking a leaking wall-mounted split aircon, I would normally start with:
Check and clean the filters.
Check airflow from the indoor unit.
Inspect the evaporator for ice.
Check the condensate tray.
Check the drain outlet.
Flush or clear the drain.
Check the drain pipe has continuous fall.
Check the indoor unit is mounted correctly.
Inspect the drain tray for cracks.
Check refrigerant pipe insulation for sweating.
A leaking indoor aircon is not always just a blocked drain.
If the evaporator is freezing or airflow is poor, clearing the drain may only fix the symptom temporarily.
Has anyone found a particular cause that seems especially common on wall-mounted split systems in tropical climates?
