An indoor unit works by passing warm room air over a cold coil. Moisture in that air condenses on the coil exactly as it does on a cold glass in summer, collects in a tray beneath it, and leaves through a drain to the outside.
So water itself is completely normal — several litres of it on a humid day. What is not normal is where it ends up. Dripping from the bottom edge of the unit, running down the wall or marking the skirting all mean the condensate has overflowed the tray rather than draining away.
Why the tray overflows
Four causes account for the overwhelming majority of indoor leaks.
The drain is blocked
Dust from the room mixes with condensate to form a slime inside the drain line, and biofilm gradually closes the bore until water backs up.
- Most common cause by a wide margin
- Typically appears in the first hot spell of the year
- Clearing the line and cleaning the tray resolves it
The drain has no fall
Condensate drains by gravity on most installations. A pipe that dips, kinks behind the unit or runs level will hold water and eventually spill back.
- An installation fault rather than a maintenance one
- Often shows up in the first season after fitting
- Needs the run corrected, or a condensate pump where fall is impossible
A filthy filter or coil
When airflow drops, the coil runs colder than intended and can ice up. Melting ice sheds more water than the tray was designed to catch.
- Filters are washable and designed to be cleaned regularly
- Ice on the coil is a symptom worth reporting, not just clearing
- A furred coil needs a proper clean rather than a rinse
Low refrigerant charge
An undercharged system runs the coil too cold, which frosts and then floods. This one is a refrigerant fault presenting as a water problem.
- Usually paired with poor cooling and longer run times
- Refrigerant work is F-Gas regulated and not a homeowner task
- We hold REFCOM registration for F-Gas refrigerant work
What to do the moment you see water
The goal is protecting the room and the electrics, not fixing the unit.
- 1
Switch the unit off at the controller
No cooling means no more condensate. The puddle stops growing immediately, which buys you time.
- 2
Isolate it if water is near electrics
Use the isolator or breaker for the unit. Water and live terminals in the same wall box is the one genuinely urgent part of this.
- 3
Catch and mop
A towel and a shallow tray under the unit protect flooring, plaster and anything beneath it while you arrange a visit.
- 4
Check the outdoor drain point
If the pipe outside is bone dry on a hot day while water appears indoors, the blockage is confirmed rather than suspected.
- 5
Wash the filters
Slide them out, rinse them, dry them fully and refit. This is the one maintenance task designed for owners, and sometimes it is the whole answer.
- 6
Report what you saw
Whether ice was present, whether cooling had got weaker, and when the drip started all shorten the diagnosis considerably.
Why the same unit leaks again next summer
Condensate drains do not stay clean on their own. Room dust keeps arriving, and the damp interior of a drain line is an ideal place for biofilm to grow, so a line cleared once will slowly close again.
That is what routine attention is for. Cleaning the tray and flushing the drain as part of a service keeps the mechanism working rather than waiting for the wall to tell you it has stopped.
It also matters because water damage is cumulative and hidden. Plaster behind a unit can be saturated for weeks before the paint blisters, and by then the repair is decorating rather than plumbing.
Things a leak is almost never caused by
- Running the system too cold — a low set point may increase condensate, but a healthy drain copes with it
- Rainwater getting in through the wall sleeve, which is sealed on a correct installation
- The unit being old, on its own; drains block on new installations just as readily
- Refrigerant dripping out, which does not happen as a liquid on your wall
- Using the unit overnight, which changes the volume of condensate rather than where it should go
Water on the wall is a drain telling you it has stopped working, and it is usually one of the cheaper faults an air conditioning system produces. Switch it off, protect the room, get the drain cleared — then keep it clear, and next summer will pass without a towel on the floor.
