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Most "air con is leaking water" guides treat it as an HVAC problem. They're missing the point. In around 75% of cases the cause is a blocked or improperly installed condensate drain - which is plumbing, not refrigeration. This guide explains the six real causes of an AC water leak, where the water is supposed to go, why Sydney coastal homes are especially vulnerable, and how to tell whether you need a plumber, an HVAC technician, or both.
It's a hot afternoon in Maroubra. You walk into the living room and there's a brown circle expanding across your ceiling. The split-system air con upstairs has been running all day, and now water is dripping onto the rug.
Before you panic-Google and book the first air-con company that returns your call, here's what's actually happening - and why the right first call is often a plumber, not an HVAC tech.
Every air conditioner is, in part, a dehumidifier. When warm humid air passes over the cold evaporator coil inside the indoor unit, the moisture in the air condenses on the coil - exactly the same way water beads on a cold glass on a humid day. That water has to go somewhere.
In a healthy system, the water collects in a tray (the condensate pan), runs out through a small pipe (the condensate drain), and exits the building - either dripping outside, draining into a stormwater point, or piped into your home's waste plumbing under AS/NZS 3500.
In an unhealthy system, that water ends up on your ceiling, in your wall, or pooling under the indoor unit. Here's why.
This is by far the most common cause, and the one most articles understate. The condensate drain is a small-diameter pipe (typically 16-25mm) that runs from the indoor unit's drip tray to an outside discharge point. Over time it accumulates:
Once the drain is blocked, the drip tray overflows and water finds the path of least resistance - usually straight down through your ceiling. This is a drainage problem. It's solved by clearing the line (a plumbing job), not by recharging refrigerant or replacing components inside the AC unit.
After 8-12 years, the metal or plastic drip tray inside the indoor unit can crack, warp, or rust through. Water bypasses the drain entirely and falls straight into the unit cavity. Replacement is an HVAC job, but the diagnosis often happens during a plumber's drain inspection.
A condensate drain needs gravity to work - it must fall at least 1:100 from the indoor unit to the discharge point. If the line was installed too flat, or with a low point that holds water, you'll get intermittent overflow even when the line is "clear". This is a common issue in retrofit installs in older Eastern Suburbs apartments where the drain has been threaded through walls and across roof spaces.
If airflow over the coil drops - usually because the filter is clogged or the fan is failing - the coil temperature can drop below freezing. Ice forms on the coil instead of running off as water. When the unit cycles off, the ice melts all at once, and the drip tray overflows faster than the drain can handle.
Solution: clean or replace the filter, and have an HVAC technician check fan operation.
Same mechanism as a frozen coil, but caused by a slow refrigerant leak rather than an airflow problem. If your AC is also blowing weaker than usual or struggling to cool, low refrigerant is likely. This needs an HVAC technician - refrigerant work is licensed and you can't DIY it.
Less common, but in older Eastern Suburbs homes with ducted systems, condensate drains often run through wall cavities and ceiling spaces. They can be knocked loose by tradespeople doing unrelated work, eaten by rats, or crushed by storage in the roof. If a previously well-functioning AC suddenly starts leaking after work has been done in your home, this is a likely cause.
Under AS/NZS 3500, condensate drains must discharge to one of:
They cannot legally discharge:
A surprising number of Eastern Suburbs AC installations don't comply, particularly in older apartment buildings where multiple split systems have been retrofitted over decades. If your AC has been leaking, getting a plumber to inspect the condensate discharge is a good chance to bring the install up to code at the same time.
The leak point is usually the indoor wall unit, and water tends to drip from the bottom edge of the unit onto whatever is below - floor, furniture, or in the worst case running down the wall behind the plasterboard. The condensate drain is usually a short, accessible run to an outside wall.
The indoor fan unit is usually in the roof space or under the floor. When it leaks, water can pool in the roof void for hours before showing up on the ceiling below - often as a brown stain or sagging plasterboard. The condensate drain is a longer, more complicated run that may pass through cavities and walls before discharging.
Older units often drain straight outside through a small hole. Leaks usually mean the unit is no longer level (often after vibration over years), or the drain hole is blocked. Often cheaper to replace than repair.
Home insurance treatment of AC water leaks varies. As a general rule, sudden damage (a drain that was clear yesterday and is blocked today) is usually covered. Gradual damage (a drain that's been slowly leaking for months) often isn't. Document the moment you noticed the leak, call your plumber promptly, and notify your insurer the same day.
If water has been pooling in a roof cavity or wall, ask the plumber to check for mould development. Mould remediation is a separate (and expensive) job that often isn't covered by standard policies if it's been growing for weeks.
Yes, in some circumstances. Reverse-cycle outdoor units produce condensate when running in heating mode (winter) and need to drain it. Cooling-only units shouldn't drip from the outdoor unit. If you're unsure, check whether the dripping correlates with cooling vs heating.
For a simple split system with an accessible drain, you can sometimes clear it with a wet/dry vacuum attached to the outdoor discharge end. For ducted systems or anything inside a wall, leave it to a plumber - you risk disconnecting the line and turning a minor blockage into a major water damage event.
A standard condensate drain clear is usually $150-$300. Replacing a drip tray or fan motor is $400-$800. Refrigerant work varies widely. If water damage to ceilings or walls has occurred, that's an additional cost - hence the value of acting fast.
We handle the plumbing side of AC water leaks: condensate drains, discharge compliance, water damage assessment, and pipe relining where the drain runs through inaccessible spaces. For refrigerant work and mechanical AC servicing, we work alongside trusted HVAC partners across the Eastern Suburbs.
Air con leaking water through your ceiling? Don't wait for the plaster to give way. Call Plumberoo on (02) 9191 8787 - same-day service across the Eastern Suburbs, $0 call-out, and a clear quote before we touch anything.