An air conditioner produces a genuinely large amount of water as a normal part of cooling — often several litres a day — and that water is supposed to leave through a condensate drain line. When that drain clogs, usually with algae growth inside the pipe, the water backs up and finds its way out somewhere you can see it. This single blockage explains the large majority of AC leaks.
Why an AC produces so much water
As warm, humid indoor air passes over the cold evaporator coil, moisture condenses out of it — the same way condensation forms on a cold glass, just continuously and at a larger scale. That water drips into a collection pan under the coil and is meant to drain away through a condensate line, typically running outside or into a household drain. On a humid day, a residential system can produce several litres of water without anything being wrong at all.
Common causes
| Cause | How likely | Typical sign |
|---|---|---|
| Algae or sludge blocking the drain line | Very common | Drain line has not been cleared in over a year |
| Drain line disconnected or kinked | Common | Especially after any recent duct or unit work nearby |
| Clogged or dirty air filter | Common | Causes the coil to over-produce condensate or ice up and then melt suddenly |
| Condensate pump failed (if fitted) | Occasional | Units without gravity drainage to the outside |
| Drain pan cracked or rusted through | Occasional | Older units; leak persists even after clearing the line |
| Unit not level | Occasional | Water overflows one side of the pan rather than draining centrally |
| Frozen coil that has just thawed | Occasional | Sudden large amount of water after the system was iced up |
Clearing the drain line
Turn the air conditioner off at the thermostat before working on the drain line or the indoor unit.
- Find the drain line
It is usually a PVC pipe running from near the indoor unit to an outside wall, or into a household drain. There is often a T-shaped access fitting with a cap along its length, specifically for clearing blockages.
- Clear it with a wet/dry vacuum
At the outside end (or the access fitting if there is one), attach a wet/dry vacuum and run it for a minute or two to pull the blockage out. This is the most effective and least messy method for algae build-up.
- Flush with a vinegar solution
If you cannot vacuum it, pour a cup of white vinegar (or a proprietary condensate line treatment) into the access point near the indoor unit and let it sit for thirty minutes before flushing with water. This breaks down algae growth along the pipe walls.
- Check the line is not kinked or disconnected
Follow it visually where you can, particularly near the indoor unit, checking for a section that has come loose or been crushed.
- Confirm the unit is level
An indoor unit that has tilted, even slightly, can cause water to overflow one side of the drain pan rather than draining through the outlet. Check with a spirit level if the leak is localised to one side.
- Test with water
Once cleared, pour a small amount of water into the drain pan (or the access point) and confirm it flows through and out the far end within a reasonable time.
Flush the drain line with a cup of white vinegar every few months during the cooling season. This single habit prevents most future algae blockages and is genuinely worth adding to a seasonal routine.
The safety switch that should have stopped this
Many systems include a float switch in the drain pan or line specifically to shut the unit down before a blocked drain causes a serious leak. If your system kept running despite a full pan, that switch may be missing, disconnected, or has itself failed — worth mentioning to a technician if you have this fitted, since it is meant to prevent exactly this situation from causing water damage.
When to call a technician
Call an HVAC technician if the drain line will not clear with a vacuum and vinegar flush, if the drain pan itself appears cracked or rusted through, if a condensate pump has failed, or if the leak continues after the line is confirmed clear — that points at a pan or coil issue rather than the drain itself.
Stop and call a qualified appliance technician or electrician if you find scorched or melted wiring, smell burning, see water that has reached electrical components, or if the fault returns immediately after a repair. Never bypass a safety device — thermal fuses, door interlocks and flood switches fail for a reason, and defeating one converts a broken appliance into a fire or flood risk.
