Air Conditioner Leaking Water? Causes and What to Do

Short answer: an air conditioner leaking water inside almost always has a drain problem. Every unit pulls moisture out of the air while it cools, and that water runs into a tray and out through a drain pipe. When the pipe blocks, the unit sits crooked or the coil freezes, the tray overflows and water drips down the wall. Switch the unit off, clean the filter, and check that the drain outlet outside isn't buried or blocked. If it still drips after that, call a technician and leave the inside of the unit alone.

Here's what causes it, in the order we find it on jobs around Newcastle and Lake Macquarie, and where the owner's part ends.

Why air conditioners make water at all

In cooling mode, warm room air passes over a cold coil inside the indoor unit. Moisture in that air condenses on the coil, the same way a cold drink sweats on a hot day. The water runs into a tray under the coil and leaves through a thin drain pipe, usually out through the wall to a garden bed or a downpipe.

Newcastle summers are humid, so a split system cooling a living room on a February afternoon keeps the outside drain dripping for hours. A small fault in the drain shows up fast here, and an air conditioner dripping inside is one of the most common calls we get in January and February.

The causes, most likely first

1. A blocked drain line

This is the one we find most. Dust, mould and a jelly-like slime build up in the tray and the drain pipe until the water backs up and spills over the front of the indoor unit. It often starts as a slow drip on a muggy day and gets worse over a few weeks. A unit that hasn't been serviced in years is the usual candidate.

2. A dirty filter freezing the coil

A clogged filter starves the indoor coil of airflow. With too little warm air passing over it, the coil gets cold enough to ice over. When the unit cycles off, the ice melts all at once and the tray can't hold it. If you see frost on the coil behind the filter, or the unit has been cooling poorly before it started dripping, suspect the filter first.

3. The unit isn't level

The tray and drain rely on gravity. If the indoor unit tilts away from the drain side, or the drain pipe has too little fall to the outside, water pools at the wrong end of the tray and runs out the front. This tends to show up on newer installs, or after a bracket has moved or someone has bumped the pipe outside.

4. A cracked or worn drain tray

Trays crack, warp and rust with age, and a unit ten to twenty years old is more likely to have one that's failing. The giveaway is a drip that keeps going even after the drain has been cleared.

5. Low refrigerant icing the coil

When a system is low on refrigerant, its pressure drops and the coil can freeze, just as it does with a dirty filter. The difference is that a clean filter doesn't fix it. Weak cooling or a hissing or bubbling sound both point this way. Low refrigerant points to a leak, and under ARC rules the leak has to be found and repaired before any refrigerant goes back in.

6. A failed condensate pump

Some units can't drain downhill, so they pump the water out instead. Ceiling cassettes usually have a drain pump built in, and other units get one where the drain has to climb before it can fall. If the pump fails, the water has nowhere to go and the tray overflows, often into the ceiling. A cassette dripping from the ceiling grille, or a ducted system leaving a stain on the ceiling below the unit, can be this.

7. Condensation on the pipes

Sometimes the unit is fine and the water is coming from the pipes. The copper refrigerant lines run cold and are wrapped in insulation. Where that lagging is split, crushed or missing, the bare pipe sweats on a humid day and drips. Check where the pipes leave the indoor unit and where they run along the wall outside. A little condensation on the valves of the outdoor unit is normal.

What you can safely do yourself

1. Switch it off. Turn it off at the remote, then at the isolator beside the outdoor unit or at the breaker. Water near electrics is the first thing to deal with.
2. Protect the floor and the wall. A towel along the skirting and a bucket under the drip. Move anything electrical out from under the unit.
3. Clean the filter. On a wall split, lift the front panel and slide the filters out. Rinse them in cool water or vacuum them, let them dry completely in the shade, and slide them back in. If the coil behind was iced, leave the unit off for a few hours so it can thaw before you try it again.
4. Check the drain outlet outside. Follow the thin pipe from the indoor unit to where it ends. Make sure the end isn't buried in mulch or soil, kinked, crushed or sitting in a puddle. Clear any garden bed away from it by hand.
5. Try it again and watch. Run it in cooling mode for half an hour. Water should start dripping from the outside outlet and nothing should come from the indoor unit.

If the breaker for the air conditioner has tripped, reset it once, and only once everything is dry. If it trips again, leave it off and call us. A breaker that keeps tripping is a fault, and resetting it over and over won't fix it.

What not to do

• Don't open the unit. Anything held on with screws is a technician's job. There are live parts behind the covers.
• Don't push wire, a coat hanger or a garden hose into the drain. It's easy to split the pipe or knock it loose inside the wall, which turns a drip into a hidden leak. General (formerly Fujitsu General) warns that home attempts at clearing a blocked drain can cause further damage or void the warranty.
• Don't touch the refrigerant pipes or valves. Handling refrigerant needs an ARC licence. If anyone offers to "top up the gas" without finding the leak first, say no. Under the ARC's own rules, topping up a system before it has been fully tested and its leaks repaired is illegal.
• Don't run it and just empty the bucket. Water that overflows the tray can also run back inside the wall, and that's how mould starts behind the plasterboard.

When to call us

Call if the unit still drips after a clean filter and a clear outside outlet, if there's ice that comes back, if you can hear hissing, if water is coming through a ceiling, or if a breaker has tripped twice. We find the cause, clear the drain properly, and tell you straight whether it's a cleaning job or a part. Our air conditioner repair page has the call-out fee and what happens on the visit.

On older units we also look at whether a repair is worth it. A ten-year-old split with a rusted tray and a refrigerant leak can be closer to replacement than repair, and if that's where it lands, our split system installation page explains the options.

How to stop it happening again

A blocked drain doesn't happen overnight. It builds up over months, so the fix is routine:

• Clean the filters every few weeks while the unit is working hard in summer, and before the first hot spell. Some manufacturers suggest every two weeks in heavy use.
• Keep the outside drain outlet clear of mulch, soil and plants.
• Have it serviced. A service flushes the drain line, cleans the tray and coil, and checks the pump on units that have one. Our air conditioning service page covers what's included and how often to book.

If your unit is old enough that you're thinking about a new system anyway, our guide to split system vs ducted is the place to start.