Why does my aircon drip water only on humid or rainy days?
A drain that leaks only sometimes is not intermittently broken. It passes ordinary flow and fails at peak flow. Humid weather and long overnight runs both push condensate past what it can clear.
By Team Snowflake | Updated 19 Aug 2026
1. A hairline gap at a concealed joint
Water appearing at the ceiling on the heaviest days, with nothing wrong the rest of the time, often means a concealed joint never bonded properly. A drain joint needs enough cement and cure time to weld solid. One closed up too early leaves a hairline gap that ordinary flow slips past.
How to tell
This path leaks out of a joint. Unlike a narrowed line, water still flows freely and nothing backs up at the tray. Unlike a slowing pump, the leak appears along the concealed run rather than at the unit.
- It only shows on the most humid days or after the longest runs.
- Water appears at the ceiling rather than at the indoor unit.
- The rest of the time there is no sign of a problem at all.
- The drain run is concealed above a false ceiling.
How we confirm it
We open the concealed run and pressure the drain line to find the joint that weeps. That joint is remade properly and left to cure before the ceiling is closed.
Do not accept a drain flush for this. The line is not blocked, so flushing changes nothing and the drip returns on the next humid stretch.
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2. A narrowed section that cannot pass peak flow
A leak that arrives after long runs, then behaves again on ordinary days, usually means the line has narrowed. Mineral deposits build on the inside wall layer by layer, or a sag holds water at one point. Steady flow still passes. The volume from a humid night or an overnight run does not.
How to tell
This path fails on volume. Unlike a hairline joint gap, water genuinely backs up before it escapes. Unlike a slowing pump, the restriction sits along the pipe run rather than in the pump chamber.
- It leaks after overnight or all-day runs, not short ones.
- Short cool-downs on ordinary days stay completely dry.
- The installation is old, or the trunking route has a visible dip.
- Any improvement after a clearing lasted only a while.
How we confirm it
We locate the narrowed or sagging section along the run and replace or re-support that length. The unit then runs long enough to push peak volume through and confirm it clears.
A repeat clearing is not the fix once a line has narrowed permanently. Mineral buildup does not flush out, so you pay twice and the leak comes back on the next long run.
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3. A drain pump slowed but not failed
On units that need a pump, leaking only on the heaviest days often means the pump chamber has silted up. The pump still runs and still moves water, just more slowly than before. On an ordinary day it keeps ahead of the condensate. On a humid one it falls behind and the tray overflows.
How to tell
This path is a pump falling behind. Unlike either pipe fault, the pump is audibly running and the water clears once demand drops. The overflow happens at the unit rather than along the concealed run.
- The unit is a cassette or ceiling type that uses a drain pump.
- You can hear the pump running, so it has not simply stopped.
- It only overflows on the most humid days or at the end of long trading hours.
- The leak stops shortly after the unit is switched off.
How we confirm it
We clean the pump chamber and float, and check the discharge line for a bend or crimp holding it back. The pump is then run under full condensate load to confirm it keeps ahead.
A pump replacement is not the fix when the pump still runs. Buildup in the chamber or a crimped discharge line gives exactly the same symptom, and the new pump slows down the same way.
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Tell us what’s going on. Symptoms, setup, photos, anything we should know. We’ll assess and come back with the right next step.