Why is my aircon trunking leaking water?
A wet trunking casing is not the same fault as a drip at the indoor unit face. It does not need rain to show up, either. The casing only carries water that started somewhere else, and three different sources inside the cover leave the identical stain outside.
By Team Snowflake | Reviewed 23 Jul 2026
1. Condensation on an exposed cold pipe
Water beads along one section of trunking after the unit has been running a while, often worse on humid days, and dries up once the unit stops. Inside, a length of copper pipe has lost its insulation wrap at a joint or bend. Room humidity meets the cold surface, condenses, and collects inside the casing until it finds a seam to escape through.
How to tell
Unlike a cracked fitting, nothing here is actually broken. The trunking stays dry near the unit and only sweats along the exposed pipe section, usually after long cooling runs rather than from the moment the unit switches on. It also does not shift position the way a backfalling drain run does.
- The damp patch sits mid-run along the trunking, not at either end.
- That one section feels cool or faintly wet to the touch.
- Cooling performance feels completely normal throughout.
- The mark dries out once the unit rests, then returns after the next long run.
How we confirm it
We open the trunking at the wet section and confirm the insulation gap on the cold pipe. Then we rewrap or replace the missing section before resealing the cover.
Replacing the drain line will not fix a condensation drip. If the pipe insulation is not checked before parts are quoted, the trunking gets reopened for work that was never the cause.
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2. Cracked or perished fitting behind the casing
The wet patch sits below a bend or an access panel in the trunking, well clear of the indoor unit itself, and often returns after a drain flush already seemed to clear it. Inside the casing, an aged elbow, coupling, or access-panel gasket has cracked or perished. Water leaves the drain hose normally, escapes at that broken point, then travels inside the trunking before dripping out somewhere lower.
How to tell
Unlike the condensation path, something here is actually broken. The drip returns even after a drain flush because flushing cannot repair a cracked joint. Unlike the backfall path, water still moves normally through the rest of the route, since only the section past the fitting is affected.
- The wet mark begins at a bend, joint, or access panel, not directly under the fan coil.
- Water keeps returning at the same spot shortly after a drain clearing visit.
- The trunking cover feels damp or slightly swollen at one seam, while the rest stays dry.
- Cooling stays normal and the indoor tray drains on schedule.
How we confirm it
We trace the wet mark back to the first point it appears, then open only that section of trunking. We replace or reseal the specific elbow, coupling, or gasket that failed, and test drain flow again.
Avoid approving an indoor unit replacement before the trunking is opened and the fitting behind it is checked. A repeat drip after a drain clearing often comes back to a cracked joint in the casing, not a failed unit.
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3. Backfall introduced by other trade work
The trunking drips only after long cooling runs, and a drain flush seems to help for a short stretch before the same patch returns. Somewhere along the run, other trade work such as a renovation or retrofit job has tipped a section the wrong way. Condensate collects in that low point and backs up once the volume builds.
How to tell
Unlike the cracked-fitting path, the pipe itself is not damaged. A flush test shows water still moving, just slowly, rather than stopping outright at a break. The issue is the angle of the whole section, not one fixed point, which is why clearing only buys a short stretch before the patch returns.
- Drain clearing helps for a short stretch, then the same wet patch returns.
- The trunking run has a visible dip near where past renovation or retrofit work took place.
- A cup of water poured into the drain pan runs out slowly instead of draining straight through.
- The drip gets worse the longer the unit runs in one stretch.
How we confirm it
We check the pipe fall along the full trunking run and find the low section where water sits. Then we correct the angle so condensate drains straight through instead of collecting.
Repeated drain clearing does not fix a backfall in the route. If the same patch returns shortly after each clearing, the next check should be pipe fall along the trunking, not another flush visit.
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