Older HDB trunking drip: cracked drain elbow behind the casing

The water was not dripping from the indoor unit face. It appeared along the trunking below it, which made the drain route the first place to check. In older flats, the indoor unit, trunking, and pipe bends can all be separate leak points.

By Team Snowflake | Updated 28 Jul 2026

Case summary

Mitsubishi Electric Wall-mounted13 years oldHDBHougang, Singapore

Concern: The homeowner thought the indoor unit might need replacing because the leak kept returning after drain clearing.
Found: Cracked drain elbow hidden behind the trunking cover
Key check: Opened the trunking section where water first appeared instead of flushing the indoor tray repeatedly
Result: The leak stopped without replacing the indoor unit or opening the full trunking run. That gave a cleaner answer than another drain clearing visit. The fault was a failed fitting in the route, not a failed fan coil.

What we were told

Water kept forming below the trunking. The drain had been flushed once before, but the leak returned whenever the bedroom unit ran for more than an hour.

What we checked

The indoor tray was draining at first, so we followed the water mark along the trunking rather than treating it as a simple choke. Repeated flushing would only prove that water could move through part of the route. It would not prove that the route was still watertight at the bend where the mark began. We opened the section around the first stain before touching the rest of the run, so the check stayed focused on where water first escaped.

Indoor drain tray was not overflowing.
Water mark started at a bend in the trunking.
Opening that section showed a cracked drain elbow.
The pipe run after the elbow was clear.

What we found

The plastic drain elbow had aged and cracked at the bend. Water entered the drain hose, then escaped at the cracked elbow and ran along the inside of the trunking before dripping out lower down. A cracked elbow behaves differently from a choke. Water can still leave the tray, so the indoor unit may not overflow. But once the water reaches the damaged bend, some escapes into the trunking and travels until it finds a low point to drip out. That is why the visible drip was lower than the actual fault. The water was following the trunking path, not falling straight from the break.

What fixed it

We replaced the cracked elbow section, sealed the joint, and ran water through the drain tray to confirm the trunking stayed dry. We kept the repair to the damaged section because the rest of the run passed the flush test. After replacement, we watched both the opened bend and the lower trunking exit during the water test. That kept the repair limited to the visible failure point while still proving the drain could carry water after the joint was replaced. The homeowner did not need a full re-run because the pipe after the elbow moved water cleanly once the crack was removed.

Outcome

The leak stopped without replacing the indoor unit or opening the full trunking run. That gave a cleaner answer than another drain clearing visit. The fault was a failed fitting in the route, not a failed fan coil.

What this case teaches us

Water leaks need the path traced

Where the first water mark appears matters more than where the unit is mounted. For trunking leaks, the first wet mark tells you which section deserves attention first.
A repeated drip after clearing can mean a cracked joint, sagged pipe, or poor drain fall. If a leak returns after clearing, the next check should include joints, bends, and pipe fall.
Photos of the trunking and drip point help narrow the first section to open. A wide photo of the trunking route helps us decide whether to open near the unit, at a bend, or near the outlet.

Questions this case answers

Water is dripping from the trunking, not from the aircon itself. What does that mean?

The drain route deserves attention before the fan coil does. Water escaping at a damaged point inside trunking runs along the inside until it finds a low spot to exit, so the visible drip sits lower than the break. Tracing back from the first wet mark located a cracked elbow here.

Why did the drip come back after the drain was flushed?

Because clearing removes dirt but cannot repair a broken fitting. The tray was draining normally, so a choke was never the whole story. Opening the trunking at the first stain exposed a plastic elbow that had aged and split at the bend. Water left the tray fine, then escaped through the crack.

Do I need a new indoor unit if the drain keeps leaking?

Rarely. A failed fitting in the route is not a failed fan coil. Once the cracked elbow was replaced and the joint sealed, a water test through the tray left the trunking dry from the bend down to the outlet. The pipe beyond the elbow moved water cleanly, so no full re-run.

Which section of trunking should be opened first?

Start where the first wet mark appears, not where the drip lands. A wide photo of the whole route helps decide between opening near the unit, at a bend, or close to the outlet. Opening one bend settled it here and kept the repair confined to the damaged section.

Ready to get started?

Tell us what’s going on. Symptoms, setup, photos, anything we should know. We’ll assess and come back with the right next step.

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