Shophouse trunking drip: drain pipe backfall after retrofit works

The unit was in a retrofitted shophouse, where drain routes often take awkward turns. The leak was not at the fan coil; it was in the fall of the drain route. In retrofit routes, the slope and bends can be the fault even when the pipe is not blocked.

By Team Snowflake | Reviewed 14 Jun 2026

Case summary

Panasonic Wall-mounted9 years oldShophouseOutram, Singapore

Concern: The leak had been cleared twice before, so the client thought a hidden pipe might need replacement.
Found: Drain pipe backfall after retrofit trunking works
Key check: Checked drain fall along the exposed trunking instead of only flushing the indoor tray
Result: The trunking stayed dry after the drain fall was corrected. The fix held because it changed the condition that made the leak repeat. Water no longer had a low point to gather in after the unit stopped. For a retrofitted shophouse, that proof mattered more than another short flush because the original problem was route geometry.

What we were told

The shophouse unit leaked from the trunking after running for a few hours. Drain clearing helped briefly, then the drip came back.

What we checked

We flushed the drain and confirmed it was not fully choked. Then we checked the pipe slope along the retrofit trunking. We watched how quickly water left the tray and where it slowed down. A slow drain after clearing points to route geometry, not just dirt inside the pipe. The retrofit history mattered because newer trunking can introduce a bad angle even when the original unit and drain tray are sound.

Drain water moved slowly even after flushing.
A trunking section near the bend had backfall.
Water sat in that section after the unit stopped.
No crack was found in the indoor drain tray.

What we found

The retrofit drain route had a slight reverse slope. Water collected in the low section and backed up when the unit produced more condensate during long runs. The reverse slope created a small holding point inside the trunking. During short runs, the water volume was low enough to pass. During longer runs, condensate collected in that low section, backed up, and eventually escaped at the nearest weak point. This also explained why previous clearing visits helped briefly but did not remove the repeat pattern. Cleaning removed dirt, but it did not change the low point that kept holding water after the unit ran for a while.

What fixed it

We adjusted the drain route to restore proper fall and flushed the line again. We explained that clearing alone would not solve a slope problem. We adjusted the route rather than selling another clearing because the earlier cleanings had only removed debris. They had not changed the angle that allowed water to sit in the pipe. The follow-up check focused on whether water left the route smoothly after the unit stopped. We also advised watching for slow return of the stain, because a slope fix should hold through long cooling runs, not only through a quick flush.

Outcome

The trunking stayed dry after the drain fall was corrected. The fix held because it changed the condition that made the leak repeat. Water no longer had a low point to gather in after the unit stopped. For a retrofitted shophouse, that proof mattered more than another short flush because the original problem was route geometry.

What this case teaches us

Water leaks need the drain route checked

A drain can be clear but still fail if the pipe falls the wrong way. A clear drain can still leak if the route has poor fall.
Retrofit layouts often create long runs, bends, and awkward slopes. Shophouse and retrofit jobs need the visible trunking route checked for bends, dips, and long flat runs.
Repeated clearing without checking fall can leave the drain route unchanged. If clearing helps only briefly, the next question is whether water is sitting somewhere it should not.

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