g/hvac • Posted by u/hvac_hero • 13 days ago

VRF branch controller condensate — 4 units, only one ever floods Diagnostics

Mitsubishi City Multi, four BC controllers across three floors. One of them, third floor east, floods its drain pan about every six weeks and drops water into the ceiling below. The other three have never done it.

Checked: trap is correct depth, drain is pitched, line is clear, pan sensor works. Blew the line out anyway. Six weeks later, flooded again.

Same install crew, same details, same drawings on all four. What is different about this one?

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u/bas_brianna • 13 days ago

What is different is almost certainly the pressure in the ceiling space it drains into, not anything about the controller.

A trap only works if the pressure difference across it stays below the trap depth. If that third floor east ceiling plenum goes more negative than the others — a return path that got blocked, a fire damper that closed, a fan that got rebalanced — the trap gets pulled and the pan cannot drain, so it backs up until it overflows.

Six week interval is a clue. Something on a schedule is changing the pressure. Check the AHU serving that zone and see whether anything about its operation changed on an interval — a filter loading up, a schedule change, an economizer cycling.

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u/hvac_hero OP • 13 days ago

Filters on that AHU are on a six week PM interval. So the pressure in that plenum walks more negative as the filter loads, floods right before the change, then resets when the filter gets swapped.

I have been chasing the drain for four months and it was never the drain.

u/duct_doctor • 13 days ago

Beautiful catch by [the reply above] and a good reminder that "same install, different behavior" almost always means the difference is in the building, not the equipment.

Fix is a deeper trap sized for the worst-case static, not the design static. Take the AHU to a dirty-filter condition, measure the actual plenum pressure, and size the trap for that plus a margin. Design static is a fantasy the moment the building is occupied.

u/freon_frank • 13 days ago

Also worth checking that the other three are not quietly on the edge. If one flooded because a filter loaded, the other three are running the same trap depth against a smaller pressure. Measure all four while you are in there.

u/minisplit_mike • 13 days ago

Six week PM interval as the diagnostic clue is the kind of thing you only find if you go look at the maintenance schedule, which is not where anyone looks for a plumbing problem. Filing this one away.