Office cassette leaked after service: pump cover left misaligned

Mitsubishi Electric Cassette·6 years old·Office·Straits View, Singapore

The office ceiling unit was serviced in the morning and began dripping over desks later that day. It had not leaked before the service.

The timing pointed to a service follow-up. We opened the cassette panel and checked the pump area before chasing the ceiling drain route. We recreated the drain condition while watching the pump area directly. That was more useful than flushing the ceiling line from a distance because the suspected escape point was inside the cassette. We also checked reassembled parts first, since the leak began after work rather than after weeks of slow drainage.

The pump cover had not seated fully after servicing. Water entered the pump chamber but escaped at the misaligned cover before being discharged into the drain line. The pump was moving water, but the cover was not seated squarely. Water found the small gap around the cover before it could be discharged cleanly. From below, that looked like a ceiling drain failure even though the fault was inside the service panel. The timing after service made reassembly the first place to prove or rule out. The ceiling route did not need to be opened because the leak appeared before water reached that part of the system.

We reseated the pump cover, checked the gasket, and ran water through the cassette until the pump cycled several times without dripping. We confirmed the gasket and cover position before closing the cassette because a misaligned cover can look fixed briefly, then leak again when the pump cycles. Repeating several pump cycles proved the seating was stable. We did not recommend ceiling work because the controlled water test showed the pump area stayed dry after the cover was seated correctly.