Kitchen sink backup

Old 07-04-18, 10:50 AM
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Kitchen sink backup

This happened in a condo unit on the 1st floor (there is a unit below in the semi-basement, they don't have backup problem). Was told by tenant that it happened randomly, not after they used the sink. There are black residues once the water went back down.

I'm kinda dumbfounded because if there is problem with the building backup valve (there are no valves for each unit), logically it should hit the unit below first. If this is a locally clogging issue then they should have draining problem but they don't. Any help about possible cause will be greatly appreciated!
Old 07-04-18, 02:01 PM
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I would suspect a partial clog. There is enough an opening to let most liquid pass through without backing up but when there is heavier than usual usage not everything can pass the partial clog fast enough and things back up.

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