We have a BIG problem with a recent kitchen renovation on a previously renovated 100+ year old building in which we installed a washer/dryer unit in the old pantry closet causing water backup with our downstairs neighbor's kitchen sink. The management for the building claims they have a plumber come out and rod the drains out to the street every year, but we still think there is some kind of blockage b/w the 1st floor neighbor's kitchen drain and the main for the building. We plan on swapping out the current washing unit with a front-load HE unit (which is what should have installed in the first place), but we were also thinking about putting in some kind of excess overflow tank so that it is never a problem again. Can you please tell me how those tanks work, where to purchase them, about how much they cost, how and where the best place to install one, and your overall thoughts on this as a solution to our problem? Any other suggestions for our issue would be much appreciated. Thank you so much for your help!