We tested the input and output before installing them... everything seemed to be fine...
Nearly one week after the trial load of laundry, our ceiling started to drip. Sorry I didn't have enough wits to grab a video camera to capture the drama... about all us adults could do was stare in disbelief, the 6-year old had to suggest that maybe we wanted to get a bucket? Of course, he was also the only one who was filled with excited energy at the prospect of water coming from the ceiling.
We were fortunate that the drain overhangs our hallway... just beyond the leak is our newly painted bedroom! Whew!
It seems that lots of people have problems with 2nd floor washers overflowing, for various reasons. People especially seem to have trouble with new washers, which use less water but apparently pump that water at an unforgiving rate for 2-inch plumbing. But there don't seem to be any ready-made solutions for anyone in our situation. The most popular solution for an occasionally slow drain seems to be to use a laundry sink as an overflow mechanism, but there isn't any room for such things in a 2nd floor laundry closet.
So David invented this contraption. It's a 6 gallon water jug mounted onto the drain pipe. We considered recessing it into the wall, but it sticks out only a tiny bit further than the dryer vent hose. We extended the outflow hose on the washer so that it reaches into the top of the 6 gallon jug. If there should be any slowness in the pipes, hopefully this will buy the pipes enough time to drain before ending up in our ceiling! Alternatively, if there was a problem, the water would spill onto the floor. Which would be a big mess to clean up, but is preferable to trapping the water between the floor and ceiling, where the only place for it to go is soaking into the drywall.
We hope it will work! We tried really hard to make the washer overflow... but we couldn't even come close, there was no indication that there was any slowness at all. Which is good!