Last week our washing machine drain overflowed. The plumber tried to snake it out, but the problem only became worse (i.e. no flow at all any more). He opened up the drywall and discovered a very corroded cast iron pipe going horizontally from the standpipe via a trap to a T at a vertical pipe (upwards a vent, down into the concrete slab). The plumber blamed the clog on the fact that the horizontal pipe had essentially no slope to it, and said the only solution was to replace standpipe, trap, and horizontal pipe with new plastic pipes. An expensive job, but we seemed to have no choice. As part of the job he installed an "eliminator outlet box" on top of the standpipe, in which the washing machine discharge hose is supposed to go (and which also has valves to connect the water hoses to). Tests with water from a faucet flowing through a hose into the drain worked fine. But as soon as we actually ran the washer, the drain immediately backed up and overflowed. Clearly it cannot handle the large volume of water that is being discharged. I did some research on this site and see two possible problems: (1) a partial blockage farther down the drain pipe (i.e. where it disappears into the concrete slab; (2) the height of the standpipe. For some reason the new setup has the standpipe at a height of only 29", much lower than it was before. The washing machine manual specificies 39".
We will obviously complain to the plumber, but I'd like to be more informed before making my complaint. In particular, is the height of the standpipe really a crucial issue? Would there be a way to test whether it really causes the problem? All I can think of is raising the end of the discharge hose by 10" and running the water through a hose into the standpipe; is that a good test?
Finally, regarding the possibility of a partial blockage in the pipe underneath the floor: I assume it can be snaked out, but I have this fear that snaking could turn the partial blockage into a complete blockage, as it seemed to do with the above-ground pipe. And that would mean jackhammering the floor to replace it. Am I wrong to worry?