My washing machine drain pipe just started backing up into the service closet where my water heater is, which is adjacent to the washing machine on the other side of the wall. It overflowso into the drip pan of my water heater but has never done it before. I've used draino, and ran a 25 foot snake down and came back clean. I can run a garden hose with a pressure nozzle and it won't back up, only when the washing machine drains. I've tried a shopvac and blew air down while blocking the other pipe outlet. Any ideas? I can't get how it takes the hose water and never backs up but can't handle a few gallons at a time from the washer?