I recently had the problem of my wash machine overflowing drain pipe plumber recommended putting a fitting on top of drain to prevent water from overflowing and it worked until about a week later when I started smelling sewer smell checked it out and found some standing water under my bathroom on the other side of house (1400 sqft home)this is also the location of the only vent so I checked all fixtures in bathroom no leaks then I noticed smell was very strong when machine was drainig so I know I need to remove the cap but why do I have the smell and standing water (ever heard of this?) and how big should I go with a new drain .I am a electrician by trade so I work around plumbers on job sites and they just scratch their heads when I ask them what to do any help you can give would be greatly appreciated