I'm working on replumbing my utility room and I'm adding a graywater system. Local codes are remarkably permissive but require an overflow to the sewer/septic system in case of graywater blockage or backup. I've come up with two designs and would like some input.
Design 1:
This design conserves fall for the graywater distribution plumbing (always a good thing) but I wonder if the overflow would be able to keep up with a really large flow. I'm pretty sure that putting the valve before the trap won't be a problem since the graywater distribution plumbing won't have any sewer gasses, but please correct me if this is wrong. My idea was to bypass the trap entirely to prevent the sewer gasses from using the overflow plumbing as a path back in the house.
Design 2:
This design squanders some fall for the graywater distribution plumbing but looks like it offers a more robust overflow control solution.
Which one of these seems more sensible, and/or what about either of them might need changing?
Best Answer
1 is better (so long as that valve has essentially no restriction). Otherwise you might need some height above the valve to keep the flow happy.
In version 2, turn the tee on the greywater line around so it goes where the overflow would go, and put in a trap (currently it allows sewer gas to vent into the greywater plumbing.) In fact, if the valve does what I expect, it forces septic venting into the greywater plumbing. So, not good.