Back valve? It’s called a check valve! and I go back to a comment I made in my post about getting the builder involved, particularly if this is new construction. Shame on the inspector who failed to do his job for the city and shame on the plumber and builder for allowing a check valve to be installed backwards. Any idiot can install it correctly but I guess it takes a real special idiot to install it backwards! It all should’ve been Detailed on the plumbing plan for the home. Jeez. Get the builder back in there; point out his failure and making him correct the issue at his expense. If he gives you any backtalk get the cities inspector involved, The reason why there are inspections is to prevent this kind of nonsense from happening. Take Jeff’s comments to heart; if there is any sewage back up you’ll be very very sorry you didn’t get this corrected.