Is it ok to start a vent on a 45 from a wye like on this drawing?
I need to clear the edge of the sink (actually a giant double compartment sink) before I can go up and start horizontal once I clear the flood rim + 6".
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A couple points, some of which include what others have said.
Can a vent be started from a wye? YES
Can you do what you were proposing? NO
As others have said, you have an S-trap there, which violates "the vent connection to a fixture drain [shall not] be below the weir of the trap" (IPC code) - the point at which air is introduced from the vent needs to be above the weir, or highest point of the seal. In your diagram, the point of air introduction is where the 45 degree part of the wye hits the drop.
You could fix this by, in effect, reversing your two connections. Instead of a wye and the 90 degree bend, use a sanitary tee to the trap and the connect the vent to the top via a 45 above to go out through your space.
As you note, the vent then needs to extend at least 6" above the sink flood rim before going horizontally.
You could also have the cleanout above the wye. But i wouldn't use a wye there anyway. I would use a san tee and trap arm over to the sink. On top of the San tee I would (if the cleanout remains on the bottom) add a St 45° that way your eliminating the illegal S trap, and can still offset over to where you need.
Remember in the plumbing world a 45° angle is still considered vertical. That's relevant because you must stay vertical with your vent until your above flood rim of the fixture its serving.
Absolutely! Most any fitting is perfectly acceptable and legal, just don't use any reducers, caps or traps in the venting.
Though, check with your local Codes, Building, Zoning, Permit or Inspection Department (whatever they're called around you). That 6-inches above the Flood Rim may need to be 36-inches and if there's a toilet on the line a 2" vent pipe may be required. Same goes for bird screening or goose-necking of the exterior pipe termination.