I have two separate vents (kitchen & vanity) that are sloped all the way up towards the main vent. Once over a horizontal section of the main vent, they both make a 90° bend and go down into a horizontal section of the main vent. That main vent then turns upward and exits the roof. The vents are all sloped toward their origin and there is nowhere for condensation to collect.
Does this work? I've been told you can't vent down into a main vent but not sure if that is true.
IPC 905.2 would prohibit that, as I understand it. Not sure what code applies where you are.
Grade: Vent and branch vent pipes shall be so connected and graded as to drain back to the drainage pipe by gravity.
The word choice of "back" making it required to drain to the drain pipe it serves, not forward to the main stack, as I read that.
There's a special case in 916 for island vents, but this isn't an island vent.
I suspect it works, but I can't see it as code-compliant, unless your local code reads differently.
