When is a separate vent not needed

drain-waste-vent

I was of the understanding that every trap needs a vent and that all the vents for all the traps tie into the vent portion of the main stack which vents through the roof.

I came across this diagram in a plumbing book and am confused why the laundry sink in the basement doesn't have its own vent? Instead, unlike the other fixtures in the image, it seems to rely on the vent portion of the main stack for its venting needs. What allows it to do this?

(uploading image isn't working right now, I'll try again later)

Best Answer

It could be that you have a single stack vent system:

The Single Stack Vent System was introduced into the IPC in the 2012 Edition and is based on a drainage stack system in the City of Philadelphia (Pennsylvania) Plumbing Code which has been used successfully in many multi-story and high-rise buildings for over 100 years.

https://wabo.memberclicks.net/assets/pdfs/Plumbing_Venting_Brochure_2018.pdf (pages 17-19)

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