First of all, I'm unfamiliar with plumbing lingo, so hopefully I can ask this question intelligently. Please be patient with me! I have a 1926 house and I recently added a second bathroom on the second floow, in part of what was once considered the attic. The plumber tied the new plumbing into the existing stack in the basement. I'm concerned now about the vent stack. Instead of installing a new stack that goes through the roof, there is a 2.5" (approx) PVC pipe with a vented cap about 4 feet tall in the open space behind the bathroom (this is the portion of the old attic that is not finished). Is this a proper way to vent the new toilet? Thanks for any help.

If the cap looks like this,(see image), then he installed a Studor Mechanical Vent. This works much the same as a regular vent. Some local codes accept them and some do not. I would have rather he had revented back to a roof vent to the outside but a Studor Vent will do the job. Good luck, Tom

It is just a code question. The plumber did an acceptable job if an AAV is allowed in your town. Call your building department and ask if they allow air admittance valves. If they say no, call the plumber back and tell him to make it right by code.