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The required diameter of a vent for a waste stack is determined by various factors, including the size of the waste stack, the number of drain fixture units it serves, and the developed length of the vent. In this case, we have a 3-inch waste stack serving 53 drain fixture units with a vent that has a developed length of 72 feet.

According to plumbing codes and guidelines, the minimum vent size for a 3-inch waste stack typically depends on the total number of drain fixture units. With 53 drain fixture units, the code suggests a vent size that can adequately accommodate the potential air flow and drainage needs associated with those fixtures.

The calculated vent size may lead you to a conclusion that a 2-inch diameter vent is appropriate for handling the venting requirements of a 3-inch waste stack serving that number of fixture units, especially considering the length of the vent as well. A larger vent would be unnecessary and could lead to waste of materials and space.

Overall, the reasoning is rooted in code adherence and practical functionality for efficiently venting the plumbing system, which is where the 2-inch designation effectively meets the requirements in this scenario.