What size must the stack vent be in relation to the waste stack?

The stack vent must be equal to the size of the waste stack to ensure proper ventilation and drainage within the plumbing system. This requirement is based on the principle that the venting system must be able to handle the air needed to maintain proper atmospheric pressure within the drainage system. If the vent size were smaller, it could create a negative pressure during drainage, potentially causing siphoning in traps and leading to drainage issues. Conversely, if the vent were larger, it could also create undue stress on the pipe system and may not provide any additional benefits. By matching the size of the stack vent to that of the waste stack, it ensures the system will function effectively, allowing for the proper flow of waste while also maintaining air pressure equilibrium. This principle is foundational in ensuring both the efficiency and reliability of a plumbing system.