What is the maximum vent size allowed for two water closets installed back to back?

The maximum vent size allowed for two water closets installed back to back is 2 inches. This size is specified because two water closets contribute to the plumbing fixture unit load, and the vent must be adequately sized to manage the potential air pressure fluctuations and the venting needs of both fixtures. A vent size of 2 inches is usually sufficient to allow for proper airflow, ensuring that wastewater can drain efficiently without causing a vacuum or pressure issues within the drainage system.

Using a vent that is larger than necessary may not provide any additional benefit, and under-sizing could lead to problems such as slow draining or gurgling sounds. Therefore, adhering to this standard helps maintain a safe and efficient plumbing system.

The other sizes would not meet the required specifications for venting when two water closets are involved. Although one may think of increasing the vent size for redundancy or additional capacity, the building codes specifically stipulate the maximum vent size to ensure optimal performance and compliance with regulations.