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

The minimum vent size for two water closets installed back to back is governed by plumbing codes that ensure adequate venting and air circulation for the drainage system. The correct answer is 2 inches because this size is necessary to effectively manage the air flow and prevent issues such as siphoning or the backflow of sewage gases.

In practice, a 2-inch vent provides sufficient capacity to accommodate the venting needs of two water closets, ensuring that any potential vacuum created by the flushing action is adequately relieved. Additionally, proper vent sizes help maintain proper drainage, allowing wastewater to flow without hindrance.

Smaller vent sizes, such as ½ inch or 1 inch, would not meet the requirements set forth in plumbing codes, likely leading to inadequate ventilation and increased risk of plumbing problems, such as clogging or odors. A 3-inch vent, while adequate, is larger than necessary for this specific application, potentially leading to unnecessary costs and complexity in the plumbing system design. Thus, the 2-inch vent size is recognized as the optimal choice for two water closets installed back to back.