What is the minimum size for a vent pipe serving a 3-inch drain?

The minimum size for a vent pipe serving a 3-inch drain is indeed 2 inches. Vent pipes are essential in ensuring that drain systems function correctly by allowing air to enter the plumbing system, which prevents the formation of negative pressure that could disrupt the flow of waste.

For a 3-inch drain, a 2-inch vent pipe is designed to adequately handle the air requirements for venting a drainage system, balancing the pressures and ensuring that the system remains functional. This dimension is determined by building codes and plumbing standards, which specify that the vent size should typically be at least half the diameter of the drain. Therefore, for a 3-inch drain, the corresponding 2-inch vent provides adequate capacity for venting.

Understanding the relationship between drain and vent sizes is crucial for effective plumbing design, ensuring the safe and efficient operation of the plumbing system. Smaller vent sizes, such as 1 inch or 1.5 inches, would not provide sufficient airflow for a drain of this size and could lead to potential problems such as slow drainage or sewer gas issues.