What is the minimum size in inches for a combination waste and vent pipe serving 31 fixture units?

The minimum size for a combination waste and vent pipe serving 31 fixture units is 3 inches. This is based on the provisions found in the International Residential Code (IRC), which outlines the sizing requirements for drainage and vent piping based on the number of fixture units.

In the context of plumbing design, fixture units are a measure of the probable demand on a plumbing system. The IRC provides guidelines for the sizing of various plumbing components, including waste and vent pipes, to ensure that the system can adequately handle the expected load. According to these guidelines, a combination waste and vent pipe must be sized appropriately to accommodate the flow of wastewater and to allow for adequate venting, which helps prevent issues like siphoning and drain line backpressure.

For 31 fixture units, the IRC specifies that a 3-inch pipe is necessary to ensure that there is sufficient capacity for both waste drainage and venting duties. A pipe of this size minimizes the risk of clogs and ensures that the plumbing system operates efficiently. If a smaller pipe were used, it may not handle the volume of discharge effectively, leading to potential plumbing problems. Therefore, for a system with 31 fixture units, the correct minimum size is indeed 3 inches.