Massachusetts Plumbers Journeyman Practice Exam

How much larger should every drainage pipe in a combination waste-and-vent system be compared to the size required?

1. Same size as required

2. One size larger

3. Two sizes larger

4. Three sizes larger

The correct answer is: Two sizes larger

In a combination waste-and-vent system, every drainage pipe should be sized larger than what is typically required to ensure proper function and to mitigate potential issues such as clogs or insufficient drainage. The correct approach is to make those pipes two sizes larger than the standard requirement. This is necessary because the combination of drainage and venting functions can lead to increased flow and pressure conditions that need to be accommodated. A larger pipe size allows for improved flow capacity, reduces friction losses, and enhances overall system efficiency and reliability. The importance of this practice is particularly relevant in systems where fixtures are connected to a single drainage line, as this can create unique challenges in maintaining adequate drainage while also providing adequate venting. Thus, ensuring the pipes are sized appropriately is crucial for the system's performance.