Massachusetts Plumbers Journeyman Practice Exam

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

1. Same size

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 two sizes larger than the drain size it connects to. This requirement is rooted in the need to effectively handle both the liquid waste and the ventilation of the system. By installing larger pipes, the flow capacity is increased, which helps prevent clogs and allows for the proper movement of air within the system. This increase in size minimizes the chances of pressure imbalances and ensures that there is adequate removal of both water and gases. This design consideration is essential for maintaining the efficiency and safety of the plumbing system, ensuring that venting allows for adequate atmospheric pressure within the pipes while also accommodating the volume and flow of waste. The specifics of the code reflect the necessity of these larger sizes to prevent potential issues associated with drainage and venting inadequacies.