Which of the following fixtures is not permitted to be served by a combination waste and vent system?

• A. Sink
• B. Toilet
• C. Standpipe
• D. Bathtub

Answer: (C)

A combination waste and vent system is designed to handle both the drainage and ventilation needs of certain plumbing fixtures. However, it is important to recognize that specific fixtures have unique requirements due to their characteristics and the nature of their waste.

The standpipe, which is typically used to accept the waste from washing machines, is not allowed to be part of a combination waste and vent system. This restriction is primarily due to the potential for excessive water flow and the need for proper venting to avoid issues such as siphoning, which can occur with fixtures that discharge significant volumes of water at once. The design of the standpipe also demands separate venting to ensure that the trap is maintained and not compromised by negative pressure.

In contrast, sinks, toilets, and bathtubs can be served by a combination waste and vent system because they generally have drainage patterns and waste volumes that are compatible with this type of venting. Their design allows for effective drainage and venting without the risks that would accompany including a standpipe in such a system.