What must be done if there are four or more water closets on a circuit vent?

When there are four or more water closets on a circuit vent, it is essential to provide a relief vent. This is because multiple fixtures, particularly water closets, can create significant airflow and backpressure within the vent system. As a result, proper venting becomes critical to maintain the functioning of the drainage system and to prevent issues such as siphoning or drainage noises.

A relief vent helps to equalize the pressure in the vent stack, allowing for better drainage performance and reducing the chance of negative pressure scenarios that can lead to the trap seals being compromised. If the system has too many fixtures connected to a single vent without appropriate pressure relief, the likelihood of trap siphonage increases, which could lead to malodorous sewer gases entering habitable spaces.

Thus, implementing a relief vent is a necessary step when dealing with a higher number of water closets to ensure that the plumbing system operates efficiently and safely.