How high above the flood rim of a fixture must a vent be extended vertically before it can be run horizontally?

The correct answer is that a vent must be extended vertically at least 3 inches above the flood rim of a fixture before it can be run horizontally. This requirement is rooted in plumbing codes, which are designed to ensure proper venting and airflow in drainage systems. A vent that is elevated above the flood rim helps to prevent any potential siphoning of water from the fixture, which could lead to sewer gases entering the living space or hinder the drainage of wastewater.

By adhering to the 3-inch vertical rise before transitioning to a horizontal run, the system maintains effective ventilation, allowing air to flow freely and assisting in the proper operation of the drainage system. This design consideration is critical in avoiding issues such as back pressure, which could compromise the efficiency of the plumbing system.

In contrast, options that suggest a height of 1, 2, or 4 inches do not align with the established guidelines for venting practices. Insufficient height may fail to provide adequate air circulation, while heights beyond the required 3 inches may be more than necessary to achieve the desired venting effect.