NCIDQ IDFX Practice Exam

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Where must a vent connect to a soil or waste stack?

At the base of the stack
Above the highest fixture in the system
At the midpoint of the stack
Below the floor level

The correct answer is: Above the highest fixture in the system

The correct answer is that a vent must connect to a soil or waste stack above the highest fixture in the system. This requirement is based on plumbing code regulations designed to ensure proper air circulation and prevent the creation of negative pressure within the drainage system. If a vent is connected below the highest fixture, it may not effectively allow air to enter the drainage system, which can lead to siphoning of the traps connected to those fixtures. Siphoning can allow sewer gases to escape into the living spaces and can introduce blockages in the waste system when water is drawn from the fixtures. By placing the vent above the highest fixture, it ensures that there is always air pressure available to balance the flow of waste and facilitates the free movement of air necessary to maintain the function of the drainage system. This connection point also helps to prevent backflow issues and ensures that any potential odors or gases are vented outside rather than into the interior of a building, maintaining health and safety standards.