Illinois Plumbing Code Practice 2025 – 400 Free Practice Questions to Pass the Exam

Question: 1 / 400

What should the size of a vent pipe be in relation to the drain it serves?

At least the same diameter as the drain pipe

At least half the diameter of the drain pipe

The size of the vent pipe in relation to the drain it serves is important for ensuring proper air flow and preventing pressure imbalances in the plumbing system. The correct choice states that the vent pipe should be at least half the diameter of the drain pipe.

This sizing is crucial because a properly sized vent allows air to enter the plumbing system when water drains, preventing suction that could lead to slow drainage or trap seal loss in the fixture traps. A vent that is too small may not allow sufficient air movement, adversely affecting the performance of the drainage system.

By specifying that the vent pipe diameter should be at least half the diameter of the drain pipe, the code balances flow dynamics while ensuring that the system remains functional and maintains sufficient air pressure. This sizing standard thus supports efficient drainage and prevents potential plumbing issues related to venting.

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At least one quarter the diameter of the drain pipe

Exact diameter of the drain pipe

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