3rd Year Electrical Trades Qualification (TQ) Practice Exam

Where should the relief vent be added on a bank of eight lavatories?

In the central utility room At the most downstream circuit vented fixture At the upstream end At the end of the main drain line

Correct Answer:

At the most downstream circuit vented fixture

Explanation:

When multiple lavatories drain into a common line, traps can siphon if the downstream flow creates a vacuum. A relief vent supplies air to the drain to break that vacuum as the fixtures drain. Placing the relief vent at the farthest downstream fixture in the vented circuit ensures air can move back along the entire run, protecting every trap along the bank. If you put the vent elsewhere, parts of the run might not receive sufficient air to prevent siphonage. So the best placement is at the most downstream fixture that is part of the vented circuit.

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