Core TradesQuestion 141 of 700
The MOST reliable method to prevent backflow contamination of the potable water supply at a fixture is:
a.Installing a single check valve
b.Increasing the supply water pressure
c.Providing a physical air gap of adequate height above the fixture flood rim
d.Adding length to the supply pipe
Explanation
A physical air gap (an unobstructed vertical space, typically at least twice the pipe diameter and not less than 1 inch, above the flood rim) cannot be defeated because there is no continuous path for backflow. Check valves are mechanical and can fail; higher pressure and longer pipe do nothing to stop back-siphonage.
Law Reference: 2025 CA Plumbing Code §603.0Practice all 700 questions free — no signup required.
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