A bathtub is specified with a deck-mounted filler whose spout outlet sits below the flood level rim of the tub, so a full tub covers the outlet. What does the plumbing code require for that installation?
- ANothing further, because the tub's overflow prevents the water level from reaching the spout outlet
An overflow is a partial safeguard against flooding a bathroom, not against back-siphonage. It also clogs, which is exactly when the level rises past the spout.
- Backflow protection for that submerged inlet, since a spout below the flood level rim is a cross connection
- CA check valve on the hot and cold supplies to the filler, installed in an accessible location under the deck
Ordinary check valves are not recognized backflow protection for a submerged inlet, and burying them under a tub deck adds an inaccessible failure point.
- DAn increase in the size of the tub's overflow so that it can carry the full flow of the filler valve
Overflow capacity is a drainage question. Enlarging it does nothing about water being pulled back into the supply piping.
Why B is correct
The hazard is defined by geometry: an outlet under water with no air gap. The code answers it with backflow protection at the fitting rather than trusting an overflow that can be blocked with hair or a stopper.
What this question is testing
Whether the candidate can recognize a submerged inlet as a cross connection and knows that an overflow and an ordinary check valve are not substitutes for backflow protection.
On the job
Deck-mounted and floor-mounted fillers with low spouts are common on high-end tubs, and the compliant products carry integral backflow protection that shows up on the submittal as a listing number. When one arrives without it, the fix is a listed vacuum breaker in the supply at the required height, which is awkward under a finished deck, so catch it at submittal review. The same reasoning covers hand showers on hoses that can reach the bottom of a full tub.
Memory technique
If the spout can go under, the supply needs protection.
Exam tip
Any outlet that can end up under water is a cross connection. Look for the answer that adds backflow protection at the fitting.
Where to look it up
IPC Chapter 6, protection of potable water supply, read with the bathtub and fixture fitting provisions in Chapter 4. The UPC handles submerged inlets in its own backflow chapter, so verify in your adopted code.