A lavatory in a remodel would have a trap arm 10 feet long, which is beyond the permitted distance for its size. The contractor proposes to install an air admittance valve instead of extending a dry vent. What does the code require?
- AThe valve solves the problem, because a valve serves as the vent regardless of how far it sits from the trap
This is the most common misconception about these valves. They relax where the vent has to go, not how far the trap may sit from it.
- BThe valve is acceptable if it is installed within 12 inches of the fixture trap it protects
No rule ties a valve to within a foot of the trap. Inventing a tighter limit is as wrong as ignoring the real one.
- CThe valve is not permitted for a lavatory, because valves may vent only fixtures without water seals
Air admittance valves are routinely used on lavatories, sinks and other trapped fixtures. The restriction on their use is about location and system pressure, not about the presence of a trap.
- The valve must still be located within the maximum distance permitted between that trap and its vent
Why D is correct
The trap arm limit exists to stop the fixture's own discharge from siphoning its seal, and that mechanism does not care what is on the far end of the vent. Putting a valve at the vent connection changes how the vent breathes, not where it is allowed to be.
What this question is testing
Whether the candidate understands that an air admittance valve substitutes for the vent terminal rather than for the vent connection, and that trap arm distance limits still apply.
On the job
Valves get treated as a universal solvent on remodels, and this is the case where they are not. A ten foot trap arm siphons its own trap whether the vent ends in a valve or on the roof. The practical fixes are the same as always: move the vent connection closer, enlarge the trap arm to buy distance, or rearrange the fixture. Watch for existing work where somebody stuffed a valve under a vanity at the end of a long run and the customer reports odor after every use.
Memory technique
The valve is the roof, not the vent. The trap arm rule never goes away.
Exam tip
A valve changes where the vent ends, never where it starts. Trap arm distance always applies.
Where to look it up
IPC Chapter 9, the air admittance valve provisions read together with the fixture vent section and its trap-to-vent distance table. The UPC limits these valves far more tightly, so verify in your adopted code.