A gas water heater sits in a small, sealed closet with no dedicated combustion-air openings. What is the most likely violation?
Not quite. The answer is B: Insufficient combustion air supply to the appliance
A sealed closet starves a gas appliance of the air it needs to burn fuel completely and vent safely, risking carbon monoxide in the space — a life-safety problem unrelated to drain proximity, relief-valve materials, or expansion control. In Washington the combustion-air rules for fuel-burning water heaters come from the Mechanical Code, not the UPC's own chapter 5.
Reference: WAC 2021 51-56-0500, §506.0 (Combustion air for water heaters is governed by the Mechanical Code in Washington)
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