General Plumbing Knowledge and Definitions
The General Plumbing Knowledge and Definitions domain tests precise code vocabulary: what makes up a plumbing system, who is the authority having jurisdiction (AHJ), and how terms such as stack, branch, riser, trap, and fixture unit change later sizing and inspection decisions. Definition questions often turn on what a pipe carries or its orientation, not on size alone.
Core terms
- A plumbing system includes potable water supply, fixtures, traps, drainage, venting, and appurtenances.
- “Approved” means accepted by the AHJ; an unapproved fitting is not code-compliant even if listed by a manufacturer.
- A soil stack carries sewage; a waste stack carries non-fecal waste; a vent stack carries air only.
- A fixture unit is a relative measure of load, used separately for drainage and water supply.
- A trap seal is the water depth in a trap, normally 2 to 4 inches.
Worked example
A 4-inch vertical pipe receives water closets on five floors. A 3-inch horizontal pipe from one closet joins it, and a 2-inch pipe continues above the roof. The vertical pipe is a soil stack because it carries water-closet discharge. The 3-inch horizontal pipe is a water-closet fixture drain or branch. The 2-inch above-roof continuation is a vent stack because it carries air, not sewage. Reasoning: classify by what the pipe carries and whether it is vertical or horizontal; diameter alone does not define it.
Traps that cost marks
- Calling every vertical pipe a riser: a riser supplies water; a stack drains it.
- Treating “branch” and “fixture drain” as interchangeable.
- Assuming a vent may carry drainage.
- Forgetting that “approved” is an AHJ decision, not a synonym for “listed.”