Master Plumber Exam DWV and Storm Drainage 4 — Questions and Answers
Question 1: What is the primary purpose of an interceptor (grease trap) in a DWV system?
- To prevent sewer gases from entering the building
- To separate fats, oils, and grease from wastewater before it enters the drain (Correct answer)
- To filter sediment from storm water
- To equalize pressure between floors
Correct answer: To separate fats, oils, and grease from wastewater before it enters the drain
Grease interceptors separate fats, oils, and grease from kitchen wastewater to prevent accumulation and blockages in the sanitary sewer system.
Question 2: Under the IPC, which of the following fixtures requires an individual vent?
- A bathtub that shares a vent with a lavatory
- A floor drain in a commercial kitchen
- A water closet in all cases (Correct answer)
- A laundry standpipe served by a wet vent
Correct answer: A water closet in all cases
A water closet must be individually vented because its 3-inch drain carries high solid loads that make shared venting unreliable for trap protection.
Question 3: What is 'siphonage' in the context of drain traps?
- Positive pressure forcing sewer gas past the trap seal
- Negative pressure that draws water out of the trap, breaking the seal (Correct answer)
- Water flowing backward through the trap
- Corrosion of trap components by sewer gases
Correct answer: Negative pressure that draws water out of the trap, breaking the seal
Siphonage occurs when negative (sub-atmospheric) pressure in the drain pipe pulls water out of the trap, destroying the water seal that blocks sewer gases.
Question 4: A roof drain leader that runs inside the building must be constructed of which material?
- PVC only
- A material approved for DWV service (Correct answer)
- Copper only
- Galvanized steel only
Correct answer: A material approved for DWV service
Inside leaders (conductors) must be made of a material approved for DWV service, such as cast iron, PVC, ABS, or copper, because they are subject to building code requirements.
Question 5: What is the maximum number of fixture units that a 4-inch horizontal branch drain may carry under the IPC?
- 90 fixture units
- 160 fixture units (Correct answer)
- 216 fixture units
- 250 fixture units
Correct answer: 160 fixture units
Per IPC Table 710.1(2), a 4-inch horizontal branch at 1/4-inch slope may carry a maximum of 160 fixture units.
Question 6: Which of the following best describes 'back-to-back' fixture venting?
- Two fixtures on opposite sides of a wall sharing a common vent (Correct answer)
- Two fixtures on the same wall sharing a drain only
- A single fixture vented in two directions simultaneously
- A vent pipe that doubles back toward the fixture drain
Correct answer: Two fixtures on opposite sides of a wall sharing a common vent
Back-to-back venting allows two fixtures installed on opposite sides of a common wall to share a single vent pipe, reducing material and labor costs.
Question 7: What device is installed on a storm drain to prevent backflow from the storm sewer into the building?
- Backwater valve (Correct answer)
- Check valve on supply line
- Pressure reducing valve
- Vacuum breaker
Correct answer: Backwater valve
A backwater valve (backflow preventer) is installed on the storm drain to stop storm sewer surcharge from flowing back into the building's drainage system.
Master Plumber Exam DWV and Storm Drainage 4
What is the primary purpose of an interceptor (grease trap) in a DWV system?