Master Plumber Exam Plumbing System Design 3 — Questions and Answers
Question 1: Which type of water heater installation requires a dedicated sealed combustion air supply from outside the building?
- Electric resistance water heaters
- Direct vent (Category IV) gas water heaters (Correct answer)
- Indirect water heaters connected to a boiler
- Solar thermal storage tanks
Correct answer: Direct vent (Category IV) gas water heaters
Direct vent Category IV appliances draw combustion air from outdoors through a sealed concentric pipe system, eliminating indoor air dependency.
Question 2: When sizing a grease interceptor for a commercial kitchen, the design capacity is primarily based on which factor?
- The number of kitchen staff
- The flow rate of fixtures served and a retention time factor (Correct answer)
- The square footage of the kitchen
- The daily meal count of the restaurant
Correct answer: The flow rate of fixtures served and a retention time factor
Grease interceptor sizing uses the total fixture flow rate (GPM) multiplied by a retention time, per PDI G101 or local code.
Question 3: In a storm drainage system design, the 100-year storm event rainfall intensity is used to calculate which parameter?
- The velocity of flow in the leader pipe
- The required pipe diameter and roof drain capacity (Correct answer)
- The depth of the sump pit
- The slope of the horizontal storm drain
Correct answer: The required pipe diameter and roof drain capacity
Roof drain and storm pipe sizing is calculated using the design rainfall intensity (inches/hour) for the local 100-year storm to ensure adequate capacity.
Question 4: A dual-check backflow preventer is the minimum required protection for which type of cross-connection?
- Lawn irrigation systems with chemical injectors
- Residential boilers with antifreeze
- Residential water softeners and humidifiers (Correct answer)
- Fire sprinkler systems with non-potable water
Correct answer: Residential water softeners and humidifiers
Dual-check assemblies are acceptable for low-hazard applications like residential water softeners and humidifiers that present no health hazard.
Question 5: The developed length of a gas supply pipe is used in conjunction with which other value to determine minimum pipe diameter from code tables?
- Operating pressure and total BTU demand (Correct answer)
- Specific gravity of the gas and line pressure only
- Number of appliances regardless of BTU input
- Meter size and gas provider specifications
Correct answer: Operating pressure and total BTU demand
Gas pipe sizing tables require both the developed length of piping and the total BTU/hr demand of all connected appliances at the design operating pressure.
Question 6: What is the primary purpose of an air gap in a plumbing system?
- To prevent water hammer in supply lines
- To provide the highest level of backflow protection by physical separation (Correct answer)
- To allow thermal expansion in hot water systems
- To equalize pressure between supply and drain lines
Correct answer: To provide the highest level of backflow protection by physical separation
An air gap provides complete physical separation between the water outlet and the flood rim of a receptor, making backflow physically impossible.
Question 7: When designing fixture rough-in locations, what minimum clearance does the IPC require from the centerline of a water closet to an adjacent side wall or obstruction?
- 12 inches
- 15 inches (Correct answer)
- 18 inches
- 24 inches
Correct answer: 15 inches
The IPC requires a minimum of 15 inches from the water closet centerline to any side wall, partition, or obstruction.
Master Plumber Exam Plumbing System Design 3
Which type of water heater installation requires a dedicated sealed combustion air supply from outside the building?