Free Master Plumber Plumbing System Design & Sizing Questions and Answers — Questions and Answers
Question 1: A master plumber is designing a natural gas piping system for a commercial building with multiple appliances. The longest pipe run from the meter to the furthest appliance is 150 feet. The total connected load is 450,000 BTU/hr. Using the longest length method and a typical sizing table for 0.5 psi gas pressure with a 0.5-inch water column pressure drop, what is the required pipe size for the main trunk line leaving the meter?
- 1-inch
- 1 1/4-inch
- 1 1/2-inch
- 2-inch (Correct answer)
Correct answer: 2-inch
Gas pipe sizing tables are based on the length of the longest run, the total BTU/hr load, and the allowable pressure drop. For a 150-foot longest run and a load of 450,000 BTU/hr (or 450 CFH for natural gas), a 1 1/2-inch pipe is typically rated for a lower capacity. Therefore, to safely supply the required volume of gas without excessive pressure drop, the next larger pipe size, which is 2-inch, must be selected. Always consult the specific sizing table in the applicable fuel gas code (like the IFGC).
Question 2: When designing a storm drainage system for a 10,000 square foot roof, which of the following is the primary factor in determining the required size of the horizontal storm drains and vertical leaders according to most model plumbing codes?
- The number of roof drains installed.
- The maximum projected roof area. (Correct answer)
- The height of the building.
- The material of the roofing membrane.
Correct answer: The maximum projected roof area.
Model plumbing codes, such as the IPC and UPC, base the sizing of storm drainage systems primarily on the maximum projected roof area that a particular drain or pipe will serve, along with the local rainfall rate. While the number of drains affects how this total area is divided, the fundamental calculation for sizing any single pipe is the area it must drain. Building height and roofing material are not the primary factors in the hydraulic sizing calculation.
Question 3: A new 80-gallon commercial water heater is being installed in a closed-loop potable water system with an incoming city water pressure of 65 psi. To accommodate thermal expansion, an expansion tank is required. What is the first and most critical step in correctly sizing this expansion tank?
- Calculating the total system volume, including all piping. (Correct answer)
- Determining the static fill pressure of the system at the tank's location.
- Setting the pressure relief valve to its maximum setting.
- Selecting the tank with the smallest physical footprint.
Correct answer: Calculating the total system volume, including all piping.
Properly sizing a thermal expansion tank requires several inputs, but the foundational value is the total volume of water in the system that will be heated. This includes the volume of the water heater itself plus the volume of the hot water piping. Once the total system volume is known, factors like water temperature change and system pressures are used to calculate the required acceptance volume of the tank.
Question 4: In a multi-story building, a plumbing system is designed using a combination waste and vent system for a stack of floor drains. Which of the following is a key requirement for this type of system under the International Plumbing Code (IPC)?
- The pipe must be sized one size smaller than a standard drainage pipe.
- The system can receive waste from toilets and urinals.
- The waste pipe must be oversized to allow for air circulation. (Correct answer)
- Air admittance valves are required at each fixture.
Correct answer: The waste pipe must be oversized to allow for air circulation.
A combination waste and vent system works by using an oversized horizontal drain pipe where the upper portion of the pipe remains dry and acts as the vent. This allows for both waste flow and air circulation within the same pipe. Because of this design, these systems are restricted to fixtures that do not produce heavy solids or suds, and sizing is critical to ensure it functions correctly without becoming self-siphoning.
Question 5: A sewage ejector pump is required for a basement bathroom group (water closet, lavatory, and shower) that is below the public sewer line. According to common plumbing code standards, what is the minimum size for the sump pit diameter and the discharge pipe for a non-grinder pump serving a water closet?
- 12-inch diameter pit, 1 1/2-inch discharge
- 18-inch diameter pit, 2-inch discharge (Correct answer)
- 24-inch diameter pit, 1 1/2-inch discharge
- 16-inch diameter pit, 3-inch discharge
Correct answer: 18-inch diameter pit, 2-inch discharge
Most plumbing codes specify a minimum diameter of 18 inches and a minimum depth of 24 or 30 inches for a sewage ejector pit to ensure adequate volume and space for the pump and float controls. For a non-grinder pump that must pass solids from a water closet, the minimum discharge pipe size is typically 2 inches to prevent clogging.
Question 6: A master plumber is sizing the main water distribution pipe for a small office building using the Water Supply Fixture Unit (WSFU) method. The total load is 50 WSFU (flush valve system), the static pressure is 70 psi, and the developed length to the most remote fixture is 200 feet. Using a standard IPC sizing table for Type L copper pipe, what is the critical calculation needed before selecting a pipe size from the chart?
- The total number of bends in the piping system.
- The pressure available for friction loss. (Correct answer)
- The average temperature of the supply water.
- The elevation of the water main in the street.
Correct answer: The pressure available for friction loss.
After determining the total WSFU, the next steps involve calculating the pressure losses. This is done by taking the available static pressure (70 psi) and subtracting all fixed pressure losses, such as pressure loss due to elevation (height of the building), pressure loss through the water meter, and the minimum required pressure at the most remote fixture. The remaining pressure is what is available to overcome friction loss in the piping. This value, along with the developed length, is used with sizing charts or formulas to determine the appropriate pipe diameter.
