I85PREP
⌘ K
MD Plumber study materials
MD Plumber study resources

MD Plumber practice questions

One question at a time. Build your MD Plumber understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1General Plumbing Knowledge

In a typical residential drainage system, what is the primary purpose of the vent stack that runs from the drainage system through the roof?

  1. To equalize air pressure in the drain pipes so trap seals are not siphoned out Correct answer.
  2. To supply fresh air to the water heater combustion chamber
  3. To provide a secondary path for sewage during a main line blockage
  4. To carry rainwater from the roof to the storm sewer

Explanation

A vent stack admits air into the drainage system, keeping pressure near atmospheric so water flowing past a fixture trap does not siphon the trap seal. Without adequate venting, gurgling and sewer gas odors appear. Vents do not carry storm water or supply combustion air.

Question 2General Plumbing Knowledge

What is the standard nominal outside diameter of 1/2-inch Type L copper tubing used in potable water lines?

  1. 7/8 inch
  2. 1/2 inch
  3. 5/8 inch Correct answer.
  4. 3/4 inch

Explanation

Copper tube sizing is by nominal inside diameter, so 1/2-inch Type L copper has an actual outside diameter of 5/8 inch. This matters when selecting fittings and hangers, which are sized to the outside diameter. The 1/2-inch figure refers to nominal bore, not the exterior.

Question 3General Plumbing Knowledge

A fixture drain must slope toward the drain outlet. What is the generally accepted minimum slope for a 3-inch horizontal building drain?

  1. 1/4 inch per foot
  2. 1/2 inch per foot
  3. 1/8 inch per foot Correct answer.
  4. 1/16 inch per foot

Explanation

Horizontal drainage piping 3 inches and larger may be installed at 1/8 inch per foot where a 1/4 inch per foot slope is not achievable, provided the pipe has adequate scouring velocity. Smaller pipe such as 1-1/2 and 2 inch generally requires 1/4 inch per foot. The larger diameter tolerates a flatter grade.

Question 4General Plumbing Knowledge

What is the principal reason a dielectric union is installed between a copper pipe and a galvanized steel pipe?

  1. To allow the joint to be disassembled without cutting
  2. To prevent galvanic corrosion caused by dissimilar metals in contact Correct answer.
  3. To dampen vibration transmitted through the piping
  4. To satisfy a requirement for a removable test point

Explanation

When copper and steel are joined directly in the presence of an electrolyte such as water, a galvanic cell forms and the less noble metal, the steel, corrodes rapidly. A dielectric union inserts an insulating barrier to break the electrical path. Removability and vibration control are secondary benefits, not the reason.

Question 5General Plumbing Knowledge

A water closet in a bathroom group is being installed. Which trap arrangement is acceptable for a floor-mounted toilet?

  1. A running trap shared with the adjacent lavatory
  2. A drum trap located within 2 feet of the fixture
  3. An integral trap built into the fixture itself Correct answer.
  4. A separate P-trap installed below the floor in the drain line

Explanation

Water closets are manufactured with an integral trap in the bowl, so no separate trap is added to the fixture drain. Adding an external trap would create a double trap and cause drainage problems. Drum traps and running traps are prohibited or restricted in modern sanitary systems.

Question 6General Plumbing Knowledge

What is the minimum air gap required between the end of a water supply faucet and the flood level rim of a fixture it serves, for a faucet with an effective opening of 1/2 inch?

  1. One half the diameter of the effective opening
  2. Twice the diameter of the effective opening Correct answer.
  3. Equal to the diameter of the effective opening
  4. One inch regardless of opening size

Explanation

The minimum air gap for a potable water outlet is twice the effective diameter of the opening, with an absolute minimum of 1 inch. This prevents backflow by ensuring a physical separation that no siphon can bridge. For a 1/2-inch opening, the required gap is 1 inch because the 1-inch floor governs.

Question 7General Plumbing Knowledge

A plumber is sizing a water heater expansion tank on a closed potable system. What condition makes an expansion tank necessary?

  1. The building has more than two stories
  2. The system has a backflow preventer or check valve that traps heated water Correct answer.
  3. The water heater is gas-fired rather than electric
  4. The water pressure exceeds 40 psi

Explanation

A closed system created by a backflow preventer, check valve, or pressure-reducing valve prevents expanded water from pushing back into the main. Thermal expansion then raises pressure until the T&P relief valve discharges. An expansion tank provides an air cushion to absorb that volume.

Question 8General Plumbing Knowledge

Which fitting is used to connect two pipes of different diameters in a drainage line while maintaining a smooth flow path?

  1. A reducing coupling Correct answer.
  2. A sanitary tee
  3. A wye branch
  4. A test tee

Explanation

A reducing coupling transitions between two different pipe sizes in a straight run and keeps the bore aligned for flow. Sanitary tees and wyes are branch fittings, not size transitions. A test tee is for pressure testing, not for changing diameter.

Question 9General Plumbing Knowledge

What is the purpose of a trap primer?

  1. To test the trap seal for leaks
  2. To inject chemicals that dissolve grease in the drain line
  3. To add water to a floor drain trap that would otherwise evaporate dry Correct answer.
  4. To maintain pressure in the water supply system

Explanation

Floor drains in infrequently used areas can lose their trap seal through evaporation, allowing sewer gas into the building. A trap primer delivers a small amount of water to the trap on a regular basis to keep the seal intact. It is not a chemical or pressure device.

Question 10General Plumbing Knowledge

A plumber is installing a water service line. What is the typical minimum burial depth for a water service pipe to protect it from frost in a cold climate?

  1. 24 inches below grade in all jurisdictions
  2. Below the local frost line Correct answer.
  3. Directly on the footing
  4. 12 inches below grade

Explanation

Water service lines must be buried below the frost line so the soil above insulates them and freezing does not rupture the pipe. The frost depth varies by region and is set locally, so no single depth applies everywhere. Installing on a footing or at a fixed shallow depth risks freezing.

Question 11General Plumbing Knowledge

What does the term "developed length" mean when calculating pipe sizing for a water distribution system?

  1. The vertical rise from the main to the highest fixture
  2. The total length of pipe including fittings and changes in direction Correct answer.
  3. The length of pipe exposed above grade
  4. The straight-line distance between the meter and the farthest fixture

Explanation

Developed length accounts for the actual path of the pipe, including elbows, tees, and offsets, which add friction loss beyond the straight-line distance. Sizing tables and pressure-loss calculations use developed length to capture real resistance. Straight-line distance ignores fitting losses.

Question 12General Plumbing Knowledge

Which of the following is the correct method for joining two lengths of ABS plastic drain pipe?

  1. Lead-oakum joints
  2. Solvent cement applied to both surfaces before assembly Correct answer.
  3. Threaded fittings with Teflon tape
  4. Compression fittings with rubber gaskets

Explanation

ABS pipe is joined with ABS solvent cement, which softens the surfaces and fuses them into a continuous joint. The cement is applied to both the pipe end and the fitting socket before assembly. Threading, lead-oakum, and compression methods are not used for ABS drainage.

Question 13General Plumbing Knowledge

A lavatory drain is gurgling and the trap seal is being pulled out when the toilet is flushed nearby. What is the most likely cause?

  1. The water pressure in the building is too high
  2. The lavatory faucet aerator is clogged
  3. The lavatory trap is too large
  4. The vent for the bathroom group is blocked or undersized Correct answer.

Explanation

Gurgling and loss of trap seal indicate that the drainage system is pulling air through the trap because the vent is not supplying enough air. When the toilet discharges, the slug of water creates negative pressure that siphons the lavatory trap. Clearing or upsizing the vent restores pressure balance.

Question 14General Plumbing Knowledge

What is the primary function of a pressure-reducing valve on a domestic water service?

  1. To increase pressure to upper floors
  2. To lower incoming street pressure to a safe usable level Correct answer.
  3. To prevent backflow into the public main
  4. To remove sediment from the water supply

Explanation

A pressure-reducing valve regulates high street pressure down to a set level, typically around 50 to 60 psi, protecting fixtures and appliances from damage. It does not boost pressure or filter water. Backflow prevention is handled by a separate device.

Question 15General Plumbing Knowledge

When installing a gas-fired water heater, what is the purpose of the draft hood?

  1. To prevent the tank from overpressurizing
  2. To mix dilution air with flue gases and stabilize draft Correct answer.
  3. To preheat incoming cold water
  4. To shut off gas if the pilot goes out

Explanation

The draft hood admits room air to dilute flue gases and breaks the draft so that changes in chimney pressure do not affect combustion. It helps maintain a stable, safe exhaust flow. Overpressure protection is the T&P valve, and flame safety is the thermocouple.

Question 16General Plumbing Knowledge

A plumber must install a cleanout on a building drain. What is the general rule for cleanout placement at the base of a vertical stack?

  1. A cleanout is required at the base of each vertical stack where it connects to the building drain Correct answer.
  2. Cleanouts are needed only on the upper floors
  3. Cleanouts are optional if the drain is plastic
  4. Cleanouts are only required at the building exterior

Explanation

A cleanout at the base of a stack gives access to the most likely blockage point where the vertical stack turns horizontal. Codes generally require it at or near the base of each stack. Exterior-only or optional cleanouts would leave the stack base inaccessible for rodding.

Question 17General Plumbing Knowledge

What is the difference between a wye and a sanitary tee in drainage piping?

  1. A wye is used only for venting, while a sanitary tee is used only for drainage
  2. A wye is a reducing fitting, while a sanitary tee is a straight coupling
  3. A wye is allowed only below grade, while a sanitary tee is allowed only above grade
  4. A wye provides a sweeping branch that promotes smooth flow, while a sanitary tee has a tighter 90-degree branch Correct answer.

Explanation

A wye branches at an angle with a sweep that directs flow smoothly in the direction of drainage, reducing turbulence and clogging. A sanitary tee has a gentler curve than a straight tee but still turns flow at 90 degrees. Both are drainage fittings, and neither is restricted to vent-only use.

Question 18General Plumbing Knowledge

A backflow preventer is required on a boiler feed line connected to a potable water system. Which device provides the appropriate protection for this cross-connection?

  1. A single check valve
  2. A reduced-pressure principle assembly Correct answer.
  3. A pressure gauge
  4. A vacuum breaker

Explanation

Boiler feed connections are a high-hazard cross-connection because boiler chemicals and hot water can contaminate the potable supply. A reduced-pressure principle assembly provides protection against both backpressure and backsiphonage. A single check valve is not adequate for high-hazard protection.

Question 19General Plumbing Knowledge

What is the purpose of a floor drain trap seal primer in a commercial kitchen?

  1. To increase drain flow rate
  2. To keep the floor drain trap from evaporating dry between uses Correct answer.
  3. To add detergent to the drain line
  4. To filter food particles from the drain

Explanation

Floor drains in commercial kitchens may go long periods without use, allowing the trap seal to evaporate and sewer gas to enter. A trap primer periodically adds water to maintain the seal. It does not clean, increase flow, or filter the drain.

Question 20General Plumbing Knowledge

Which type of valve is best suited for throttling flow in a water distribution line?

  1. A gate valve
  2. A ball valve used only fully open or closed
  3. A check valve
  4. A globe valve Correct answer.

Explanation

A globe valve has a plug that moves against the flow, allowing gradual and precise throttling. Gate valves are designed for isolation and should be fully open or closed. Check valves allow flow in one direction only and are not throttling devices.

Question 21General Plumbing Knowledge

In a water distribution system, what does "fixture unit" represent?

  1. A measure of the probable demand a fixture places on the supply system Correct answer.
  2. The number of fixtures allowed on one branch
  3. The pressure drop across a fixture
  4. The physical size of the fixture in square inches

Explanation

A fixture unit is a dimensionless measure of the load a fixture places on the water supply or drainage system, based on its flow rate and usage pattern. It is used to size pipes and determine allowable loads. It is not a physical dimension or a pressure measurement.

Question 22General Plumbing Knowledge

A plumber is replacing a section of cast iron soil pipe with PVC. What must be done at the transition to maintain proper support and alignment?

  1. Weld the PVC directly to the cast iron
  2. Use a transition coupling with appropriate clamps and support both pipe sections independently Correct answer.
  3. Suspend the PVC from the cast iron with wire
  4. Fill the joint with concrete

Explanation

A transition coupling with suitable clamps accommodates the different outside diameters of cast iron and PVC and provides a watertight joint. Each pipe material must be supported independently because PVC is lighter and more flexible. Welding, wire suspension, and concrete are not acceptable joining methods.

Question 23General Plumbing Knowledge

What is the minimum size of a building sewer serving a single-family dwelling, per typical plumbing code?

  1. 6 inches
  2. 3 inches
  3. 4 inches Correct answer.
  4. 2 inches

Explanation

A building sewer serving a single dwelling typically must be at least 4 inches in diameter to handle the combined drainage load and provide adequate scouring. Smaller sizes are used for branch drains, not the main sewer. Six-inch pipe is larger than required for a single residence.

Question 24General Plumbing Knowledge

A water heater's temperature and pressure relief valve discharges continuously. What is the most appropriate first diagnostic step?

  1. Cap the relief valve to stop the leak
  2. Reduce the size of the expansion tank
  3. Increase the thermostat setting
  4. Check whether the valve is stuck open or the system is overpressure or overheating Correct answer.

Explanation

Continuous discharge indicates either a failed relief valve or a real overpressure or overheating condition. The plumber must determine whether the valve is stuck or whether thermal expansion, a failed thermostat, or a blocked system is raising pressure. Capping the valve is dangerous and prohibited.

Question 25General Plumbing Knowledge

What is the purpose of a P-trap on a fixture drain?

  1. To filter debris before it enters the drain
  2. To increase drain velocity
  3. To provide a cleanout access point
  4. To create a water seal that blocks sewer gas from entering the building Correct answer.

Explanation

A P-trap holds a water seal in its lower bend that prevents sewer gases from traveling back into the occupied space. The depth of the seal is regulated to resist evaporation and siphonage. It is not a filter, a velocity booster, or a cleanout.

Question 26General Plumbing Knowledge

When installing a horizontal branch drain in a building, what is the standard slope requirement for a pipe 3 inches (76 mm) in diameter or smaller?

  1. 1 inch per foot (about 8 percent)
  2. 1/8 inch per foot (about 1 percent)
  3. 1/2 inch per foot (about 4 percent)
  4. 1/4 inch per foot (about 2 percent) Correct answer.

Explanation

Plumbing codes require pipes 3 inches and smaller to slope at 1/4 inch per foot, which provides roughly 2 percent fall and keeps solids moving. Larger pipes may use 1/8 inch per foot. Too little slope allows solids to settle, while excessive slope lets liquids drain away from waste.

Question 27General Plumbing Knowledge

A plumber is installing a water closet (toilet) in a residential bathroom. What is the typical minimum distance required from the centerline of the toilet to any side wall or partition?

  1. 15 inches (380 mm) Correct answer.
  2. 30 inches (760 mm)
  3. 10 inches (250 mm)
  4. 24 inches (610 mm)

Explanation

Most plumbing codes require at least 15 inches of clear space from the centerline of a water closet to any side wall or partition. This gives the user adequate elbow room and clearance for installation. The 30-inch dimension refers to the clear space in front of the fixture, not the side clearance.

Question 28General Plumbing Knowledge

What is the primary purpose of installing a vent through the roof on a drainage system?

  1. To allow rainwater to flush the drains during storms
  2. To equalize air pressure in the drainage piping so trap seals are not siphoned Correct answer.
  3. To provide a connection point for a garden hose
  4. To increase the velocity of wastewater in the building drain

Explanation

A vent admits air into the drainage system so that pressure stays balanced as wastewater flows. Without proper venting, a siphon can pull the water seal out of a trap and allow sewer gas into the building. Vents also help gases escape safely above the roof.

Question 29General Plumbing Knowledge

Which material is typically used for the water supply piping inside a home where copper is not desired and chlorinated potable water is carried?

  1. ABS (acrylonitrile butadiene styrene)
  2. CPVC (chlorinated polyvinyl chloride) Correct answer.
  3. PVC schedule 40
  4. Cast iron

Explanation

CPVC is rated for hot and cold potable water distribution and withstands the temperatures and pressures found in domestic supply lines. Standard PVC is limited to cold water and drainage use, and ABS is a drainage material. Cast iron is used for drain, waste, and vent piping, not supply.

Question 30General Plumbing Knowledge

A plumber is replacing a lavatory faucet. What does the term "aerator" refer to on a faucet?

  1. A fitting that connects the faucet to the drain
  2. A valve inside the wall that controls water temperature
  3. A type of backflow preventer installed on the supply line
  4. A screen device at the spout that mixes air into the water stream Correct answer.

Explanation

An aerator is the small screen-and-mesh fitting threaded onto the end of the spout. It mixes air with the water to reduce splashing and can restrict flow to a rated rate. It is a common source of reduced flow when clogged with mineral deposits.

Question 31General Plumbing Knowledge

What is the minimum size of a vent stack serving a single bathroom group in a typical one-family dwelling?

  1. 1/2 inch (15 mm)
  2. 3 inches (76 mm)
  3. 1-1/4 inches (32 mm) Correct answer.
  4. 4 inches (100 mm)

Explanation

A vent serving a bathroom group is commonly sized at 1-1/4 inches minimum, though the exact size depends on the fixture units and developed length. A 1/2-inch vent is too small for a bathroom group. Three- and four-inch vents are far larger than needed for a single bathroom.

Question 32General Plumbing Knowledge

Which tool is used to cut and ream the inside of a copper tube after cutting so the fitting will seat properly?

  1. A deburring/reaming tool Correct answer.
  2. A soil pipe cutter
  3. A pipe wrench
  4. A flaring block

Explanation

After cutting copper tube, the inside burr must be removed with a reamer or deburring tool so the tube seats fully in the fitting and flow is not restricted. A pipe wrench is for turning threaded pipe, a flaring block is for flare fittings, and a soil pipe cutter is for cast iron.

Question 33General Plumbing Knowledge

A plumber notices a persistent sewer odor in a bathroom that is rarely used. Which cause is most likely?

  1. A failed pressure-reducing valve
  2. A loose toilet seat bolt
  3. A clogged aerator on the lavatory faucet
  4. An evaporated trap seal in a floor drain or unused fixture Correct answer.

Explanation

Traps in fixtures that are rarely used can dry out as water evaporates, breaking the seal and allowing sewer gas to enter. The usual remedy is to refill the trap or install a trap primer. Aerators, pressure-reducing valves, and seat bolts have no connection to sewer gas.

Question 34General Plumbing Knowledge

A plumber is calculating the total fixture units for a building's water distribution system. Which fixture is assigned the highest demand value among these?

  1. Lavatory faucet
  2. Bathtub with a standard faucet
  3. Flushometer valve water closet Correct answer.
  4. Dishwasher

Explanation

Flushometer-valve water closets demand a large instantaneous flow and are assigned a high fixture-unit value that drives supply pipe sizing. Tank-type fixtures and ordinary faucets draw far less. This is why flushometer systems require larger supply mains.

Question 35General Plumbing Knowledge

What is the purpose of a dielectric union when connecting copper pipe to a steel pipe?

  1. To allow the pipe to expand and contract freely
  2. To increase the flow rate through the transition
  3. To provide a removable joint for cleaning
  4. To prevent galvanic corrosion by electrically isolating the two metals Correct answer.

Explanation

When copper and steel are joined directly in the presence of water, galvanic corrosion attacks the less noble metal. A dielectric union inserts an insulating barrier so the two metals are electrically separated. This prolongs the life of the joint and the piping.

Question 36General Plumbing Knowledge

A plumber is installing a sump pump in a basement. Where should the discharge line's check valve be located?

  1. At the highest point of the discharge run
  2. On the discharge line above the pump, before the line exits the pit Correct answer.
  3. On the inlet side of the pump
  4. At the bottom of the pit below the pump

Explanation

A check valve on the discharge line just above the pump prevents water from flowing back into the pit when the pump shuts off. It should be accessible for service and located before the line leaves the pit. Placing it below the pump or on the inlet side would not stop backflow.

Question 37General Plumbing Knowledge

What is the function of a relief valve on a boiler or water heater?

  1. To remove air from the system
  2. To prevent backflow of potable water
  3. To open and release pressure when the system exceeds its set limit Correct answer.
  4. To regulate the incoming water temperature

Explanation

A temperature and pressure relief valve is a safety device that opens when pressure or temperature exceeds its setpoint, protecting the tank from rupture. It must be piped to a safe discharge point. It does not regulate temperature, prevent backflow, or remove air.

Question 38General Plumbing Knowledge

A plumber is installing a water heater in a garage. What is the typical requirement for protecting the ignition source from gasoline vapors?

  1. The water heater must be elevated so the ignition source is at least 18 inches above the floor Correct answer.
  2. The water heater must be vented through a side wall
  3. The water heater must be placed inside a closet
  4. The water heater may sit directly on the floor if it has a sealed burner

Explanation

Gas-fired water heaters installed in garages must have their ignition source elevated, commonly 18 inches above the floor, to reduce the risk of igniting gasoline vapors that collect near the ground. Sealed-combustion units may have different rules, but the general requirement is elevation. Closet location and side-wall venting do not address vapor ignition.

Question 39General Plumbing Knowledge

Which type of pipe is typically required for underground building sewer lines because of its strength and resistance to root intrusion at joints?

  1. SDR-rated PVC sewer pipe with solvent-welded or gasketed joints Correct answer.
  2. Thin-wall copper tubing
  3. Galvanized steel pipe
  4. PEX tubing

Explanation

Underground building sewers are commonly installed with SDR-rated PVC sewer pipe joined by solvent welding or gaskets, which provides strength and tight joints that resist root intrusion. Copper, galvanized steel, and PEX are not typical sewer materials.

Question 40General Plumbing Knowledge

A plumber is troubleshooting low water pressure at a showerhead while other fixtures in the house have normal pressure. What is the most likely cause?

  1. A failed pressure-reducing valve at the service
  2. A worn toilet flapper
  3. A clogged building sewer
  4. A clogged showerhead or a faulty pressure-balancing cartridge Correct answer.

Explanation

When only one fixture has low pressure, the problem is local to that fixture, such as a clogged showerhead, a restricted cartridge, or a partially closed stop valve. A failed pressure-reducing valve would affect the whole house. Sewer clogs and toilet flappers are unrelated to supply pressure.

Question 41General Plumbing Knowledge

What is the purpose of a cleanout in a drainage system?

  1. To connect a fixture drain to the trap
  2. To vent sewer gas above the roof
  3. To provide access for clearing blockages in the drain line Correct answer.
  4. To measure flow in the building drain

Explanation

A cleanout is an accessible opening fitted with a removable cap that allows a plumber to rod or auger a blocked drain. It is required at certain locations such as the base of a stack and at changes in direction. It is not a vent, a trap connection, or a flow-measuring device.

Question 42General Plumbing Knowledge

A plumber is installing a floor drain in a commercial kitchen. Why is a trap primer often required?

  1. To prevent grease from entering the drain
  2. To increase the flow rate of the drain
  3. To provide a secondary vent for the drain
  4. To keep the floor drain trap seal from evaporating between uses Correct answer.

Explanation

Floor drains in commercial kitchens may go long periods without receiving water, allowing the trap seal to evaporate and sewer gas to escape. A trap primer automatically adds a small amount of water to maintain the seal. It does not increase flow, block grease, or act as a vent.

Question 43General Plumbing Knowledge

What is the minimum size of a water service pipe for a single-family dwelling, per typical plumbing code?

  1. 1-1/2 inches (40 mm)
  2. 2 inches (50 mm)
  3. 1/2 inch (15 mm)
  4. 3/4 inch (20 mm) Correct answer.

Explanation

A 3/4-inch water service is the common minimum for a single-family dwelling, though sizing depends on fixture units and developed length. A 1/2-inch service is generally too small for a whole house. Larger sizes are used when demand or run length requires it.

Question 44General Plumbing Knowledge

A plumber is joining two lengths of PEX tubing for a potable water line. Which method is appropriate?

  1. Threading the tubing and using a coupling
  2. A mechanical push or crimp fitting rated for PEX Correct answer.
  3. Solvent welding with PVC cement
  4. Soldering with lead-free solder

Explanation

PEX is joined with mechanical fittings such as crimp, clamp, or push-to-connect fittings that are rated for the tubing and for potable water. PEX cannot be solvent welded, soldered, or threaded. Using the wrong method would leak or contaminate the water.

Question 45General Plumbing Knowledge

A plumber is installing a backwater valve on a basement floor drain. What is its purpose?

  1. To vent the floor drain trap
  2. To prevent potable water from flowing backward into the main
  3. To prevent sewage from the public sewer from backing up into the basement Correct answer.
  4. To reduce water pressure to the fixtures

Explanation

A backwater valve is a check device installed on a drainage line to stop flow from reversing when the public sewer surcharges. It protects basements from sewage backups. Backflow prevention on potable supply, pressure reduction, and venting are handled by different devices.

Question 46General Plumbing Knowledge

What is the purpose of a water hammer arrestor in a plumbing system?

  1. To remove minerals from the water
  2. To stop the flow of water to a fixture
  3. To absorb the shock wave created when a valve closes quickly Correct answer.
  4. To increase the water pressure at the fixture

Explanation

When a valve closes suddenly, the momentum of moving water creates a pressure surge that travels through the piping and causes banging. A water hammer arrestor contains a cushion of air or a piston that absorbs the surge. It does not stop flow, filter water, or boost pressure.

Question 47General Plumbing Knowledge

A plumber is sizing a gas line for a residential range. Which factor most directly determines the required pipe diameter?

  1. The total BTU input of the appliances and the length of the run Correct answer.
  2. The number of electrical outlets in the kitchen
  3. The diameter of the water service
  4. The color of the appliance

Explanation

Gas pipe sizing depends on the total demand in BTUs per hour and the equivalent length of the run, including fittings. Longer runs and higher demand require larger pipe to keep pressure drop within limits. Appliance color, water service size, and electrical outlets are irrelevant.

Question 48General Plumbing Knowledge

A building has a 4-inch building drain and a 3-inch soil stack. What is the minimum size of the building sewer serving the building, per typical code?

  1. 3 inches
  2. 6 inches
  3. 4 inches Correct answer.
  4. 2 inches

Explanation

The building sewer must be at least as large as the building drain it receives, so a 4-inch building drain requires a sewer of at least 4 inches. A 3-inch sewer would be smaller than the drain. Six inches is larger than the minimum but not required.

Question 49General Plumbing Knowledge

A plumber is testing a new DWV system with a water test. What must be done before the test is considered valid?

  1. The system must be pressurized with air to 100 psi
  2. The system must be filled with water and held under the required head for the specified time with no drop in level Correct answer.
  3. The system must be flushed with hot water for one hour
  4. The system must be smoke tested only

Explanation

A water test on a DWV system requires filling the piping with water to the specified head and holding it for the required duration without loss of water level. This confirms that joints are tight. Air testing at 100 psi is dangerous and not the standard water test, and smoke testing is a separate method.

Question 50General Plumbing Knowledge

A plumber observes that a water heater's temperature and pressure relief valve is discharging continuously. What is the most appropriate first step?

  1. Install a larger expansion tank without further checks
  2. Increase the thermostat setting
  3. Shut off the heat source and investigate the cause before replacing parts Correct answer.
  4. Immediately cap the relief valve to stop the leak

Explanation

Continuous discharge indicates excess temperature or pressure, a failed valve, or a thermal expansion problem. The safe first step is to shut off the heat source and diagnose the cause. Capping the valve is dangerous, and changing the thermostat or adding a tank without diagnosis may not solve the problem.