306A water service, distribution and wells practice questions.

Five free practice questions on water service, distribution and wells: block D of the Red Seal 306A Plumber exam, about 24 of its 125 questions. They are in the exam's format: four options, one correct answer. Answer each one before opening the explanation. No account, no email.

01The questions

1. Procedural · Installs cross connection controls

   The AHJ rates a connection as a moderate hazard: a potable supply feeding a fire-sprinkler system with a fire department connection that holds only potable water, with no antifreeze or other additives. Which device suits it per the code and CSA B64.10?

   1. A double check valve assembly (DCVA)
   2. A pressure vacuum breaker (PVB)
   3. A hose-connection vacuum breaker
   4. A dual check valve (DuC)
   Show the answer

   A. Correct. For a moderate, non-health hazard where back-pressure is possible (the fire department connection can pressurize the system above the supply), the testable double check, in its fire-protection version (CSA B64.5.1), is the NPC's selection for sprinkler systems without additives. Only a small all-potable-material system with no fire department connection may take a single check; add antifreeze or chemicals and that portion needs an RP.

   Why not the others

   B. A PVB protects against back-siphonage only. A sprinkler system can create back-pressure, which a PVB cannot stop.

   C. A hose-connection vacuum breaker is for hose bibbs and hose outlets that are not under continuous pressure. It is not a device for a pressurized piping system.

   D. A dual check is for low (minor) hazard, such as a residential service, and it cannot be field-tested. A moderate-hazard fire line needs a testable assembly such as a DCVA.

2. Procedural · Installs piping for private water pressure systems

   You are installing the suction line for a shallow-well jet pump drawing from a dug well. How must the line be laid?

   1. Rising steadily toward the pump, airtight, with no high spots
   2. Sloping down toward the pump so water drains to it
   3. Level, with an air vent at its highest point
   4. One size smaller than the pump inlet to speed the water up
   Show the answer

   A. Correct. Any high spot traps air that breaks the prime, so the line rises continuously from the foot valve to the pump. It is at least the pump's suction size, the foot valve sits below the lowest water level and clear of the bottom, and every joint is airtight — a suction leak draws in air rather than dripping water.

   Why not the others

   B. A line sloping down to the pump leaves its high point at the well end, where air collects and breaks the prime. It must rise toward the pump.

   C. A suction line is below atmospheric pressure, so a vent would let air in and lose the prime. The line is sloped up toward the pump so air moves to the pump and out.

   D. A smaller suction line adds friction and reduces the lift the pump can manage, inviting cavitation. The suction line is at least the pump's inlet size.

3. Recall · Installs components for water service

   Why is a copper or PE water service connected to the corporation stop at the main with a gooseneck or expansion loop rather than a straight run?

   1. It lets the main and service move or settle without straining the tap
   2. It gives the tracer wire a place to connect to the metal main
   3. It adds length so friction lowers the pressure entering the building
   4. It forms a trap that keeps sediment from the main out of the service
   Show the answer

   A. Correct. Mains and services move with settlement, frost, traffic loads and temperature. The loop (gooseneck) gives the service slack right at the corporation stop so that movement bends the loop instead of pulling on the tap or cracking the connection.

   Why not the others

   B. Tracer wire is brought up to accessible points such as the curb box and building, not bonded to the main at the tap. The gooseneck's purpose is to absorb movement.

   C. A loop adds a negligible length and is not a pressure control. Where supply pressure is too high, a PRV is installed inside the building.

   D. A water service under pressure does not trap sediment the way a drain trap holds a seal; water flows through the loop at full velocity. The loop is there for movement.

4. Recall · Installs components for potable water distribution systems

   A drain-water heat recovery unit is a vertical copper section of the drain stack with a coil wrapped around it. Why does it recover the most heat from showers?

   1. It stores warm drain water in the coil for the next hot draw
   2. Warm water drains while cold flows to the shower at the same time
   3. It heats the drain water to kill bacteria before the sewer
   4. Water pools in the section and gives the coil more contact time
   Show the answer

   B. Correct. The unit has no storage: it transfers heat only while warm water is falling down the stack as a film on the pipe wall and cold water is flowing through the coil at the same moment. A shower does both for minutes at a time, so the preheated cold water feeds the heater or the shower's cold side.

   Why not the others

   A. Drain water passes straight through the stack section and is never stored; nothing is held for later. The unit works only during simultaneous drain and supply flow.

   C. It takes heat out of the drain water to preheat the incoming supply; it adds no heat to the drain and plays no role in disinfection.

   D. The unit is installed vertically because drain water clings to the inside wall as a thin falling film; it does not pool. A horizontal section would transfer far less heat.

5. Recall · Installs components for private water pressure systems

   On a conventional (air-over-water) pressure tank with no diaphragm, what is the job of the air volume control?

   1. It maintains the air cushion so the tank does not waterlog
   2. It stops the pump from running when the well is dry
   3. It sets the pump's cut-in and cut-out pressures
   4. It relieves the tank if the pressure switch sticks closed
   Show the answer

   A. Correct. In a plain tank the water slowly absorbs the air cushion; as the air shrinks the tank waterlogs and the pump short-cycles. The air volume control admits or releases air to hold the cushion at the right level. Diaphragm and bladder tanks separate air from water and need no air volume control.

   Why not the others

   B. Dry-run protection comes from a low-water cut-off or pump protection relay. The air volume control deals with the tank's air, not the well level.

   C. Cut-in and cut-out are set on the pressure switch. The air volume control manages the amount of air in the tank.

   D. Over-pressure protection is the relief valve's job. The air volume control only keeps the air cushion at the right size.

02Answer key

03What block D covers

This block covers bringing potable water in and getting it to every outlet. It includes sizing water services and distribution piping, thrust restraint, meters and PRVs, hot-water recirculation, water heaters, T&P valves and expansion tanks, tempering valves, cross connection control with RP, DCVA and vacuum breakers, disinfection and pressure testing, and private wells with submersible and jet pumps and pressure tanks.

The Red Seal Occupational Standard names this block “Installs, tests and services water service and distribution”. It carries 19.2% of the exam, about 24 of the 125 questions, split across three tasks. The share column is each task's weight within the block, as the standard publishes it.

04What the questions turn on

Pressure left for friction

Start with the available pressure and subtract elevation at 0.433 psi per foot, meter and backflow losses, and the pressure the fixture needs. What remains is the friction allowance. The chosen size must also stay within the job's velocity limit.

Matching the backflow device

Back-pressure from pumps, boilers or pressurized systems needs an RP, DCVA, dual check or air gap. AVBs and PVBs stop back-siphonage only. An AVB can have no shut-off downstream, and chemically treated boiler make-up takes an RP.

Closed systems and expansion

A PRV with no thermal bypass closes the system, so each heating cycle raises pressure until the T&P valve relieves at 150 psi. A thermal expansion tank on the heater's cold supply, precharged to the incoming static pressure, absorbs it.

Pressure tank precharge

Set a diaphragm tank's precharge to cut-in minus 2 psi with the tank empty of water. A reading at the air valve under system pressure only shows system pressure. A pump that cycles on every draw points to lost precharge or a failed diaphragm.

05The other blocks

The 306A exam has seven blocks. Each has its own page of free questions:

The 306A Plumber practice exam page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.