306A DWV, sewers and septic systems practice questions.

Five free practice questions on DWV, sewers and septic systems: block C of the Red Seal 306A Plumber exam, about 32 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 above-ground piping and components for interior drainage, waste and vent (DWV) systems

   A 3-inch (75 mm) PVC drain passes through a 1-hour fire-rated concrete floor in an apartment building. How is the penetration sealed?

   1. A listed firestop system for plastic pipe, e.g. a collar
   2. Non-shrink grout packed solid around the pipe
   3. Fire-rated caulk as used for a copper pipe
   4. Mineral wool packed tight and topped with latex caulk
   Show the answer

   A. Correct. Combustible pipe needs a listed (ULC) system matched to the pipe material, size and assembly rating. An intumescent collar or wrap expands in the heat and crushes the softening pipe to seal the opening.

   Why not the others

   B. Grout seals the annular space, but the plastic pipe itself burns out and leaves an open hole through the floor. A listed system for plastic pipe closes that hole.

   C. Metal-pipe systems assume the pipe stays in place. PVC softens and burns away, so a system listed for metal does not work. Use a system listed for the plastic pipe.

   D. When plastic pipe burns away, it leaves a 3-inch hole that mineral wool and ordinary caulk cannot close. Plastic pipe needs a listed system that closes the opening, such as an intumescent collar.

2. Procedural · Installs maintenance holes and catch basins

   A new 6-inch (150 mm) storm lateral from a hospital parking structure must enter an existing precast concrete maintenance hole. How is the new penetration made?

   1. Core-drill the wall and fill the gap around the pipe with spray foam
   2. Break an opening with a hammer drill and grout the pipe in solid
   3. Core-drill the wall and seal the pipe in a flexible gasketed connector
   4. Cut the opening at the joint between two riser sections
   Show the answer

   C. Correct. A cored hole keeps the structure's integrity, and a flexible rubber connector clamped to the pipe stays watertight while the lateral and the structure settle. Then reshape the channel in the benching so the new flow is directed to the outlet.

   Why not the others

   A. Coring is right, but spray foam is not a watertight, durable seal below grade and lets groundwater and fines in. Use a gasketed connector made for the pipe and the wall thickness.

   B. Hammering spalls and cracks the precast wall, and a rigid grout joint cracks as the pipe and structure settle differently. Core the hole cleanly and use a flexible, gasketed connector.

   D. A riser joint is where the structure is sealed; cutting into it breaks that seal and weakens the section. The penetration is cored through the wall of a section, clear of joints, as the structure maker allows.

3. Procedural · Installs components for sewage treatment systems

   You are setting the float switches in the pump chamber for a pumped leaching bed at a rural cottage. From the bottom of the chamber upward, what is the order of the float levels?

   1. Pump off, then high-level alarm, then pump on
   2. Pump on, then pump off, then high-level alarm
   3. High-level alarm, then pump off, then pump on
   4. Pump off, then pump on, then high-level alarm
   Show the answer

   D. Correct. The off float keeps the pump submerged for cooling; the volume between off and on is the dose sent to the bed; the alarm float sits above the on level and warns of pump failure. Wire the alarm on a circuit separate from the pump so a tripped pump breaker still alarms.

   Why not the others

   A. With the alarm below the pump-on level it sounds on every normal cycle, before the pump starts. The alarm goes above the on level.

   B. With the on float below the off float the pump would never be told to stop at the right level. The off float is lowest so the pump stays submerged; the on float above it sets the dose.

   C. An alarm float at the bottom would sound whenever the chamber had any liquid in it. The alarm sits above the pump-on level so it sounds only if the pump fails to keep up.

4. Procedural · Installs underground piping and components for interior drainage, waste and vent (DWV) systems

   A floor drain in a condo parking-garage vestibule is getting a pressure-drop trap seal primer. Which installation is correct?

   1. Valve on an accessible cold line, tubing to the drain's primer inlet
   2. No primer, but a deeper trap with a 150 mm (6 in) seal
   3. Valve buried in the slab beside the drain, fed from the hot line
   4. Tubing into the trap arm downstream of the seal
   Show the answer

   A. Correct. The primer valve sits on a cold-water line where it can be reached and adjusted, and its tubing, sleeved through the concrete, runs to the primer inlet on the drain body so the water lands in the trap.

   Why not the others

   B. The code sets only a minimum seal depth (38 mm); it has no 100 mm cap, and a deeper trap only delays evaporation while collecting sediment. A little-used floor drain needs a primer to replace what evaporates.

   C. A buried primer valve cannot be reached to service or adjust, and primer valves are fed from a cold-water line as the maker specifies. The valve goes on a cold line in an accessible spot.

   D. Water added downstream of the seal runs straight to the drain and never refills the trap. The primer water must enter the drain body upstream of the trap.

5. Procedural · Installs piping for sewers

   A school's basement washrooms sit below the building sewer, so they drain to a sealed, vented sewage sump with an ejector pump. Starting from the pump, in what order are the fittings on its discharge line?

   1. Shut-off valve, then check valve, then the gravity drain
   2. Shut-off valve only; the sealed lid stops any backflow
   3. Check valve, then shut-off valve, then the gravity drain
   4. Check valve only, so no one can close a valve with the pump on
   Show the answer

   C. Correct. The check valve next to the pump stops the discharge column from draining back into the sump, and the full-port shut-off above it lets you isolate the check valve and pump for service. Add a union for pump removal and a high-level alarm.

   Why not the others

   A. With the shut-off below the check valve, closing it does not hold back the column of sewage above the check, so the check valve cannot be serviced without draining that column onto the floor. The check valve goes first, the shut-off above it.

   B. The lid seals the sump against gas; it does nothing to stop the discharge line draining back into the sump after every cycle. Without a check valve the pump re-pumps the same sewage and short-cycles.

   D. The code calls for a shut-off valve as well, so the check valve and pump can be isolated and removed without draining the discharge line. The risk of closing it in error is managed by placing and labelling it, not by leaving it out.

02Answer key

03What block C covers

This block covers everything that carries waste away. It includes building sewers, inverts and grade, maintenance holes and catch basins, septic tanks, leaching beds, percolation tests and pump chambers, and interior DWV piping: fixture-unit sizing, trap arms, wet vents, circuit vents and air admittance valves, backwater valves, sewage sumps and interceptors, DWV water, air and smoke tests, and drain cleaning.

The Red Seal Occupational Standard names this block “Installs, tests and services sewers, sewage treatment systems and drainage, waste and vent (DWV) systems”. It carries 25.6% of the exam, about 32 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

Trap arm fall and crown vents

The total fall of a trap arm may not exceed its diameter. More fall puts the vent opening below the trap weir, and the arm siphons its own trap. A vent within two pipe diameters of the weir is a crown vent and fouls.

Sizing by fixture units

Add the fixture units, then read the table at the slope the site allows. If the load exceeds a size's limit at that slope, go up a size. A branch serving a WC is at least 3 in, and drains never get smaller downstream.

Rough-in test versus final test

A rough-in water test needs at least 3 m of head above the highest joint in the section, held for 15 minutes. The final test runs at only 25 mm of water column, because the trap seals are in place and must not be forced.

Septic tanks and pump chambers

A tank is due for pumping when sludge and scum together fill about a third of the liquid depth, measured from the riser and never by entering. In a pump chamber the floats go pump off, pump on, then high-level alarm, bottom up.

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.