CSLB Trade — Plumbing (C-36) — All Questions
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105 questions
A combination waste-and-vent system is chiefly intended for…
- a.floor drains, sinks, and similar fixtures where conventional venting is impractical, using oversized horizontal piping✓
- b.water-heater relief piping
- c.roof (storm) drainage
- d.a battery of water closets
A combination waste-and-vent system uses oversized horizontal drain piping that carries both waste and air, providing venting for fixtures like floor drains and sinks in large open areas where a conventional vent cannot be run. Water closets and other high-discharge fixtures are generally not permitted on this system. The oversized pipe flows only partially full so air can move above the waste.2022 California Plumbing Code (CPC) §910.0
As a vent extends upward toward its terminal, its required diameter…
- a.may be reduced by half at the roof
- b.may be reduced to 1/2 inch above the roof
- c.must double at every branch
- d.must not be reduced (no decrease in size in the direction of the terminal)✓
A vent may not be reduced in size in the direction of flow toward the outlet; the required diameter is maintained or increased, never decreased, up to the terminal. Reducing the vent would restrict airflow and could allow frost or debris to close it. This is separate from the frost-closure rule that enlarges the terminal in cold climates.
Two fixtures set on the same floor level and connecting to a common vertical drain within the required distance may share…
- a.a common vent, provided the drain and vent are sized for both fixtures✓
- b.one cleanout only
- c.a single trap for both
- d.one backwater valve
Two fixtures at the same level connecting to a shared vertical drain within the code distance may be served by a common vent, with the connecting drain and vent sized for the combined load. Each fixture still has its own trap; a single trap serving two fixtures is generally prohibited. Common venting saves piping for back-to-back layouts.
Where a building (house) trap is installed on the building drain, it is commonly accompanied by…
- a.a sump pump
- b.a grease interceptor
- c.a fresh-air inlet on the upstream (building) side to ventilate the drainage system✓
- d.a backwater valve on the vent
A building (house) trap isolates the building drainage from the public sewer, and when used it is paired with a fresh-air inlet on the building side so the drainage system stays ventilated. Because a house trap can collect debris and block flow, many modern codes discourage or prohibit it. The fresh-air inlet terminates in open air, screened against pests.
In a horizontal wet-vent system, how must the wet-vented pipe (which carries both waste and air) be sized?
- a.at 4 inches regardless of load
- b.at 1-1/4 inch minimum only
- c.the same as the smallest fixture drain
- d.Per the DFU load of the wet-vented section, and at least 2 inches where a water closet drains through it✓
The wet-vented pipe must be sized for the drainage fixture unit (DFU) load of the fixtures it serves, and must be at least 2 inches where a water closet discharges into it. Adequate size keeps the pipe partially full so air can pass above the waste to vent the group. Undersizing would let the pipe flow full and lose its venting action.2022 California Plumbing Code (CPC) §908.0