or less the low end of the drain is not flooded and air can get in Without an

Or less the low end of the drain is not flooded and

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If the drain is 5 ft. or less, the low end of the drain is not flooded, and air can get in. Without an effective plumbing drain vent, a vacuum forms in the pipebetween the water going down the drain and the water that needs to stay in the trap. A siphon is created and the atmospheric air pressure pushes the water out of the trap to satisfy the vacuum. This allows sewer odors to back up into the house. The gurgling sound is air forcing its way through the water in the trap.
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If the drain pipe (trap arm) is longer than 5 feet distance from the vertical plumbing vent stack, or the plumbing drain pipe slope is morethan ¼ inch per foot, a 1 ¼ inch diameter pipe will flood. That’s the same as having no vent. The illustration below shows that air from the stack can’t get into the trap arm, and a siphon results.
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TYPES OF PLUMBING VENTS
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Individual vents Individual vents serve individual fixtures. This vents only one fixture but may connect into an other vent that extend to the open air. an individual vent should be at least one-half the size of the drain it vents (no less than 1 1/4", 32 mm)
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Relief vents Relief vents provide additional air to the drainage system where the primary vent is too far from the fixture. a relief vent should be at least one-half the size of the drain it vents (no less than 1 1/4", 32 mm)
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Circuit vents Circuit vents are used with more than one fixtures. A circuit vent is installed before the last fixture extend up to open air or connects to an other vent that extend to the outside a circuit vent should be at least one-half the size of the drain it vents (no less than 1 1/4", 32 mm)
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Non-Conventional Venting Methods
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Island Fixture Venting This is a specific method for venting an island sink, one limited to sinks and lavatories. Residential kitchens sinks with a dishwasher waste connection, a food waste grinder, or both, in combination with the kitchen sink waste, shall be permitted. The island fixture vent shall connect to the fixture drain as required for an individual or common vent. The vent shall rise vertically to above the drainage outlet of the fixture being vented before offsetting horizontally or vertically downward. The vent or branch vent for multiple island fixture vents shall extend to a minimum of 6 inches (152 mm) above the highest island fixture being vented before connecting to the outside vent terminal. The lowest point of the island fixture vent shall connect full size to the drainage system. The connection shall be to a vertical drain pipe or to the top half of a horizontal drain pipe. Cleanouts shall be provided in the island fixture vent to permit rodding of all vent piping located below the flood level rim of the fixtures. Rodding in both directions shall be permitted through a cleanout.
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