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This page defines plumbing vent system terms, distances, and functions, including how plumbing vents work on buildings, why plumbing vent piping is needed, and what happens to the building drains when the vent piping is not working. We define the soil stack, waste stack, wet vents and dry vents, and we summarize the distances permitted between plumbing fixtures and their vent piping. Our page top sketch of a plumbing stack vent and other sketches included below are provided courtesy of Carson Dunlop.
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to the author. Technical review by industry experts has been performed and is ongoing - reviewers welcomed and are listed at "References."
Plumbing Vent System Terms, Definitions, & Types of Vents
How & Why Building Plumbing Vent Piping Works
The plumbing vent system is part of the building plumbing drain system and serves two basic functions:
1. Allow building drains to flow freely by allowing air into the drain system, avoiding the vacuum and slow drainage that would otherwise occur at fixtures.
Imagine a full soda bottle with its cap off, turned upside down: the soda does not flow nicely out of the bottle. As some soda spills out, the spillage has to nearly stop to let some air into the bottle to fill the vacuum created by the soda leaving.
Now perform the same experiment, but punch a hole in the bottom of your soda bottle just before you turn it upside down. Because air can easily enter the bottle the soda flows nicely out of the bottle mouth.
2. Allow sewer gases to be vented safely outdoors. Because sewer gases may flow back up into the building drain piping from a public sewer or private septic system, and because some sewer gases are included in building waste flowing through the piping, the plumbing vent system needs to carry these gases outside, usually above the building roof, where they are disposed-of safely and without leaving unpleasant, or possibly dangerous smells and gases inside the building.
The basic plumbing vent terms are shown in the sketch at left. In these notes, the plumbing stack vents and other sketches included below are provided courtesy of Carson Dunlop.
What is the Plumbing Stack Vent?
Here we show the main building vent pipe, the plumbing stack vent, connecting inside plumbing drains to a vent pipe that extends above the building roof in order to let needed air into the drains and to vent sewer gases harmlessly above the building.
Below we explain how building plumbing vent systems work and why plumbing vents are needed for drain function and plumbing drain safety.
What is the Difference Between a Soil Stack & a Waste Stack?
The soil stack pipe, as shown in Carson Dunlop's sketch, carries waste from toilets to the house trap (if one is installed) and there connects to the sewer line extending outside the building and on to a public sewer or private septic system.
The soil stack is the large-diameter main vertical waste pipe or building drain, or vertical portion of the "main drain" in the building.
The soil stack pipe is normally extended outdoors above the building roof, as shown in the page top sketch.
Other main building drain piping sections that slope closer to horizontal are connected to the soil stack but move waste horizontally where needed in a building.
The waste stack pipe shown in the sketch refers to any other vertical drain piping in the building that does not carry soil (sewage) from a sanitary fixture (toilet). Typically waste stack piping carries drainage away from sinks, tubs, and showers.
How far can the plumbing vent piping be located from a plumbing fixture?
As we discuss at DRAIN NOISES, if the horizontal distance between a plumbing fixture and the vertical vent piping is too great, the fixture may not drain properly, producing slow drainage or gurgling noises.
Poor drainage is not just an annoyance, it can be unsafe since there is also the risk that the poorly-vented plumbing fixture will lose the water from its plumbing trap, then permitting sewer gases into the building.
As we show in Carson Dunlop's sketch, the distance allowed between a plumbing fixture (actually the fixture plumbing trap) and the vertical vent piping varies between a minimum and maximum as a function of the pipe diameter.
What is Direct Venting of Plumbing Fixtures?
Basically,larger piping diameter allows longer distances between a plumbing fixture and its vent stack. If a plumbing fixture is located too far from the main building vent stack, then its own drain pipe must have its own vent stack connection piping.
But if a plumbing fixture is close enough (five feet or less) to the main waste stack pipe (vent), the fixture does not usually require its own plumbing vent piping, and it is considered a direct-vented plumbing fixture.
Of course this rule presumes that the drain piping between the fixture trap and the waste stack is properly installed and properly sloped. The usual slope on the fixture drain piping is 1/4" of slope per 12" (foot) of horizontal distance or "run" of piping.
What is a Wet Vented Plumbing Fixture?
In many buildings we find that the toilet is located quite close (within 5 feet) of the main building waste stack. This makes sense because the toilet needs really effective venting. Our sketch above shows a toilet located close to the waste stack - an installation that should work fine.
When you flush a toilet it sends a sudden large volume of waste and wastewater into the building drain waste vent (DWV) piping. This surge of wastewater can certainly create a vacuum problem in the waste line if the vent piping system is inadequate, blocked, or missing entirely. It is exactly this condition that produces the gurgling or even siphonage out and loss of water in nearby sink or tub traps when you flush a toilet in a building where the vent piping is inadequate. See Plumbing Drain Noises.
A toilet that is located too far from the soil stack can be wet vented as shown in Carson Dunlop's sketch. The drain piping for a sink (basin) or other fixture located closer to the soil stack than is the toilet can provide a pathway to let air into the horizontal waste piping used by the toilet to carry waste to the soil stack.
But a wet vented fixture requires a larger drain pipe diameter in its wet portion as we show in the sketch. This diameter increase helps assure adequate air flow into the drain system in the event that the sink basin (in this example) happens to be draining at the same moment that the toilet is flushed.
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Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
What is a Septic System An Engineer's View - Types of treatment tanks, adsorption systems, pumps, and other special equipment are discussed in some further detail in this text
Components of a Septic System- the Basic Parts of a Conventional Septic Tank and Leachfield, a chapter in the Home Buyers Guide to Septic Systems
A Toxic Gas Testing Sampling Plan for Residential Indoor Air Investigations
Pennsylvania State Fact Sheets relating to domestic wastewater treatment systems include
Pennsylvania State Wastewater Treatment Fact Sheet SW-161, Septic System Failure: Diagnosis and Treatment
Pennsylvania State Wastewater Treatment Fact Sheet SW-162, The Soil Media and the Percolation Test
Pennsylvania State Wastewater Treatment Fact Sheet SW-l64, Mound Systems for Wastewater Treatment
Pennsylvania State Wastewater Treatment Fact Sheet SW-165, Septic Tank-Soil Absorption Systems
Document Sources used for this web page include but are not limited to: Agricultural Fact Sheet #SW-161 "Septic Tank Pumping," by Paul D. Robillard and Kelli S. Martin. Penn State College of Agriculture - Cooperative Extension, edited and annotated by Dan Friedman (Thanks: to Bob Mackey for proofreading the original source material.)
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