Drain-waste-vent system
Encyclopedia
In modern plumbing
, a drain-waste-vent (or DWV) is part of a system that removes sewage
and greywater
from a building and regulates air pressure in the waste-system pipes, facilitating flow. Waste is produced at fixtures
such as toilets, sinks and showers, and exits the fixtures through a trap
, a dipped section of pipe that always contains water. All fixtures must contain traps to prevent sewer gases from leaking into the house. Through traps, all fixtures are connected to waste lines, which in turn take the waste to a soil stack, or soil vent pipe. At the building drain system's lowest point, the drain-waste vent is attached, and rises (usually inside a wall) to and out of the roof. Waste is removed from the building through the building drain and taken to a sewage line, which leads to a septic system or a public sewer
. Cesspit
s are generally prohibited in developed areas.
The venting system, or plumbing vents, consists of pipes leading from waste pipes to the outdoors, usually through the roof. Vents provide a means to release sewer gases outside instead of inside the house. Vents also admit oxygen to the waste system to allow aerobic sewage digestion. Vents provide a way to equalize the pressure on both sides of trap, thereby allowing the trap to hold water, which is needed to maintain effectiveness of the trap. Every fixture is required to have an internal or external trap
; double trapping is prohibited by plumbing codes
. With exceptions, every plumbing fixture must have an attached vent. The top of stacks must be vented too, via a stack vent, which is sometimes called a stink pipe.
DWV systems maintain neutral air pressure
in the drains, allowing flow of water
and sewage
down drains and through waste pipes by gravity. As such, it is critical that a downward slope be maintained throughout. In relatively rare situations, a downward slope out of a building to the sewer cannot be created, and a special collection pit and grinding lift 'sewage ejector' pump
are needed. By contrast, potable water supply systems operate under pressure to distribute water up through buildings.
pipe is normally at neutral air pressure compared to the surrounding atmosphere
. When a column of waste water flows through a pipe, it compresses air in the pipe, creating a positive pressure that must be released or it will push back on the waste stream and downstream traps' water seals. As the column of water passes, air must flow in behind the waste stream or negative pressure results. The extent of these pressure fluctuations is determined by the fluid volume of the waste discharge.
Excessive negative air pressure, behind a 'slug' of water that is draining, can siphon water from trap seals at plumbing
fixtures. Generally, a toilet
outlet has the shortest trap seal, making it most vulnerable to being emptied by induced siphonage. An empty trap can allow noxious sewer gas
ses to enter a building.
On the other hand, if the air pressure within the drain becomes suddenly higher than ambient, this positive transient could cause waste water to be pushed into the fixture, breaking the trap seal, with dire hygiene
and health
consequences if too forceful. Tall buildings of three or more stories are particularly susceptible to this problem. Vent stacks are put in parallel to waste stacks to allow proper venting in tall buildings.
or PVC
DWV-rated plastic pipe equipped with a flashing
to prevent rainwater from entering the buildings. Older homes may use copper
, iron
, lead
or clay
pipes, in rough order of increasing antiquity.
Under many older building codes, a vent stack, a pipe leading to the main roof vent, is required to be within a five foot radius of the draining fixture (sink, toilet, shower stall, etc.). To allow only one vent stack, and thus one roof protrusion as permitted by local building code, sub-vents may be tied together and exit a common vent stack.
A blocked vent is a relatively common problem caused by anything from leaves, to dead squirrel
s, to ice dam
s in very cold weather. Symptoms range from bubbles in the toilet bowl when it is flushed, to slow drainage, and all the way to siphoned (empty) traps and sewer gases entering the building. When a fixture trap is venting properly, a "sucking" sound can often be heard as the fixture empties out.
While some state and local building departments prohibit AAVs, the International Residential and International Plumbing Codes allow it to be used in place of a vent-through-the-roof. AAV's are certified to reliably open and close a minimum of 500,000 times, (approximately 30 years of use) with no emanation of sewer gas; and some manufacturers claim their units are tested for up to 1.5 million cycles, or at least 80 years of use. Air Admittance Valves have been effectively used in Europe for more than two decades. US manufacturers offer warranties that range from 20 years to lifetime.
, but also many specialized fittings
which add considerably to their cost of construction. But fittings such as "clean-outs" enhance the maintainability of the systems.
Plumbing
Plumbing is the system of pipes and drains installed in a building for the distribution of potable drinking water and the removal of waterborne wastes, and the skilled trade of working with pipes, tubing and plumbing fixtures in such systems. A plumber is someone who installs or repairs piping...
, a drain-waste-vent (or DWV) is part of a system that removes sewage
Sewage
Sewage is water-carried waste, in solution or suspension, that is intended to be removed from a community. Also known as wastewater, it is more than 99% water and is characterized by volume or rate of flow, physical condition, chemical constituents and the bacteriological organisms that it contains...
and greywater
Greywater
Greywater is wastewater generated from domestic activities such as laundry, dishwashing, and bathing, which can be recycled on-site for uses such as landscape irrigation and constructed wetlands...
from a building and regulates air pressure in the waste-system pipes, facilitating flow. Waste is produced at fixtures
Fixtures
A fixture is a legal concept that includes any physical property that is permanently attached or fixed to real property which once removed will cause [permanent] damage to the real property, usually land. If the property is not affixed to real property it is considered chattel property...
such as toilets, sinks and showers, and exits the fixtures through a trap
Trap (plumbing)
In plumbing, a trap is a U-, S-, or J-shaped pipe located below or within a plumbing fixture. An S-shaped trap is also known as the S-bend invented by Alexander Cummings in 1775 but became known as the U-bend following the introduction of the U-shaped trap by Thomas Crapper in 1880. The new U-bend...
, a dipped section of pipe that always contains water. All fixtures must contain traps to prevent sewer gases from leaking into the house. Through traps, all fixtures are connected to waste lines, which in turn take the waste to a soil stack, or soil vent pipe. At the building drain system's lowest point, the drain-waste vent is attached, and rises (usually inside a wall) to and out of the roof. Waste is removed from the building through the building drain and taken to a sewage line, which leads to a septic system or a public sewer
Sanitary sewer
A sanitary sewer is a separate underground carriage system specifically for transporting sewage from houses and commercial buildings to treatment or disposal. Sanitary sewers serving industrial areas also carry industrial wastewater...
. Cesspit
Cesspit
A cesspit, or cesspool is a pit, conservancy tank, or covered cistern, which can be used to dispose of urine and feces, and more generally of all sewage and refuse. It is a more antiquated solution than a sewer system. Traditionally, it was a deep cylindrical chamber dug into the earth, having...
s are generally prohibited in developed areas.
The venting system, or plumbing vents, consists of pipes leading from waste pipes to the outdoors, usually through the roof. Vents provide a means to release sewer gases outside instead of inside the house. Vents also admit oxygen to the waste system to allow aerobic sewage digestion. Vents provide a way to equalize the pressure on both sides of trap, thereby allowing the trap to hold water, which is needed to maintain effectiveness of the trap. Every fixture is required to have an internal or external trap
Trap (plumbing)
In plumbing, a trap is a U-, S-, or J-shaped pipe located below or within a plumbing fixture. An S-shaped trap is also known as the S-bend invented by Alexander Cummings in 1775 but became known as the U-bend following the introduction of the U-shaped trap by Thomas Crapper in 1880. The new U-bend...
; double trapping is prohibited by plumbing codes
Building code
A building code, or building control, is a set of rules that specify the minimum acceptable level of safety for constructed objects such as buildings and nonbuilding structures. The main purpose of building codes are to protect public health, safety and general welfare as they relate to the...
. With exceptions, every plumbing fixture must have an attached vent. The top of stacks must be vented too, via a stack vent, which is sometimes called a stink pipe.
DWV systems maintain neutral air pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
in the drains, allowing flow of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
and sewage
Sanitary sewer
A sanitary sewer is a separate underground carriage system specifically for transporting sewage from houses and commercial buildings to treatment or disposal. Sanitary sewers serving industrial areas also carry industrial wastewater...
down drains and through waste pipes by gravity. As such, it is critical that a downward slope be maintained throughout. In relatively rare situations, a downward slope out of a building to the sewer cannot be created, and a special collection pit and grinding lift 'sewage ejector' pump
Pump
A pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...
are needed. By contrast, potable water supply systems operate under pressure to distribute water up through buildings.
Purpose
A sewerSanitary sewer
A sanitary sewer is a separate underground carriage system specifically for transporting sewage from houses and commercial buildings to treatment or disposal. Sanitary sewers serving industrial areas also carry industrial wastewater...
pipe is normally at neutral air pressure compared to the surrounding atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
. When a column of waste water flows through a pipe, it compresses air in the pipe, creating a positive pressure that must be released or it will push back on the waste stream and downstream traps' water seals. As the column of water passes, air must flow in behind the waste stream or negative pressure results. The extent of these pressure fluctuations is determined by the fluid volume of the waste discharge.
Excessive negative air pressure, behind a 'slug' of water that is draining, can siphon water from trap seals at plumbing
Plumbing
Plumbing is the system of pipes and drains installed in a building for the distribution of potable drinking water and the removal of waterborne wastes, and the skilled trade of working with pipes, tubing and plumbing fixtures in such systems. A plumber is someone who installs or repairs piping...
fixtures. Generally, a toilet
Toilet
A toilet is a sanitation fixture used primarily for the disposal of human excrement, often found in a small room referred to as a toilet/bathroom/lavatory...
outlet has the shortest trap seal, making it most vulnerable to being emptied by induced siphonage. An empty trap can allow noxious sewer gas
Sewer gas
Sewer gas is a complex mixture of toxic and non-toxic gases produced and collected in sewage systems by the decomposition of organic household or industrial wastes, typical components of Sewage....
ses to enter a building.
On the other hand, if the air pressure within the drain becomes suddenly higher than ambient, this positive transient could cause waste water to be pushed into the fixture, breaking the trap seal, with dire hygiene
Hygiene
Hygiene refers to the set of practices perceived by a community to be associated with the preservation of health and healthy living. While in modern medical sciences there is a set of standards of hygiene recommended for different situations, what is considered hygienic or not can vary between...
and health
Health
Health is the level of functional or metabolic efficiency of a living being. In humans, it is the general condition of a person's mind, body and spirit, usually meaning to be free from illness, injury or pain...
consequences if too forceful. Tall buildings of three or more stories are particularly susceptible to this problem. Vent stacks are put in parallel to waste stacks to allow proper venting in tall buildings.
Venting mechanisms
To prevent the problems of high pressure in a drain system, sewer pipes will usually vent via one of two mechanisms.Venting to atmosphere
Most residential buildings' drainage systems in North America are vented directly through the buildings' roofs. The DWV pipe is typically ABSAcrylonitrile butadiene styrene
Acrylonitrile butadiene styrene is a common thermoplastic. Its melting point is approximately 105 °C ....
or PVC
PVC
Polyvinyl chloride is a plastic.PVC may also refer to:*Param Vir Chakra, India's highest military honor*Peripheral venous catheter, a small, flexible tube placed into a peripheral vein in order to administer medication or fluids...
DWV-rated plastic pipe equipped with a flashing
Flashing (weatherproofing)
Flashing refers to thin continuous pieces of sheet metal or other impervious material installed to prevent the passage of water into a structure from an angle or joint...
to prevent rainwater from entering the buildings. Older homes may use copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
or clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
pipes, in rough order of increasing antiquity.
Under many older building codes, a vent stack, a pipe leading to the main roof vent, is required to be within a five foot radius of the draining fixture (sink, toilet, shower stall, etc.). To allow only one vent stack, and thus one roof protrusion as permitted by local building code, sub-vents may be tied together and exit a common vent stack.
A blocked vent is a relatively common problem caused by anything from leaves, to dead squirrel
Squirrel
Squirrels belong to a large family of small or medium-sized rodents called the Sciuridae. The family includes tree squirrels, ground squirrels, chipmunks, marmots , flying squirrels, and prairie dogs. Squirrels are indigenous to the Americas, Eurasia, and Africa and have been introduced to Australia...
s, to ice dam
Ice dam
An ice dam occurs when water builds up behind a blockage of ice. Ice dams can occur in various ways.-Caused by a glacier:Sometimes a glacier flows down a valley to a confluence where the other branch carries an unfrozen river...
s in very cold weather. Symptoms range from bubbles in the toilet bowl when it is flushed, to slow drainage, and all the way to siphoned (empty) traps and sewer gases entering the building. When a fixture trap is venting properly, a "sucking" sound can often be heard as the fixture empties out.
Air admittance valve
Air admittance valves (AAVs or Durgo valves) are negative pressure-activated, one-way mechanical vents, used in a plumbing system to eliminate the need for conventional pipe venting and roof penetrations. A discharge of wastewater causes the AAV to open, releasing the vacuum and allowing air to enter plumbing vent pipe for proper drainage. Since AAVs will only function under negative pressure situations they are not suitable for all venting applications, such as venting a sump, where positive pressures are created when the sump fills. Using AAVs can significantly reduce the amount of venting materials needed in a plumbing system, increase plumbing labor efficiency, allow greater flexibility in the layout of plumbing fixtures, and reduce long-term roof maintenance problems associated with conventional vent stack roofing penetrations.While some state and local building departments prohibit AAVs, the International Residential and International Plumbing Codes allow it to be used in place of a vent-through-the-roof. AAV's are certified to reliably open and close a minimum of 500,000 times, (approximately 30 years of use) with no emanation of sewer gas; and some manufacturers claim their units are tested for up to 1.5 million cycles, or at least 80 years of use. Air Admittance Valves have been effectively used in Europe for more than two decades. US manufacturers offer warranties that range from 20 years to lifetime.
Fittings
Drainage and venting systems require not only pipePiping
Within industry, piping is a system of pipes used to convey fluids from one location to another. The engineering discipline of piping design studies the efficient transport of fluid....
, but also many specialized fittings
Piping and plumbing fittings
Fittings are used in pipe and plumbing systems to connect straight pipe or tubing sections, to adapt to different sizes or shapes, and for other purposes, such as regulating or measuring fluid flow...
which add considerably to their cost of construction. But fittings such as "clean-outs" enhance the maintainability of the systems.
See also
- Fuel gas pipingPipingWithin industry, piping is a system of pipes used to convey fluids from one location to another. The engineering discipline of piping design studies the efficient transport of fluid....
- PlumberPlumberA plumber is a tradesperson who specializes in installing and maintaining systems used for potable water, sewage, and drainage in plumbing systems. The term dates from ancient times, and is related to the Latin word for lead, "plumbum." A person engaged in fixing metaphorical "leaks" may also be...
- Potable cold and hot water supply
- Rainwater, surface, and subsurface water drainageDrainageDrainage is the natural or artificial removal of surface and sub-surface water from an area. Many agricultural soils need drainage to improve production or to manage water supplies.-Early history:...
- ZeocarbonZeocarbonZeocarbon includes material mostly derived from charcoal and volcanic rock. It denotes a material which has an exceptionally high surface area, typically determined by nitrogen adsorption, and includes a large amount of microporosity...