An Atmospheric Vacuum Breaker (AVB) is a backflow prevention device

Backflow prevention device

A backflow prevention assembly is used to protect water supplies from contamination or pollution.In water supply systems, water is normally maintained at a significant pressure to enable water to flow from the tap, shower etc...

 used in 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...

 to prevent backflow of non-potable liquid

Liquid

Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

s into the drinking water

Drinking water

Drinking water or potable water is water pure enough to be consumed or used with low risk of immediate or long term harm. In most developed countries, the water supplied to households, commerce and industry is all of drinking water standard, even though only a very small proportion is actually...

 system. It is usually constructed of brass

Brass

Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...

 and resembles a 90-degree elbow with a hood on its top to allow air to enter the water system if a siphon

Siphon

The word siphon is sometimes used to refer to a wide variety of devices that involve the flow of liquids through tubes. But in the English language today, the word siphon usually refers to a tube in an inverted U shape which causes a liquid to flow uphill, above the surface of the reservoir,...

 attempts to form. Inside this elbow is a poppet valve

Poppet valve

A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...

 that is held "up" by the water pressure found in the system, closing the air entrance to the device. If the pressure in the "upstream side" is reduced to atmospheric pressure or below, the poppet valve drops and allows air to enter the system, breaking the siphon.

These devices, since they work on atmospheric principles, cannot be installed in an enclosure containing air contaminants

Air pollution

Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or cause damage to the natural environment or built environment, into the atmosphere....

. As air is a fluid, those contaminants could be drawn into the device, thus fouling the pipe

Water pipe

Water pipes are pipes or tubes, frequently made of polyvinyl chloride , ductile iron, steel, cast iron, polypropylene, polyethylene, or copper, that carry pressurized and treated fresh water to buildings , as well as inside the building.-History:For many centuries, lead was the favoured material...

s. AVBs must be installed a minimum of 6 inches above the highest usage point in the system (i.e. sprinkler, drip emitter, etc.) Underground installation of AVBs is entirely ineffectual in providing backflow protection as groundwater in the underground vault could be drawn into the water system, contaminating it.

The AVB should not be used for high hazard applications and must not be used with continuous pressure on the device, as the poppet would likely stick and the AVB would no longer function properly. A shutoff valve should never be placed downstream of any AVB, as this would result in continuous pressure on the AVB. The AVB is not a testable device.

See also

• Air gap
• Chemigation valve
  Chemigation valve
  A chemigation valve is an apparatus designed to protect water supplies from agricultural chemicals used during chemigation, the application of chemicals such as fertilizers and pesticides through irrigation water...
  
  
• Double check valve
  Double check valve
  A double check valve or double check assembly is a backflow prevention device designed to protect water supplies from contamination. It is also a valve used in air brake systems on heavy trucks.Usage in Water Supply:...
  
  
• Hydrostatic loop
  Hydrostatic loop
  A hydrostatic loop, though not often used in plumbing practice, is an arrangement of pipes formed into a vertical loop to prevent backflow of water within the plumbing potable water system. Since a siphon has a maximum height that it can work , a hydrostatic loop is built higher than 33 feet...
  
  
• Pressure vacuum breaker
  Pressure vacuum breaker
  A Pressure Vacuum Breaker is a type of backflow prevention device, used to keep non-potable water from entering the water supply. A PVB is similar to an atmospheric vacuum breaker , except that the PVB contains a spring-loaded poppet. This makes it acceptable for applications that are high...
  
  
• Reduced pressure zone device
  Reduced pressure zone device
  A Reduced Pressure Zone Device is a type of backflow prevention device used to protect water supplies from contamination...