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IAPMO Uniform Codes Spotlight

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From the 2015 UPC Illustrated Training Manual

603.3.2 Atmospheric Vacuum Breaker (AVB).
An atmospheric vacuum breaker consists of a body, a checking member, and an atmospheric port.

The purpose of a vacuum breaker is to stop backsiphonage. The atmospheric vacuum breaker (AVB) consists of a valve that allows air into the piping system for the purpose of stopping a downstream siphon (see Figure 603.3.2a). At the same time, a check valve is closed to the supply piping. A common design element is that the airinlet valve and check valve are nearly always the same mechanical component. A float rises with water pressure to seal the system for water to flow and drops with a lack of water pressure opening to the atmosphere.

Although simple in its design, the AVB has several installation requirements in order to function properly:

  • Because the AVB provides backsiphonage protection only, it is considered isolation protection only. These devices are located on individual plumbing fixtures and appliances. They will not be installed at the service line to a building, for example, where the device may be subject to backpressure.
  • All vacuum breakers are considered high-hazard protection. These devices protect the potable system from toxic materials. Remember that any backflow preventer that provides high-hazard protection will be acceptable for lowhazard applications.
  • The AVB must be installed with its critical level a minimum of six inches above the highest downstream usage (see Figure 603.3.2b).
  • The AVB must not be subjected to continuous pressure. An industry and plumbing code accepted definition of "continuous pressure" is, "water pressure applied for not more than 12 hours in a 24-hour period." If the AVB is used with continuous pressure, the valve may become stuck or sealed closed and not open on demand to atmosphere. The reasons for this may include mineral deposits or chemical bonding from water quality conditions, or a spider's web holding the valve closed to atmosphere.
  • Valves are not permitted downstream of an AVB. Components such as check valves, gate valves, solenoid valves or pressure-regulating devices can trap or suspend pressure in the AVB, thereby allowing continuous pressure.
  • An AVB must not be subjected to backpressure. This valve allows air into the system to stop a siphon with a water pressure loss. The device cannot determine the water's direction of flow, as long as water pressure holds the valve closed to atmosphere. Therefore, elevated piping, auxiliary sources of water and pressure pumps, for example, are not permitted downstream.
  • AVBs must be installed upright. The valve that allows air into the downstream piping relies on gravity to function properly. If the device is installed out of plumb, the valve may not fully open.

The 2015 Uniform Plumbing Code Illustrated Training Manual eBook is available for purchase here.

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