What Is A Vacuum Breaker?

According to Wikipedia: A vacuum breaker is an attachment commonly placed on a hose bibb or exterior water faucet to prevent water from being siphoned backward (backsiphonage) into the public drinking water system.
This prevents contamination should the public drinking water system's pressure drop.
The following information is from Watts, one of the largest manufacturer’s of the valves:
1. What is backsiphonage?
Backsiphonage is the reversal of normal flow in a system caused by a negative pressure (vacuum or partial vacuum) in the supply piping.
2. What factors can cause backsiphonage?
Backsiphonage can be created when there is a stoppage of the water supply due to nearby fire-fighting, repairs or breaks in city main, etc. The effect is similar to the sipping of an ice cream soda by inhaling through a straw, which induces a flow in the opposite direction.
3. What is pressure backflow?
Backpressure backflow is the reversal of normal flow in a system due to an increase in the downstream pressure above that of the supply pressure.
4. What factors can cause a backpressure backflow condition?
Backpressure backflow is created whenever the downstream pressure exceeds the supply pressure which is possible in installations such as heating systems, elevated tanks, and pressure producing systems. An example would be a hot water space-heating boiler operating under 15-20 lbs. pressure coincidental with a reduction of the city water supply below such pressure (or higher in most commercial boilers). As water tends to flow in the direction of least resistance, a backpressure backflow condition would be created and the contaminated boiler water would flow into the potable water supply.
5. What is a cross-connection?
A cross-connection is a direct arrangement of a piping line which allows the potable water supply to be connected to a line which contains a contaminant. An example is the common garden hose attached to a sill cock with the end of the hose lying in a cesspool. Other examples are a garden hose attached to a service sink with the end of the hose submerged in a tub full of detergent, supply lines connected to bottom-fed tanks, and supply lines to boilers.
6. What is the most common form of a cross-connection?
Ironically, the ordinary garden hose is the most common offender as it can be easily connected to the potable water supply and used for a variety of potentially dangerous applications.
7. Where is a hose bibb vacuum breaker used?
Hose bibb vacuum breakers are small inexpensive devices with hose connections which are simply attached to sill cocks and threaded faucets or wherever there is a possibility of a hose being attached which could be introduced to a contaminant. However, like the atmospheric vacuum breaker, they should not be used under continuous pressure.
Go outside and look at your hose bibbs. If your home is over five years old and you do not see a brass device attached to the threads, then you may not be protected. If you own a newer home, the vacuum breakers could be built into the hose bibbs.
When you close down the water to your hose bibbs in the winter to prevent freezing, you must go to the hose bibbs and open them up and press the drain release located inside the threaded part to drain the water.

Vacuum breakers can be purchased from Lowes or Home Depot for under $7.00 each. If you are not protected, add them to your house to keep your drinking water safe.
Source: http://www.watts.com/pages/faq.asp?catId=65&faqId=6

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