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What is backflow and why do I need a preventer?

Backflow is the unintended flow of untreated, polluted, or contaminated water in a reverse direction . That would be from an end user back into the water distribution system. This distribution system could be the public water supply or the plumbing in your business or home.
There are two types of backflow, back-siphonage and back-pressure.
• Back-siphonage occurs when a drop in pressure happens on the upstream side of a cross-connection. The pressure drop could be caused by firemen opening a hydrant or a water main breaking, among others.
• Back-pressure is when the pressure downstream, towards the end user, of the service connection becomes greater than that supplied. It is possible for this to happen when a pump is used to transfer fluids, a hot water heater or boiler malfunctioning or an elevated storage tank.
These examples aren’t dangerous until a “cross-connection” exist.
Backflow Preventers are a special type of control valve and have special requirements for their installation and use. If installed and maintained properly a backflow preventer assembly will prevent both types of backflow protecting your drinking water from contamination. 
There are four types of approved backflow preventers:
    Reduced Pressure Zone Assembly: Designed to protect against both back-pressure and back-siphonage by keeping the water pressure lower than the supply water pressure. A RPZ is used to protect against high level health hazards that could cause illness or even death if the contaminate was consumed.
    Pressure Vacuum Breaker: Will protect against back-siphonage only and therefore may not be used where there is any chance of back pressure. Usually installed out doors or where there is adequate drains due to it’s design. When pressure is applied it will spill a small amount of water as it activates.
    Spill Resistant Vacuum Breaker: Very simular to the pressure vacuum breaker the SVB is designed not to spill water and therefore may be used indoors reducing the need for freeze protection.
    Double Check Valve Assembly: The DCVA is the most common type of backflow preventer. Used on landscape irrigation systems it provides protection against both back-pressure and back-siphonage and is the only assembly that may be installed below the surface grade. Being installed below grade reduces the need for freeze protection in areas that are subject to very cold weather. The DCVA may only be used on systems with a low heath hazard, that is if the device fails the pollutant will not cause illness or death.