A backflow preventer device is used to protect water supplies from contamination or pollution. Many types of backflow prevention devices also have test cocks so that they can be tested or examined to ensure that they are functioning properly.
In the United States, the Environmental Protection Agency (EPA) holds local water suppliers responsible for maintaining a certain amount of purity in potable water systems. Many states and/or local municipalities require annual testing of backflow prevention assemblies. A check valve is a common form of backflow prevention.
You must have a backflow preventer on your irrigation system. In most places this is the law, everywhere else it’s just the smart thing to do. Almost everywhere the local authorities will dictate that certain types of backflow preventers may not be used with irrigation systems because they do not provide adequate protection. In some cases, the authorities will dictate the exact type of backflow preventer you MUST use. You may hear or see the term “Cross-Connection Control” used, this essentially is referring to backflow prevention.
Your landscape has all kinds of nasty things in it that will make you sick or worse if you drink them. Thus irrigation water is considered a contaminant (creates a health hazard) rather than just a pollutant (is objectionable in color or odor). Toxic chemicals (fertilizers, pesticides, etc.) and animal waste can come back up your irrigation pipes and into your and others water supply. The purpose of the backflow preventer is to protect you when the valve breaks or leaks, which all valves will do eventually.
There are times when the water pressure drops in the supply system, and this is when the backflow occurs. No, it is not a frequent occurrence. But it does happen more often than you think. Such as when they have to shut off the water to repair a water pipe, or hook up a new pipe. This makes construction projects easily the most common cause of backflow problems. Fire fighting is another common cause of backflow. Fire trucks use huge pumps to suck the water out of the fire hydrants. This often causes the water pressure in the water system to drop, and backflow will occur in the surrounding neighborhoods. You can do a quick experiment yourself and create backflow. Simply turn off the water valve leading to your house. Next have someone turn on a faucet. Now turn on a different faucet that is higher than the first. You will hear air being sucked into the higher faucet. You just created backflow in your house piping.
Another common argument against the need for backflow preventers is that if all the valves are closed the water can’t go backwards through them. So the valves will prevent backflow. The obvious problem with this is that if the backflow occurs at a time when the valve is open, the valve will not stop backflow.But even when the valve is closed it may not prevent backflow. A standard manually operated valve will stop backflow when it is closed– if the valve if fully closed, has good seals, and does not leak.