Backflow is the reversal of the normal and intended direction of water flow in a water system. Devices and assemblies known as backflow preventers are installed to prevent backflow, which can contaminate potable water supplies.

Backflow is a potential problem in a water system because it can spread contaminated water back through a distribution system. For example, backflow at uncontrolled cross connections (cross-connections are any actual or potential connection between the public water supply and a source of contamination or pollution) can allow pollutants or contaminants to enter the potable water system. Sickness can result from ingesting water that has been contaminated due to backflow.

Back-pressure is the reverse from normal flow direction within a piping system as the result of the downstream pressure being higher than the supply pressure. This reduction in supply pressure occurs whenever the amount of water being used exceeds the amount of water being supplied (such as during water-line flushing, fire-fighting, or breaks in water mains).

Back-siphonage is the reverse from normal flow direction within a piping system that is caused by negative pressure in the supply piping (i.e., the reversal of normal flow in a system caused by a vacuum or partial vacuum within the water supply piping). Back-siphonage can occur when there is a high velocity in a pipe line, when there is a line repair or break that is lower than a service point, or when there is lowered main pressure due to high-water withdrawal rate (such as during fire-fighting or water-main flushing).

Backflow prevention for residences is most commonly accomplished through the use of atmospheric vacuum breakers (AVBs). AVBs operate by allowing the entry of air into a pipe so that a siphon cannot form. AVBs are bent at 90 degrees and are usually composed of brass. Compared with backflow preventer assembles, AVBs are small, simple and inexpensive devices that require little maintenance or testing. They have long life spans and are suitable for residential purposes such as sprinkler systems. InterNACHI inspectors can check for the following:

• The AVB must be at least 6 inches above any higher point downstream of the device. For this reason, they can never be installed below grade. Even if they are installed 6 inches above grade, inspectors should make sure that they are not installed less than 6 inches above some other point in the system downstream of the device.
• The AVB cannot be installed in an enclosure containing air contaminants. If contaminated air enters the water piping, it can poison the potable water supply.
• A shut-off valve should never be placed downstream of any AVB, as this would result in continuous pressure on the AVB.
• AVBs cannot be subject to continuous pressure for 12 hours in any 24-hour period or they may malfunction.
• Spillage of water from the top of the AVB is an indication that the device has failed and needs to be replaced.

Some types of assemblies are common in commercial and agricultural applications but are rare for residential uses. The appropriate type of backflow preventer for any given application will depend on the degree of potential hazard. The primary types of backflow preventers appropriate for use at municipalities and utilities are:

• Double check valves: These are commonly used in elevated tanks and non-toxic boilers. Double check-valve assemblies are effective against backflow caused by back-pressure and back-siphonage and are used to protect the potable water system from low-hazard substances. Double-checks consist of two positive-seating check valves installed as a unit between two tightly closing shut-off valves, and are fitted with testcocks.
• Reduced pressure principle assemblies: These are commonly used in industrial plants, hospitals, morgues, chemical plants, irrigation systems, boilers, and fire sprinkler systems. Reduced pressure principle assemblies (RPs) protect against back-pressure and back-siphonage of pollutants and contaminants. The assembly is comprised of two internally loaded, independently operating check valves with a mechanically independent, hydraulically dependent relief valve between them.
• Pressure vacuum breakers: These are commonly used in industrial plants, cooling towers, laboratories, laundries, swimming pools, lawn sprinkler systems, and fire sprinkler systems. Pressure vacuum breakers use a check valve designed to close with the aid of a spring when water flow stops. Its air-inlet valve opens when the internal pressure is one psi above atmospheric pressure, preventing non-potable water from being siphoned back into the potable system. The assembly includes resilient, seated shut-off valves and testcocks.

A number of organizations, such as the American Water Works Association (AWWA) and the American Backflow Prevention Association (ABPA) offer certification courses designed to train professionals to test backflow preventers. Requirements for training vary by jurisdiction. Inspection of backflow preventers requires knowledge of installation requirements, although inspectors are not required to become certified.

In summary, backflow preventers are designed to prevent the reverse flow of water in a potable water system. They come in a number of different types, each of which is suited for different purposes.