A cross-connection is any temporary or permanent connection between a public water system or consumer's potable (i.e., drinking) water system and any source or system containing non-potable water or other substances. An example is the piping between a public water system or consumer's potable water system and an auxiliary water system, cooling system, or irrigation system.

Backflow is the undesirable reversal of flow of non-potable water or other substances through a cross-connection and into the piping of a public water system or consumer's potable water system. There are two types of backflow... backpressure backflow and back-siphonage.

Backpressure backflow is backflow caused by a downstream pressure that is greater than the upstream or supply pressure in a public water system or consumer's potable water system. Backpressure (i.e., downstream pressure that is greater than the potable water supply pressure) can result from an increase in downstream pressure, a reduction in the potable water supply pressure, or a combination of both. Pumps can create increases in downstream pressure; temperature increases in boilers, etc. Reductions in potable water supply pressure occur 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 backflow caused by a negative pressure (i.e., a vacuum or partial vacuum) in a Public water system or consumer's potable water system. The effect is similar to drinking water through a straw. Back-siphonage can occur when there is a stoppage of water supply due to nearby fire fighting, a break in a water main, etc.


Backflow into a public water system can pollute or contaminate the water in that system (i.e., backflow into a public water system can make the water in that system unusable or unsafe to drink), and each water supplier has a responsibility to provide water that is usable and safe to drink under all foreseeable circumstances. Furthermore, consumers generally have absolute faith that water delivered to them through a public water system is always safe to drink. For these reasons, your water district must take precautions to protect its public water system against backflow.

The Texas Commission on Environmental Quality gives public water suppliers two options:


1. Require a backflow prevention assembly at each service connection that may create a threat to the public water supply.

2. Require backflow prevention assemblies and assemblies within a premises as outlined in the state required plumbing codes. The water suppliers usually do not have the capability to repeatedly inspect every consumer's premises for cross-connections and backflow protection. Generally, this would include the water service connection to each dedicated fire protection system or irrigation piping system and the water service connection to each of the following types of premises: (1) premises with an auxiliary or reclaimed water system; (2) industrial, medical, laboratory, marine or other facilities where objectionable substances are handled in a way that could cause pollution or contamination of the public water system; (3) premises exempt from the State Plumbing Code inspection and premises where an internal backflow prevention assembly required under the State Plumbing Code is not properly installed or maintained; (4) classified or restricted facilities; and (5) tall buildings.


A backflow preventer is a means or mechanism to prevent backflow. The basic means of preventing backflow is an air gap, which either eliminates a cross-connection or provides a barrier to backflow. The basic mechanism for preventing backflow is a mechanical backflow preventer, which provides a physical barrier to backflow. The principal types of mechanical backflow preventers are the reduced-pressure principle assembly, the pressure vacuum breaker assembly, and the double check valve assembly.

An air gap is a vertical, physical separation between the end of a water supply outlet and the flood-level rim of a receiving vessel. This separation must be at least twice the diameter of the water supply outlet and never less than one inch. An air gap is considered the maximum protection available against backpressure backflow or back-siphonage but is not always practical and can easily be bypassed.

An RPBA is a mechanical backflow prevention assembly that consists of two independently acting, spring-loaded check valves with a hydraulically operating, mechanically independent, spring-loaded pressure differential relief valve between the check valves and below the first check valve. It includes shutoff valves at each end of the assembly and is equipped with test cocks. An RPBA is effective against backpressure backflow and back-siphonage and may be used to isolate health or non-health hazards.

Two styles of PVBs are available: (1) Pressure Vacuum Breaker and (2) Spill Resistant Vacuum Breaker. A PVB is a mechanical backflow prevention assembly that consists of an independently acting, spring-loaded check valve and an independently acting, spring-loaded, air inlet valve on the discharge side of the check valve. It includes shutoff valves at each end of the assembly and is equipped with test cocks. A PVB may be used to isolate health or non-health hazards but is effective against back-siphonage only.


A DCVA is a mechanical backflow prevention assembly that consists of two independently acting, spring-loaded check valves. It includes shutoff valves at each end of the assembly and is equipped with test cocks. A DCVA is effective against backpressure backflow and back-siphonage but should be used to isolate only non-health hazards.

In order to insure the proper operation of a backflow prevention assembly, it must be tested and certified upon installation and at least once a year thereafter if installed to protect form a health hazard situation. The Texas Commission on Environmental Quality and local water suppliers oversee these state requirements. Mechanical backflow prevention assemblies have internal seals, springs, and moving parts that are subject to fouling, wear, or fatigue. Also, mechanical backflow preventers and air gaps can be bypassed. Therefore, all backflow prevention assemblies have to be tested periodically to ensure that they are functioning properly. Mechanical backflow prevention assemblies have to be tested with properly calibrated gauge equipment.

Soft drink dispensers (post-mix carbonators) use carbonated water mixed under pressure with syrup and water to provide soft drinks beverages. Many, if not most, water pipes are made of copper. When carbonated water comes into contact with copper it chemically dissolves the copper from the pipe. This copper-carbonate solution (i.e. Carbonic Acid) has been proven to be a risk to the digestive system. The Universal Plumbing Code requires that the approved backflow prevention for a post-mix carbonator be a reduced pressure backflow assembly (RPZ). Additional requirements: no piping containing copper shall be installed downstream of the RPZ backflow assemblies supplying carbonators.

One excellent reference manual is the second (1990) edition of the American Water Works Association's (AWWA's) Manual M14, Recommended Practice for Backflow Prevention and Cross-Connection Control, which is available from the AWWA Bookstore; 6666 West Quincy Avenue; Denver, Colorado 80235; 800/926-7337; http://www.awwa.org (Exit to AWWA).

Another excellent reference manual is the ninth (1993) edition of the University of Southern California's Manual of Cross-Connection.