Educational Information



 In order to survive all living things need safe drinking water. In fact water is the most essential nutrient a person needs. There are many cases where cross-connections in the public water system have been responsible for the contamination of potable water, and therefore triggering the dispersion of disease. For this reason the City of Houston has implemented a Cross-Connection Control program. Click on any of the following topics for further detailed information on cross-connection control and backflow prevention.

FAQ'S: In order to fully understand how the Cross-Connection Control Group protects the public water supply several key questions must be answered. Click here for these questions along with their answers.

LINKS: Click here for links to related websites



What is a cross-connection? An actual or potential physical connection between a potable water system and any other source or system through which it is possible to introduce into any part of the potable system any contaminant and/or pollutant. An example of a cross-connection would be a garden hose submerged in a swimming pool.

What is backflow? Under normal operation water flows from the City of Houston waterlines to the water customers. However, certain pressure variations in the public water supply can cause water to flow in the opposite direction. This reversal of flow direction is known as backflow.

What are the different causes of backflow? Two circumstances can cause backflow to occur, backpressure backflow and backsiphonage backflow.

What is backpressure? This is a type of backflow, which occurs when the pressure in the potable water system is less than the pressure in the downstream piping system. For instance, backpressure backflow may take place if the public water supply is connected to a high-pressure boiler. Once the pressure of the boiler exceeds the pressure of the potable waterline backpressure will occur and the non-potable water will enter the supply lines.

What is backsiphonage? This is a second type of backflow, which occurs when a vacuum is generated in a piping system, and thus contaminates are sucked into the potable water supply. This condition may occur if there is a water main line break.

What is the danger of a cross-connection? If a cross-connection exists and backflow occurs the non-potable water will enter the public water supply. Depending on the degree of hazard of the non-potable water, the contaminated water supply may put the public's health in jeopardy.

How can backflow be prevented? Although backflow itself is difficult to prevent there are ways to protect the water supply from the dangers of cross-connections. A cross-connection may be eliminated by providing an adequate air gap or installing an approved backflow prevention assembly.

What is an air gap? This is a physical separation between the lowest point of a water supply outlet and the flood level rim of the receiving vessel. The distance must be at least twice the diameter of the water supply outlet, but never less than one inch.

What are some of the approved backflow prevention assemblies? There are several different types of backflow prevention assemblies. The specific type of assembly to be used is based on the degree of hazard present and the type of backflow condition the cross-connection may be subject to. Here are some of the most common assemblies used:

back to: FAQ Menu

Reduce Pressure-Principle (RP) Backflow Prevention Assembly


RP Backflow Prevention Assembly


Double Check (DC) Backflow Prevention Assembly


DC Backflow Prevention Assembly


Pressure Vacuum Breaker (PVB) Backflow Prevention Assembly


PVB Backflow Prevention Assembly


Hose Bibb Vacuum Breaker (HBVB) Backflow Prevention Assembly




back to: FAQ Menu


American Backflow Prevention Association (ABPA)

American Water Works Association (AWWA)

University of Southern California-Foundation for Cross-Connection Control & Hydraulic Research

Environmental Protection Agency (EPA) Cross-Connection Control Manual