Backflow Prevention


What should you know about backflow prevention?

  • What is back-siphonage?

    Back-siphonage is the reversal of normal flow in a system caused by a negative pressure (vacuum or partial vacuum) in the supply piping.

  • What factors can cause back-siphonage?

    Back-siphonage can be created when there is a stoppage of the water supply due to nearby fire fighting, repairs or breaks in city main, etc. The effect is similar to the sipping of an ice cream soda by inhaling through a straw, which induces a flow in the opposite direction.

  • What is backpressure backflow?

    Backpressure backflow is the reversal of normal flow in a system due to an increase in the downstream pressure above that of the supply pressure.

  • What factors can cause a backpressure backflow condition?

    Backpressure-backflow is created whenever the downstream pressure exceeds the supply pressure, which is possible in installations such as heating systems, elevated tanks, and pressure-producing systems. An example would be a hot water space-heating boiler operating under 15-20 lbs. pressure coincidental with a reduction of the city water supply below such pressure (or higher in most commercial boilers). As water tends to flow in the direction of least resistance, a backpressure-backflow condition would be created and the contaminated boiler water would flow into the potable water supply.

  • What is a cross connection?

    A cross connection is a direct arrangement of a piping line which allows the potable water supply to be connected to a line which contains a contaminant. An example is the common garden hose attached to a sill cock with the end of the hose lying in a cesspool. Other examples are a garden hose attached to a service sink with the end of the hose submerged in a tub full of detergent, supply lines connected to bottom-fed tanks, supply lines to boilers.

  • What is the most common form of a cross connection?

    Ironically, the ordinary garden hose is the most common offender as it can be easily connected to the potable water supply and used for a variety of potentially dangerous applications.

  • What is a cross connection control program?

    This is a combined cooperative effort between plumbing and health officials, waterworks companies, property owners and certified testers to establish and administer guidelines for controlling cross connections and implementing means to ensure their enforcement so that the public potable water supply will be protected both in the city main and within buildings. The elements of a program define the type of protection required and responsibility for the administration and enforcement. Other elements ensure continuing education programs.

  • What is the difference between pollution and contamination?

    Pollution of the water supply does not constitute an actual health hazard, although the quality of the water is impaired with respect to taste, odor or utility. Contamination of the water supply, however, does constitute an actual health hazard in which the consumer is subjected to potentially lethal water borne disease or illness.

  • What is meant by “Degree of Hazard?”

    The degree of hazard is a commonly used phrase referring to cross connection programs, and is simply a determination on whether the substance in the non-potable system is toxic (health hazard) or non-toxic (non-health hazard).

  • What is the difference between a toxic and a non-toxic substance?

    A toxic substance is any liquid, solid or gas which, when introduced into the water supply, creates or may create a danger to the health and well being of the consumer. An example is treated boiler water. A non-toxic substance is any substance that may create a non-health hazard, is a nuisance or is aesthetically objectionable. Examples of non-toxic substances are foodstuffs such as sugar, soda pop, etc. Therefore, you must select the proper device according to the type of connection and degree of hazard.

  • Are there any regulations in OSHA regarding cross connections?

    Yes, OSHA requires that no cross connection be allowed in an installation unless it is properly protected with an approved backflow preventer. These requirements are also covered in B.O.C.A., Southern Standard Building Code, Uniform Plumbing Code and City, State and Federal Regulations.


–Excerpt from Watts Industries, Inc. public service booklet 50 Cross-Connection Questions, Answers, & Illustrations. © Watts Regulator Co., 1997