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Cross Connections

The following article is an excerpt of an article that appeared in the Fall 2007 edition of On Tap magazine. Cross connections are a critical system component that is a requirement for anyone installing a rainwater system that will interconnect with a municipal or other common water source (e.g. shared well). Installing a system with a correct cross connection device will greatly improve the safety of the system.

by Caigan McKenzie

A cross connection is a physical link, such as a jumper connection or swivel arrangement, between a potable water supply and a source of contamination. A backflow is a change of pressure in a water pipe that forces water to flow opposite its intended direction, allowing contaminants to enter the potable water system through unprotected cross connections. Cross connections occur around the home as well as in municipal water systems and can involve low-or high-hazard contaminants.

There are two types of backfow: backpressure and backsiphonage. Backpressure occurs when downstream pressure is greater than the potable water supply pressure. Some causes of backpressure are booster pumps and elevated plumbing. Backsiphonage is caused by a negative pressure or vacuum in a water system, just like when you suck a beverage through a straw. Some causes of backsiphonage are water line breaks and low pressure in a water distribution system because of fire fighting.

An example of a direct cross connection is the makeup water line feeding a recirculating system. This setup creates a direct pathway between the two separate systems, making it possible for backflow to enter the potable water system.

An example of an indirect cross connection is a garden hose connected to a water supply line and submerged in a bucket of soapy water. Other examples of direct and indirect cross connections are lawn irrigation systems, hot tubs, swimming pools, boilers, fire protection systems, film processors, and service wash basins.

When cross connections are used, correctly installed backflow prevention devices stop contaminated water from flowing back into the potable water supply.

There are five basic types of backflow control method, and prevention devices:

  • air gaps,
  • reduced-pressure-zone backflow preventors,
  • double check valves,
  • vacuum breaker, and
  • barometric loops.

The method or device used to reduce the contamination risks of a cross connection depends on whether the backflow is caused by back pressure or backsiphonage and on the degree of hazard, as defined by the industry, to public health. With the exception of the air gap, backflow prevention devices are mechanical and need to be periodically tested to ensure that the internal check valves and mechanics are properly working.

Air GapAn air gap is a physical, vertical separation between a potable and non potable system. Air gaps should be twice the diameter of the supply pipe, but not less than one inch. This method is one of the simplest and most effective for preventing backflow and backsiphonage.

Air gaps can be can be used in all hazard levels and is the only acceptable method for protecting against lethal hazards, according to the Foundation for Cross-Connection Control and Hydraulic Research. They are commonly found in the standard design of household bathtubs and kitchen and bathroom sinks(See Note below, also common in Rainwater Harvesting Systems).

A reduced-pressure-zone backflow preventer consists of two spring-check valves separated by a pressure-relief valve that can vent to the atmosphere. This assembly is effective against both backpressure and backsiphonage and can be used in both non-health and health hazard situations, according to ABPA (American Backflow Prevention Association). They are often used to protect municipal systems from commercial or industrial connections, such as a hospital or factory.

A double check valve consists of two check valves coupled together in one body, test cocks to evaluate whether each check valve is watertight, and a closing gate valve at each end. Double check valves are effective against backpressure and backsiphonage but should be used only for non-health hazards, according to ABPA. They are used for low- to medium-hazard installations, such as lawn irrigation and fire sprinkler systems.

A vacuum breaker is an assembly with a check valve that seals the water supply entry and opens an air vent when the normal flow of water is stopped. These assemblies only prevent backsiphonage.

A barometric loop is "U" shaped and is 35 feet in height, allowing the water to flow over it. It only protects against backsiphonage since backpressure could drive water over the top of the arrangement.


In rainwater catchment systems it is not uncommon for an air gap cross connection device to be installed. Well or city water is fed into the storage tank when the tanks become low and the air gap is connected to the water source, prior to the water being fed into the storage tank. This prevents contamination from the tank flowing back into the source water supply. As mentioned above, this method generally requires no maintenance once installed.

Related Links

On Tap Magazine
Frequently Asked Questions
List of Backflow Prevention Videos
Introduction to Backflow
University of California's Manual of Cross-Connection Control
National Environmental Service Center Cross Connnection Tech Brief
Listing of Vendors and Suppliers by State and Country







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