Skip Ribbon Commands
Skip to main content An Official Website of the Commonwealth of Kentucky

Energy and Environment Cabinet

Division of Water

Division of Water
Backflow and Backsiphonage

What is backflow?

Backflow means the reversal of water flow from its normal or intended direction of flow. Whenever a water utility connects a customer to the utility's distribution system, the intention is for the water to flow from the distribution system to the customer.

However, the flow of water could be reversed from the customer back into the distribution system. If cross-connections exist within the customer's plumbing system when backflow occurs, then it is possible to contaminate the public water supply. There are two types of backflow -- backpressure backflow and backsiphonage.

What is backpressure backflow?

Backpressure backflow occurs when the pressure of the nonpotable system exceeds the positive pressure in the water distribution lines; that is, the water pressure within an establishment's plumbing system exceeds that of the water distribution system. For example, there is a potable water connection to a hot water boiler system that is not protected by an approved backflow preventer. If pressure in the boiler system increases to a point that it exceeds the pressure in the water distribution system, a backflow from the boiler to the public water system may occur. 

A downstream pressure that is greater than the potable water supply pressure causes backpressure backflow. Backpressure can result from an increase in downstream pressure, a reduction in the potable water supply pressure or a combination of both. Boiler pumps, pressure pumps or temperature increases in boilers can create increases in downstream pressure. 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.

What is backsiphonage? 

Backsiphonage occurs when there is a partial vacuum (negative pressure) in a water supply system, which draws the water from a contaminated source into a potable water supply. The water pressure within the distribution system falls below that of the plumbing system it is supplying. The effect is similar to siphoning or drinking water through a straw. For example, during a large fire, a pump is connected to a hydrant. High flows pumped out of the distribution system can result in significantly reduced water pressure around the withdrawal point. A partial vacuum has been created in the system, causing suction of contaminated water into the potable water system. During such conditions, it is possible for water to be withdrawn from nonpotable sources located near the fire -- for example, air-conditioning systems, water tanks, boilers, fertilizer tanks and washing machines -- into buildings located near a fire. The same conditions can be caused by a water main break.    

Garden hoses, toilets or similar devices create most household cross-connections. Under certain conditions, the flow in household water lines can reverse and siphon contaminates into the water supply. A toilet installed incorrectly without a "plumbing-code approved" toilet ballcock (air gap) will allow contaminated water to backflow to other water outlets in your house, including the kitchen sink.

What is a Backflow Preventer?

A backflow preventer is a method or mechanical device to prevent backflow. The basic method of preventing backflow is an air gap, which either eliminates a cross-connection or provides a barrier to backflow. Mechanical backflow preventers are devices that provide a physical barrier to backflow. There are four devices commonly used -- the reduced pressure principle assembly, the double check valve assembly, the pressure vacuum breaker and the atmospheric vacuum breaker. All of these devices require periodic maintenance and testing.

Kentucky plumbing codes and standards [815 KAR 20:120, Section 2 (7)] for backflow preventers include:

  1. Air gap
  2. Reduced pressure principle back pressure backflow preventer
  3. Double check valve assembly
  4. Pressure type vacuum breaker
  5. Atmospheric type vacuum breaker
  6. Barometric loop

Other types of mechanical devices, such as the barometric loop, superior pressure type device or the venturi type vacuum breaker, are used for backflow prevention.

The double check valve assembly was one of the first designs during the early 1900s to prevent backflow. Improvements in the early designs of double check valve assemblies ranged from the early metal-to-metal seats to resilient facing on the clapper assembly. 

What is an Air Gap?

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 backsiphonage, but is not always practical and can easily be bypassed.

In Kentucky, refer to plumbing regulations in 815 KAR 20:120, Section 2 (7)(a) for specific uses of air gaps.

What is a Reduced Pressure Principle Assembly (RP or RPBA)?

An RP is a mechanical backflow preventer 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 RP is effective against backpressure backflow and backsiphonage and may be used to isolate health or nonhealth hazards.

The RP may be used on all direct connections which may be subject to backpressure or backsiphonage and where there is the possibility of contamination by the material that does constitute a potential health hazard. A health hazard or high hazard is a cross-connection involving any substance that could cause death, illness, spread disease or have a high probability of causing such effects. The degree of hazard refers to a contaminant being toxic on nontoxic, whereby a health hazard involves a toxic substance.

In Kentucky, refer to plumbing regulations in 815 KAR 20:120, Section 2 (7)(c) for specifics on a DC. Applicable to low level of hazard back pressure backflow conditions.

Are a Double Check and Dual Check Backflow Preventer the same thing?

NO. They sound very similar, and they are "relatives," but they are not the same. A Double Check will ALWAYS have two manual shutoff valves -- one on the inlet and one on the outlet. These manual valves are used as emergency shut-offs and are also necessary to properly test the operation of the backflow preventer. A Double Check will also have test cocks (small outlets) for connecting the test gauges. If it doesn't have those shut off valves and test cocks, it is NOT a Double Check Backflow Preventer.

A dual check is more of a flow-control device rather than a backflow preventer. Dual check devices do not have shut-off valves or test cocks.

What is a Pressure Type Vacuum Breaker (PVB)?

A PVB is an assembly consisting of an independently operating, internally loaded check valve and an independently operating, loaded air-inlet valve located on the discharge side of the check valve. The device includes tightly closing shut-off valves on each side of the check valves and properly located test cocks for the testing of the check valve(s).

PVBs may be used as protection for connections to all types of nonpotable systems where the vacuum breakers are not subject to backpressure. These units may be used under continuous supply pressure. They must be installed above the usage point. This type of vacuum breaker can be used for lawn sprinkler systems under continuous pressure. Therefore, if properly installed, it will protect the potable water supply. The device shall be installed 12 inches above the highest sprinkler head.

In Kentucky, refer to plumbing regulations in 815 KAR 20:120, Section 2 (7)(d) for specifics on pressure vacuum breakers.  Applicable to backsiphonage conditions.

What is an Atmospheric Type Vacuum Breaker (AVB)?

The purpose of the AVB is to prevent a siphon from allowing a contaminant or pollutant into the potable water system. They do not prevent backflow from backpressure. 

The most commonly used atmospheric type antisiphon vacuum breakers incorporate an atmospheric vent in combination with a check valve. Its operation depends on a supply of potable water to seal off the atmospheric vent, admitting the water to downstream equipment. If a negative pressure develops in the supply line, the loss of pressure permits the check valve to drop, sealing the orifice, while at the same time the vent opens, admitting air to the system to break the vacuum. AVBs can be used on most inlet type water connections which are not subject to backpressure, such as low inlet feeds to receptacles containing toxic and nontoxic substances, valve outlet or fixture with hose attachments, lawn-sprinkler systems and commercial dishwashers.

In Kentucky, refer to plumbing regulations in 815 KAR 20:120, Section 2 (7)(e) for specifics and critical level settings for atmospheric type vacuum breakers. Applicable to backsiphonage conditions.  

What is a Barometric Loop?

A barometric loop is a looped piping arrangement 35 feet (11 meters) in height in which the water flow goes over the loop at the top. This method of backflow prevention is only capable of protecting against backsiphonage, since backpressure could drive water backward over the loop.

In Kentucky, refer to plumbing regulations in 815 KAR 20:120, Section 2 (7)(f) for specifics on barometric loop. Applicable to backsiphonage conditions. The use of a barometric loop shall not be acceptable as the primary backsiphonage preventer.

Back to Cross-Connections Main Page