Backflow Prevention


From the time the Romans built the aqueducts, all through the Industrial Revolution, right up to our 21st century present day, water cross contamination has been a key safety concern. Our modern day plumbers are tasked with “protecting the health of the nation.” Safe drinking water is not possible without proper prevention of clean, potable, water supplies from being contaminated by a cross connection to polluted water. Backflow devices had to be created to prevent all types of cross contamination that can occur in plumbing systems.

In 1878 the National Quarantine Act was passed by Congress which gave the Marine Hospital Service, which later became the Public Health Service (PHS), the task of taking steps to prevent sailors from bringing diseases ashore when ships arrived at ports. The Public Health Service also determined Drinking Water Standards under this authority. But it wasn't until 1914 that the first official federal drinking water standard was established. It was called the Treasury Standard because it was developed by a commission appointed by the Secretary of the Treasury. Despite these standards, much about plumbing technology and cross connections was still being learned.

Consequences Of Hazardous Cross Contamination

During an outbreak of dysentery in Chicago in 1933, during the Worlds Fair, investigators discovered an issue of cross contamination among two hotels across the street from one another sharing one water supply was to blame for 98 deaths and 1400 people getting sick. Sometime long before the outbreak it is believed a workman, at the Congress Plaza hotel, attempting to run a rod or cable through a cleanout wye, was not able to remove the cleanout cap. He improvised and made a hole in the center of the cap in order to put the rod into the sewer. When he was finished, he did not properly seal the hole with a metal threaded plug, but drove a wooden plug into the hole. Over time the wooden plug deteriorated, and when found, was so loose it was easily removed by finger tips. Around the wooden plug was a rust stain surrounded by a black crust where the wood made contact with the iron. The dried crust was determined to be sewage solids that had accumulated as leakage escaped through the plug. Unfortunately this leak was directly above a drinking water cooling tank made of steel with an inspection opening about two feet square in the top of the tank.

The cover over this opening was wood which was condsiderably deteriorated and not tight-fitting. Any leakage onto the top of this tank near the opening could drain into the tank. An engineering report stated that on June 29th, 1933 conditions at the Congress Plaza hotel were bad enough, due in part to excessive rainfalls, to allow gross polution of the cooled drinking water. Even without the storm, the high number of of guests staying at the hotel (approx.1044) would put the sewer under pressure which would then cause leakage from the wooden plug. Tests later determined as much as two-tenths of a gallon per minute had leaked into the tank. Between the period of June 1, 1933 to June 30, 1934 there were 1,409 cases of illness that were ultimately related to the Century of Progress (Worlds Fair in Chicago). A little more than two-thirds of those infected were visitors from out-of-town. 75% of those infected had contact with one or the other of the two hotels. Less than one-half of the infected residents of Chicago reported being in contact with either hotel.

After this tragic epedidemic, health officials in the United States realized many deadly diseases such as cholera, polio, typhoid fever, hepatitis A, leptospirosis, and others could spread through water systems. Technologies and plumbing standards advanced over time with special factions dedicated specifically to creating and maintaining cross connection codes. During World War II, in 1943, a supply ship discovered salty sea water from the harbor in their ships potable water tanks. It was determined this was caused by a cross connection between the city potable water supply and the harbor water. A group of concerned citizens believed an educational institute would be the best way to approach the goal of protecting potable water supplies. This group of individuals worked out an agreement with the University of Southern California to establish a laboratory to research cross contamination solutions. In 1944 the Foundation for Cross-Connection Control Research was organized by the USC Board of Trustees. But it wasn't until the 1950's the first double check valve assemblies and pressure vacuum breakers were used in plumbing applications.

Backflow prevention products come in a variety of types, models, brands and sizes. While various options are available for various plumbing situations, the basic science behind how backflow valves prevent contamination is the same despite which type is used. Backflow prevention devices are installed on piping systems where water flows in one direction and at the point where possible cross connection could occur on the property. They prevent any water, contaminated or not, from flowing back in to the main water supply line. Negative pressure can cause contaminated water to be back-siphoned in to a potable water supply line. If your water main is flowing normally, a sudden negative pressure can be caused by a break in the pipe, or a stoppage in the municipal water supply, or an increased water demand elsewhere on the same water supply, such as a firefighter opening a fire hydrant down the street. This negative pressure creates a vacuum in the pipe, commonly referred to as back-siphonage, and will reverse the normal flow of water in the piping system. If a backflow preventer is not installed, contaminants can be pulled into your drinking water.

Basic backflow prevention is already built into the principles of plumbing systems. For example, sink faucets are required by plumbing code to have at least a one inch clearance above the sink fill line to prevent backflow. Most residential plumbing fixtures do not require additional backflow protection. Only a few fixtures, such as: handheld showers, hose bibbs, irrigation sprinklers, and bottom inlet water heaters, require at least a vacuum breaker.

As backflow prevention technology advanced, different types of backflow prevention emerged. There are four basic types of backflow prevention devices:

  • Atmospheric Vacuum Breaker (AVB) – Also known as anti-siphon vacuum breakers. The simplest of backflow prevention devices, it consists of a check seat, an air inlet valve, and an air inlet port. The air inlet valve at the top of the vacuum breaker is forced to close upwards when the flow of water pressurizes into the body of the vacuum breaker as water flows through and exits out. When the water flow stops, the air inlet valve drops the check seat effectively closing off the inlet and preventing foreign material from entering. When this happens it simultaneously opens the air inlet port allowing air to enter and eliminate the vacuum created by the termination of the water flow and prevents any back-siphon. Atmospheric Vacuum Breakers are available in pipe sizes from 1/4" up to 3" inside diameters. The AVB must be installed in a vertical position for the device to work properly and may not be subjected to continuous water pressure. It must be installed at least six inches above all downstream outlets and no shutoff valves are allowed downstream of the device. Installation examples include process tanks, dishwashers, washing machines, and lawn sprinklers.
  • Double Check Valve Assembly (DCVA) – The DCVA is comprised of two positive seating check valves (hence, the double check) within one backflow prevention assembly body. These assemblies are designed for use in non-health hazard cross-connections and continuous pressure applications subject to backpressure or backsiphonage incidents, such as lawn sprinklers, fire sprinkler lines, commercial pools, or industrial processing tanks and vats. Double check valve assemblies includes two shut-off valves and four test cocks. Double check valve assemblies require annual testing by a certified backflow prevention inspector to ensure they are working correctly. Faulty assemblies can be repaired with replacement parts.
  • Double Check Detector Assembly (DCDA) – The DCDA is a souped-up version of the double check valve assembly (DCVA). Designed exclusively for water utilities for non-health hazard containment requirements. This device prevents reverse flow from non-potable water sources from being pumped or siphoned into potable drinking water lines. Usually necessary for fire protection systems, it incorporates a meter to detect underground water leaks and unauthorized illegal taps. This feature greatly reduces unauthorized water use and annual water loss expenses. Like the double check valve assembly (DCVA) it requires annual testing by a certified backflow prevention inspector to ensure it is working correctly.
  • Reduced Pressure Zone Device (RPZ) – These crucial assemblies keep reverse flow of contaminated water out of residential drinking water and municipal water supplies. Reduced pressure zone valves provide the best level of protection from back flow or back-siphonage. Once water flows out of the reduced pressure zone device it cannot get back through it into the water supply. The water flowing through and out of the double check valves is kept at a lower pressure than the pressure coming in from the water supply. If the pressure on the outlet side of the valve ever begins to exceed the supply, or inlet, side, the two check valves close to prevent any possible backflow. If either check valve were to leak, a relief valve would open and release water to the outside. Since this device has the potential to dump profuse amounts of water where it is located, it should be installed in an area that allows for ample drainage. Reduced pressure zone valves are ideal for any type of potential back flow situation and must be installed in a horizontal position with the relief valve at the bottom for easy discharge. This device also requires annual testing by a certified backflow prevention inspector to ensure it is working correctly.

Backflow prevention assemblies are available in a variety of brands, models and pipe sizes. Parts for backflow devices can be obtained, but it is imperative to know the brand, model and size when looking for replacement parts or kit assemblies. There are different types of backflow prevention assembly parts that can be repaired or replaced:

  • Backflow Prevention Shut Off Valves – Can be engaged to turn off the flow of water at the shut off valve location. These valves can be used in case repairs are needed to the assembly or to shut off the flow of water.
  • Test Gauges – Most backflow prevention assemblies need to be tested annually. To do so, test gauges must be purchased and registered with TCEQ testing and inspections. The test gauges must be tested themselves annually to ensure proper function.
  • Wye strainers – Are usually purchased separately to protect the backflow equipment by filtering sediment and debris that could cause the pricey backflow prevention devices to fail prematurely.
  • Pipe supports – Supports help to secure the backflow prevention device and ensure the proper clearance underneath the device, by code.
  • Relief Valves – These valves are meant to open and allow water to vacate the device. Prevents cross contamination by back-sigphonage and backpressure.
  • Repair Kits – Can be found for almost every part of any backflow device. Kits can include repair parts for the entire valve, or for specific internal parts of the backflow assembly. Typical kits include o-rings, springs, discs, seats, diaphragms, pistons and more, or less, depending on the model of backflow prevention assembly you have.
  • Obtaining safe drinking water and safe public water systems would not be possible without backflow prevention. The devices and assemblies that prevent cross contamination are true lifesavers. Due to their importance, backflow prevention devices must be checked annually or as governed by your local code authority to ensure their operability. Not all licensed plumbers are licensed and qualified to install or service backflow prevention devices and assemblies. Confirm that a plumbing professional possesses all proper qualifications and certifications before allowing them access to your backflow prevention device.

Related Items & Articles



return to top ↑

Copyright© 1995-2022 PlumbingSupply.com.
All Rights Reserved.