FAQ - Repairing Backflow Preventers

Print this Article

Repair of Backflow Prevention Assemblies


The purpose of a repair on a backflow preventer is to return the assembly back to its original factory working specifications. The need for a repair can only be determined by conducting a field test on the assembly and knowing when the data indicates the backflow prevention assembly is in need of repair.

Backflow preventers should only be repaired when the data collected from the field test indicates the assembly has degraded in its performance below the established minimum standard. The test procedure is a diagnostic evaluation of the workings of a backflow preventer. When the backflow preventer fails to meet this minimum standard, the backflow preventer must be repaired or if necessary, replaced. These minimum standards are set at a level that as the backflow prevention assembly begins to deteriorate in its performance, it can still prevent backflow. The purpose of a repair is to return the assembly back to its original factory working specifications. This means the assembly is repaired so that it can perform above the minimum standard.

Sometimes, an untrained person will see the relief valve of an RP discharging water and make the assumption the backflow prevention assembly is not working. This assumption could be wrong when pressure fluctuations or other hydraulic conditions may cause the relief valve to open. This discharge from the relief valve can happen from a working RP as well as a non-working one and this is why the field test must be performed to determine the condition of the assembly. The repair may be as simple as replacing a few rubber parts (eg. discs or o'rings) or may require the replacement of some of the hardware parts (eg. seats or stems) inside the backflow prevention assembly. When a tester performs a field test, he generates data on the workings of the assembly. It becomes the tester's duty to know what the field test data means for that particular assembly. When a trained tester interprets the data, he can determine what parts of the backflow prevention assembly failed the test and needs repair.

Repair Frequency

Backflow prevention assemblies are designed to provide decades of protection. There are many regional conditions that can shorten the working life of a backflow preventer. Conditions such as excessive pressure, water quality, temperature or turbidity can all cause the assembly to deteriorate in its performance. Some backflow prevention assemblies may rarely need repair while some are repaired on a regular basis. The test data from the field test will determine the repair frequency. If the repair frequency becomes excessive it is important to determine the factors that are causing the unusual repair frequency. If the cause is due to excessive turbidity (dirt and debris), then a strainer may provide a longer duration between repairs. If the backflow prevention assembly is failing due to water quality conditions such as chloramines or temperature, replacing the existing assembly with an assembly with different rubber and material composition may solve the problem. If excessive pressure is damaging the software parts, then a pressure regulator may be needed. Evaluating not only the field test data but also the cause of the failure is important to ensure a proper repair frequency.


Backflow prevention testers usually have attended a course on the subject and they also may choose to be certified by completing an examination process. Once a person becomes a certified tester, there is a good chance the person knows how to test a backflow preventer. Unfortunately, this does not mean the person knows how to repair a backflow preventer. Many times it is assumed if you can test them, you can repair them. This is not true. The repair technician has a long list of things he must learn to be able to repair a backflow prevention assembly and return it to its original factory specifications.


We need to clarify a little terminology before we proceed with a backflow prevention assembly repair. A backflow prevention assembly consists of an inlet and outlet shut off, appropriately located test cocks, and the necessary components of the assembly (check valves, relief valve or air inlet). Each component can further be broken down into its parts. For example a first check is a component. A component can further be broken down into its individual parts (check discs, springs, disc holder etc). Backflow parts are assembled to make components, and components are assembled to make a backflow prevention assembly.

Each component of the assembly must be tested to determine what is in need of repair. The field test may not always show you the severity of the failure or whether repair parts beyond the basic rubber repair kit are necessary. There may be additional test procedures, beyond those established in the tester certification program, that can provide data showing the severity of the failure that could help in the diagnostic analysis of the backflow prevention assembly.

A review of the hydraulic conditions present at the assembly must then be evaluated. There are hydraulic conditions such as pressure fluctuations and disc compression that could lead to false data on the workings of the backflow prevention assembly. Hydraulic conditions such as water hammer can lead to extreme pressure spikes that could damage components. The effects of elevated temperature can also cause excessive pressure conditions which could damage the parts in the components. These and other negative effects from the hydraulic conditions must be recognized and their cause corrected. If parts are replaced and the hydraulic conditions are not corrected, the new parts can soon be made inoperative.

The next criteria to evaluate is how the backflow prevention assembly is installed. It is important that the repair technician knows the correct installation orientation and can evaluate the installation conditions to ensure there are no conditions circumventing the working of the assembly. The backflow prevention assembly must be installed in the installation orientation for which it was approved. Most backflow preventers are installed in a horizontal orientation to the surrounding grade. Some (not all) backflow preventers can be installed in a vertical orientation, but it is important that the assembly only be installed if it is approved for that orientation. To arbitrarily take an RP and stand it on its end in a vertical orientation may cause the backflow prevention assembly to not work properly and no matter how many parts you put inside the assembly, it still may not work because the installation orientation is causing the failure.

The backflow prevention assembly must also be correctly installed based on the sizing, temperature and pressure parameters for which the assembly was approved. Placing an excessive pressure or temperature on an assembly could damage internal parts. Replacing the damaged parts without fixing these mentioned conditions, will only lead to another failure of the new parts.

The repair technician must be able to identify the many different manufacturers and the particular models produced over the years. Some models may seem similar on the outside but could require different repair parts on the inside, so the repair technician must be able to identify the make, model and size of the backflow prevention assembly and any variations that may have been produced. Sometimes the serial number of the assembly will be required to differentiate one model from another. It is important to know the correct model to assure you get the correct repair parts.

Correctly identifying the make model and size of any backflow prevention assembly will allow the repair technician to determine that the particular assembly is approved for use in this jurisdiction. There are different approval processes that may be allowed in an area; be sure the particular model has gone through the appropriate approval process. The repair technician needs to have a clear understanding of how the different types of backflow prevention assemblies operate. It is not enough to know the minimum testing standards for the assembly; you mustalso have a clear idea of how each component is constructed and operates, in order to repair it properly.. Manufacturers produce specification literature that can help the technician learn about how the particular backflow prevention assembly is built and what materials it is constructed of. There are also flow charts available that can help the repair technician learn the important head loss data of a particular assembly. Manufacturers also provide repair and maintenance manuals that can help a technician complete a successful repair on their backflow prevention assembly.

Proper repair tools are needed to complete the repair; usually standard hand tools are all that is needed. Some models of backflow prevention assemblies, however, may also require special tools. Special tools are tools that are constructed for a specific task on a specific model of backflow preventer. These special tools are usually not commercially available but can be fabricated or purchased from the backflow prevention assembly manufacturer in order to complete the repair. Determine if you will need special tools in addition to standard tools, before you begin your repair procedure.

The repair of a backflow preventer will usually involve the replacement of damaged rubber parts. Backflow manufacturers all produce basic repair parts for their models. Most components of backflow preventers are field-repairable. This means the individual component (check valve, relief valve, air inlet) can be rebuilt by replacing the rubber goods. On some backflow prevention assemblies, the components are not field-repairable and the whole component will have to be replaced as part of a normal repair. The basic repair kits will contain either the rubber parts for the component or the whole component. Manufacturers also sell additional parts in addition to the rubber parts (eg springs, guides). In most cases, these additional parts are not needed for an average repair. Only if the hardware parts are damaged should they be replaced. The repair technician must be sure he is using only original factory repair parts. This will help guarantee that the repair technician can return the backflow prevention assembly to its original factory working specifications. In some older assemblies, repair parts may no longer be available and the backflow prevention assembly may have to be replaced.

Repair Preparation

The repair of a backflow preventer requires the technician to have the necessary knowledge of how to diagnose the problem and restore the backflow prevention assembly to its original factory specifications. Even if the technician has the necessary background, some steps must be taken first before beginning the actual repair.

Before the actual repair can begin, the repair technician must notify the water user that the assembly is in need of repair. During the repair process, the water will need to be turned off and the water user needs to be made aware of this. Sometimes repairs may have to be done at special hours to accommodate the water user in their normal course of business. Be sure to tell the water user how long you expect the water to be off and what equipment in the facility the water outage could effect. Most of the time there is equipment that must be isolated when the water is turned off. This equipment could be pumps, cooling towers or other water using equipment that must be protected when the water to the backflow preventer is turned off. Most wet-charged fire systems have alarms attached to them; when the shut-offs to the backflow preventer are turned, the alarm will send a signal to a monitor station that could cause the arrival of emergency equipment such as the Fire Department. To avoid any equipment damage or improper alarm signals, be sure the water user knows risks and effects of shutting off the water to the backflow preventer.

When a water system is being de-pressurized for a repair, it is important to make sure a backflow condition is not created. The repair technician must be careful to not create a backsiphonage condition by draining the water from a higher elevations down to lower elevation. If the backflow prevention assembly was installed as a service protection assembly, this backsiphonage condition could lead to a contamination of the plumbing system.

Before the repair technician begins the repair process, he must be sure he has evaluated the work area around the backflow prevention assembly in order to identify any restrictive conditions. If the area is considered a confined space, proper safety precautions must be followed for proper entry. A proper and safe work environment may entail the placement of safety barricades or entry restrictions around the work area so the repair technician or the general public around your work area are not harmed. If the backflow preventer is installed in a restrictive area, this could affect the technician's ability to successfully complete the repair.

Have all tools needed on site and a proper work area available. This will vary on the size or location of the assembly. A small assembly may only require a few hand tools, while a 10" assembly could require a large work bench to complete the repair.

Repair Procedure

First, shut off the backflow prevention assembly. The downside shut off should be closed first, followed by the inlet shut off. Open all test cocks fully to assure all water is shut off and all water pressure is relieved; in addition, some models require the test cocks open for the removal of components from the body.

Make sure you have a full understanding of how to disassemble the backflow preventer. Without full knowledge, you could release spring tension prematurely, or disassemble parts that do not need to be removed, or remove the parts in the wrong order.

Most backflow preventers are designed to allow the components to be removed for repair while the main body stays installed in the piping system. Removing the backflow preventer to repair it is not necessary in most cases. Some components such as relief valve bodies may need to be removed from the main body to facilitate a repair. Knowing the correct order for disassembly of the backflow preventer can be important.

The next step in the repair process is to remove the components from the assembly. Be sure all pressure is relieved and the disassembly order is understood. Each component should be removed in its entirety if possible. It is good to evaluate the working of each component once it is removed from the backflow prevention assembly body. Looking for what caused the failure of the component can help assure you repair the assembly properly. For instance, removing a check assembly from the body and then finding a piece of dirt or debris lodged between the disc and the seat is the likely cause of the check failure and will ensure a proper repair.

The next step is to disassemble the component down into its separate parts. Each part must be inspected to assure there is not any excessive wear and tear that could render the part unusable. There are several different materials that parts can be made of (bronze, stainless steel, iron, or plastic). These materials have different hardness characteristics and must be cleaned in a way that does not damage them any more than they already are. A good rule of thumb is to clean the parts with a material softer than what you are cleaning. This will minimize any scratching or galling of the parts. Learning the properties of the different materials is important so that you can evaluate whether the materials can be cleaned or they have been damaged and in need of replacement.

Metal alloys are susceptible to an electro-chemical condition called corrosion. Corrosion can literally eat a metal away causing the metal to not be as strong as it originally was or alter the tolerances causing parts to not work in a backflow preventer. The effect of corrosion on a metal alloy must be evaluated to determine if the part is usable or must be replaced.

The other condition metal alloys are susceptible to is the growth of scale on the parts. Scale is the accumulation of dissolved solids from the water onto the alloy. This accumulation is a growth on the alloy. Like a barnacle, this accumulation grows but does not damage the host it is attached to. When removing the scale, care must be taken not to damage the material under the scale.

Plastic materials will not have corrosion, but the effects of erosion, scale or stress can cause these non-metal parts to need cleaning or in extreme cases, replacement.

In the average repair, the old rubber parts must be removed and replaced with new rubber parts. It is important that only original factory repair parts are used when putting the backflow prevention assembly back together whether they are hardware or rubber parts. The rubber parts the manufacturer produces for the assembly are the only ones the assembly was designed to use and changing the rubber from the original factory parts will affect the operation of the backflow prevention assembly and void any approvals.

Once the hardware parts have been cleaned and inspected and the rubber parts have been replaced, now reassemble the parts back into their components. Some components may have very tight tolerances that have to be maintained. An important thing to identify on each component is the guide. The guide is the part that directs or steadies the motion of the component. The guide is a very critical part of the design of each component. If the component is not repaired properly, the guide could restrict the motion (rather than direct or steady) of the component, which could cause the component to not work to its capabilities. Many times springs are erroneously blamed when there is a low differential on a check valve, or a relief valve will not open. In most cases the guides are restricting the travel of the component causing lower test results than what is needed.

Next, inspect the body. Depending on the size and manufacturer, the bodies can be made of bronze, plastic, stainless steel, cast iron or ductile iron. It is again important for the repair technician to be aware of the hardness of the materials so that their cleaning process is not destructive. In the larger backflow prevention assemblies (2 - 10") the iron bodies will be coated to minimize any scale growth or corrosion. It is important that the repair technician is sensitive to the hardness and type of the different coatings used on iron body units so as not to damage the protective coating.

Evaluate the component seats. The seats may be mounted in the body, or some seats are part of the component. In either case it is important that the seats are properly cleaned. Many repair technicians make the fatal error of trying to aggressively sand or file the seats in a backflow preventer. Be aware of two important conditions in the design of the seat: the flatness across the seat face, and the shape or profile of the seat. If one side of the seat becomes lower than the other due to an aggressive filing, the seat will not seal against the disc. If the seat profile is changed, there may not be enough loading in the component to seal the disc against the seat. For instance, if the design of the component requires a very sharp or pointed profile seat, and a repair technician files the seat and creates a flat profile, the spring loading may not seal the disc against the newly re-shaped seat and the backflow prevention assembly will not work properly. If the seats shape or profile is changed, the backflow preventer may not be repairable, and the seat may have to be replaced.

The next step we must perform in the repair process, is to flush the incoming water line. After we have cleaned the body, and the repaired components are still out of the body, and we have completed all of our part replacement and cleaning, we must open the inlet shut off to flush any rust or debris. This can be tricky, especially when the backflow prevention assembly is installed inside an area where water cannot be splashed around. It is important that the inlet valve be opened to flush this dirt and debris even if some sort of water collection system has to be utilized to assure the line is properly flushed.


We now have cleaned and inspected the components parts, replaced the rubber repair parts, re-assembled the parts and the rubber parts back into their components, cleaned the body and the seats and flushed the incoming water line. We are now ready to begin the re-assembly process. Some models require the components to be removed in a particular order. These assemblies will also require the re-assembly in a particular order. Many components cannot be re-installed into the body unless the test cocks are left open. As each component is put back into the body, it is important to put them back in the correct location. It is also important that as they are placed back into the body, that no foreign matter is attached on the components or accidentally placed in the body. The components must be placed so that the guides are able to maintain their designed tolerances.

Lubrication may help with the placement of some components within the body of the backflow prevention assembly. Lubricant does not help the assembly work any better, it is only there to help the technician assemble the components into the body without creating damage from friction. The lubricant must be a food grade lubricant to assure the water system is not contaminated. In many cases the best lubricant is water. The usual use for lubricant in the repair process, is on o'rings (not guides). All o'rings should be lightly lubricated to assure they do not roll out of their retainer. The use of too much lubricant can cause the backflow prevention assembly to fail or wear out sooner by attracting dirt and debris or restricting the motion of the component. The proper amount of lubricant should make the surface of the o'ring appear wet. You should never see chunks or globs of lubricant.

Once the components are placed back into the body, the backflow prevention assembly is ready to be pressurized. It is important to turn the inlet shut-off valve on slowly while the downside shut off remains in the closed position. This allows the parts to move to their pressurized positions in a precise movement. Once the assembly is pressurized, then any accumulated air must be removed from the body. Most backflow prevention assemblies have air bleeds or air screws so that air can be removed from critical areas in the assembly. In some models, test cocks can be used to remove air from an assembly if they are near the top of the body or in a location where the air will accumulate.

Once the air is removed, it is time to test the backflow prevention assembly once again. The testing of a repaired assembly is not done just to see that the assembly meets the minimum test requirements of the test procedure, but to see that it meets the original factory working specifications. The headloss across an assembly is shown in the manufacturers literature in their flow chart. To assure you have repaired the assembly properly, you must compare the headloss of the backflow prevention assembly to the headloss shown in the flow chart. Remember, the headloss in the chart is the pressure loss across the whole backflow preventer, not just one component. The final test results should be entered on the test report. On the test report, a notation should be made as to which parts had to be replaced during the repair.

Once the test results show the repair is complete, it is time to re-pressurize the plumbing system that was isolated during the repair process. It is important that the plumbing system be turned on very slowly to assure no damage is done to the piping system. Once the system is pressurized, air must be removed from the plumbing system. Each outlet on the plumbing system should be checked to ensure no valves were accidentally left open or did not close properly when re-pressurized. Once the plumbing system is pressurized, be sure all valves are completely open and notify the water user that the plumbing system has been returned to service.

The proper repair of a backflow preventer requires many items to be evaluated. The hydraulic conditions present at the backflow prevention assembly must be evaluated. The installation must be evaluated to ensure it does not circumvent the workings of the assembly. Test data must be accumulated to determine the workings of each component. The backflow prevention assembly must be disassembled and parts replaced with original factory parts. The backflow prevention assembly must be tested again to be sure it was properly repaired. Comparing the test results from the repaired backflow prevention assembly to the manufacturers flow chart is necessary to ensure it was repaired properly. A properly repaired assembly should provide years of protection.