Top Causes Behind Nuisance Backflow Assembly Failures

Determining the precise source of backpressure in a water system can be challenging. Here is some guidance.

Top Causes Behind Nuisance Backflow Assembly Failures

Anthony Pacilla

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Most plumbers are familiar with backflow assemblies, devices meant to prevent the unwanted reversal of flow in a water system.

If you look deeper into the issue of backpressure, you might find yourself wondering about the potential sources. You could be called upon to diagnose the cause of backpressure for a commercial building that is having routine failures of backflow assemblies. Looking through the standard list of typical culprits, you may start to realize how difficult it can be to figure out precisely what is causing the issue. There are so many devices and controls within a commercial building, that it can be a challenge to track down the source of backpressure. Here are some tips to keep in mind.

Talk with any maintenance staff available to get an idea of where they think the backpressure might be coming from. Be patient with maintenance staffers. They are not as knowledgeable about plumbing matters as we are, but they might be able to provide clues to get you started.

Consistently high backpressure can wear down the second check valve of a backflow assembly quickly. You may have 120 pounds of water pressure rushing in through a 6-inch pipe when the building is in full use. If you have a source within the building providing high backpressure, you could severely damage the second check valve when the incoming water shuts off. When the check valve violently slams shut due to the enormous backpressure, it will wreak havoc on the seats and discs. I have seen cases during service where a check valve became dislodged from its retainer due to extreme pressures.

Thermal Expansion

The first thing you should be looking for in a high backpressure situation is a boiler. It could be a domestic hot water boiler or it could be a hydronic boiler providing heat to the building. A domestic hot water boiler is a glorified water heater. As with any water heater, thermal expansion occurs as the water heats up. This expansion will build pressure higher than the incoming water pressure, hence the invention of the expansion tank. The same principle applies to boilers. Boilers require properly sized expansion tanks to control over-pressurization within the closed piping system.

If you check the incoming water pressure from the main and the pressure reads 120 pounds, and you follow up by checking the pressure at the water heater and it reads 200 pounds, you have a problem. The relief valves will intermittently discharge to relieve itself; the expansion tanks will rupture or flood if they are not sized properly, and this puts tremendous pressure on the system and pushes back on the check valves, causing them to prematurely fail.

Installing a check valve at the boiler will solve the pressure problem pushing against your backflow assembly, but the tremendous pressure still has nowhere to go except push on the entire piping system, causing all kinds of failures within the building. You have to fix the real issue in this case, which is the boiler and its undersized expansion tank.

Water Hammer

Another major cause of premature failure to a backflow assembly is water hammer due to a quick-acting valve of some kind. This could be pumps slamming on and off, solenoid valves of all shapes and sizes, or automatic controls. 

When you are doing your initial walkthrough, you should be looking for quick-acting valves and what they service. Buildings with large irrigation systems and car washes with quick on/off valves are a couple examples.

Main Line Breaks

When the water utility has a main leak, it will shut down a section of the main to make a repair. Upon completion of the repair, it will restore water pressure. Assuming you’ve made a repair on a residential home before, you are aware of the amount of dirt that comes out of a standard spigot; imagine that times 10. The debris that can come out of a city main can be the size of small calcium deposits to as large as rocks. When water service is restored, this debris can shoot into your device like a bullet and destroy internal parts. This can be especially damaging if it happens to destroy the guts of an RP assembly that goes into full discharge, possibly flooding a facility.

Water Quality

Poor water quality is another major factor behind assemblies routinely failing. Small amounts of debris regularly flow through water mains, which is why most manufacturers suggest installing a strainer before every backflow assembly. 

The strainer will allow the debris to collect into its fall point and can be routinely blown out if there is a ball valve installed on the fall point. This should be done at least annually when the assembly is tested. If during your assembly repair you find a large amount of silt, debris, or scratches to the rubbers and discs, you should address the issue by installing a strainer.


You should pay attention to the size of the incoming water line compared to the size of the water meter and the size of the backflow assembly. The sizes of these devices and assemblies should be consistent. Every meter, valve, assembly, and pipe have flow rates. If you significantly reduce any of these items, you cause the velocity to increase, which can screech, drag, and tear the guts of a backflow assembly. 

About the Author

Anthony Pacilla is a registered master plumber for McVehil Plumbing in Washington, Pennsylvania. He has 23 years' experience in the plumbing and HVAC trades, and has a bachelor’s in business and economics from Thiel College. 


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