Backsiphonage vs. Back-Pressure: What’s the Difference?

As a business owner or building manager, you probably don’t think about acute water service complications daily. Furthermore, specific backflow terms like “negative pressure” or “double-check valve” likely aren’t a part of your regular vocabulary. However, understanding the key difference between backsiphonage and back-pressure is vital for ensuring safer plumbing conditions. Read on to learn about these types of backflow, common causes and prevention methods for each, and a breakdown of the most effective backflow prevention equipment for these specific complications.


To truly understand backsiphonage, you must first know about vacuum pressure and its significant role in backflow. A great example of vacuum pressure is, well, the standard home vacuum cleaner currently sitting in your storage closet! Vacuum cleaners create pressure differences (positive pressure) between suction tubes and various dirty surfaces to “suck up” dirt and debris.

This very same phenomenon occurs within pressurized plumbing systems all the time—in fact, it’s crucial for providing proper water services and clean water supplies! Water flows freely without steady pipeline pressure throughout your plumbing infrastructure, potentially contaminating potable public resources. Therefore, modern plumbing systems maintain constant pressure levels to ensure optimal directional flow.

However, this steady directional flow is vulnerable to negative pressure, which can instantly reverse the flow and create backflowing conditions. This is called backsiphonage, and it works similarly to a plastic drinking straw. When negative pressure is present, water gets sucked backward and heads for the supply. But what causes this negative pressure, and how can businesses protect their plumbing systems from backsiphonage conditions?

Common Causes of Backsiphonage

Since negative pressure is the root cause of backsiphonage, interruptions to water supply systems are undoubtedly the most common cause of these backflow conditions. For example, water main breaks and high-water demand (firefighters accessing fire hydrants) create large amounts of negative pressure that can impact plumbing infrastructure throughout entire communities. Other water service interruptions, like power outages, pump failures, and preventer malfunctions, all reduce supply line pressure and create ideal backsiphonage conditions. Finally, misaligned backflow prevention equipment or other plumbing fixtures can create negative pressure and contaminate potable water supplies.

Preventing Backsiphonage Backflow

Both types of backflow conditions are prevented through special equipment known as backflow preventers. These devices include pressure-sensitive check valves and, sometimes, relief valves to effectively respond to and eliminate backflowing water before it contaminates potable water resources. The two most popular types of commercial backflow preventers are double-check valve assemblies (DCVAs) and reduced pressure zone (RPZ) devices—importantly, both are effective at preventing backsiphonage! However, RPZs are the most suitable devices for high-hazard and high-pressure applications and, unlike DCVAs, protect against back-pressure conditions. You can always opt for a DCVA to prevent backsiphonage in low-hazard environments, such as fire suppression systems.


Back-pressure is very similar to backsiphonage and can impact any pressurized system, from plumbing infrastructure to chemical pumps at a processing refinery. Yet, unlike backsiphonage complications, back-pressure is caused by excessive positive pressure. When pressure from the downstream piping exceeds that of the water supply, back-pressure occurs and pushes the directional flow back toward the supply. This, in essence, is the true difference between these types of water system issues—backsiphonage “sucks” water, while back-pressure “pushes.” Furthermore, the common culprits and causes for back-pressure are less common than those for backsiphonage.

Common Causes of Back-Pressure

More often than not, back-pressure conditions are triggered by pump malfunctions of downstream piping obstructions. Both of these complications increase downstream pressure tremendously and increase the chances of backflow conditions. Other common culprits of back-pressure issues include the presence of an unprotected cross-connection (which is also illegal according to the Clean Water Act of 1972) and damaged or malfunctioning plumbing infrastructure. Piping that suffers from water hammer—a surge of positive pressure that can rattle and move plumbing systems—are more likely to experience back-pressure due to the extensive misalignment and damage.

Preventing Back-Pressure Backflow

Unfortunately, DCVAs do not provide adequate protection against back-pressure conditions. While these assemblies can stop backsiphonage in low-hazard applications, they do not include any pressure relief elements to reduce downstream pressure. Conversely, RPZs are incredibly versatile and feature pressure-sensitive components designed to combat all forms of backflow conditions. If your building requires a high-pressure prevention device, we highly recommend opting for an RPZ unit.

DCVAs vs. RPZs

As mentioned before, double-check valves and reduced pressure zone assemblies are ideal backflow prevention equipment for combatting backsiphonage and back-pressure. However, these two devices are not simply interchangeable—rather, each features specific capabilities and advantages designed to ensure superior protection and performance for certain applications and environments. Learn more about the differences between these two backflow preventers to determine which is best for your specific needs.

Advantages of DCVAs

A standard double-check valve assembly features four distinct features—four total test valves, multiple outlet shutoff valves, a singular inlet shutoff valve, and one valve body with two independently spring-loaded check valves. Additionally, DCVAs always run parallel to the connected plumbing infrastructure, either horizontally or vertically. Ultimately, this specific design is optimal for low-pressure and low-hazard (non-health hazard) applications, including installation in fire suppression systems or appliances that use chemical products.

Advantages of RPZs

Reduced pressure zone backflow preventers, similar to DCVAs, feature two spring-loaded valves. However, these specific valves are separated by a continuous low-pressure zone that is incredibly flexible for vertical, horizontal, underground, and aboveground installation. RPZs also boast two unique elements not found on most DCVAs—a bypass relief valve and a pressure sensing line. Together, these features increase your backflow prevention equipment’s performance, accuracy, and response. More importantly, the low-pressure zone design is ideal for high-hazard and high-pressure applications, including installation in chemical processing plants or for hospital water supply systems.

Understanding the difference between backsiphonage and back-pressure conditions is crucial for preventing these hazardous water service complications. Backflow of any kind threatens the health and safety of building occupants, nearby communities, and surrounding ecosystems alike. Ensure you protect those around your business by investing in quality backflow prevention equipment for your existing plumbing infrastructure. Our team at Backflow Direct proudly carries industry-leading Deringer backflow preventers designed for superior performance and reliability. The Deringer brand is the industry’s only 100 percent stainless steel backflow preventer product line—contact our team today to learn more about the advantages of a Deringer device.

Backsiphonage vs. Back-Pressure: What’s the Difference?Backsiphonage vs. Back-Pressure: What’s the Difference?