Backflow refers to the unwanted reversal of fluid flow in a piping system — essentially when water moves in the opposite direction from what’s intended. One specific type of backflow is backsiphonage, which occurs when pressure in the system drops below atmospheric levels, creating a siphon effect. This can happen during events like water main breaks, heavy fire hydrant usage, or any situation where water demand suddenly exceeds supply.
Because fluids naturally move from areas of high pressure to low pressure, a reversal in pressure differential can trigger backflow. The most reliable way to prevent this is by maintaining a physical separation between potable and non-potable systems — known as an air gap. When an air gap isn’t feasible, a backflow prevention device must be installed.
There are many mechanical devices designed to protect against backflow. Some guard against backpressure, others against backsiphonage, and some do both. Common examples include reduced pressure principle assemblies (RPZ), double check valve assemblies (DCVA), and vacuum breaker assemblies.
Vacuum breakers, equipped with check valves, are specifically engineered to prevent siphoning. Since this article focuses on backsiphonage, let’s explore how ASSE standards test these devices.
The backsiphonage test evaluates whether a backflow prevention device can resist siphoning when both internal check valves fail. To simulate this failure, testers intentionally foul the check valves—using a small wire to keep them from closing properly.
The fouled device is then installed in a test setup where a vacuum can be applied. A sight tube is connected to the outlet, with its end submerged in a water reservoir to detect any reverse flow.
ASSE standards require a series of vacuum tests, each designed to mimic real-world pressure fluctuations.
These tests are:
- Sustained Vacuum Test: Holds a full vacuum (up to 25 in-Hg)
for a set duration. - Intermittent Vacuum Test: Applies varying vacuum levels between
0 and 25 in-Hg. - Instantaneous Vacuum Test: Rapidly surges vacuum from
0 to 25 in-Hg and back. - Slowly increases and decreases vacuum in small steps.
Each test is conducted according to specific time frames and vacuums outlined in each specific ASSE standard.
During testing, the sight tube is monitored for water rise — an indicator of backsiphonage. ASSE standards define a maximum allowable rise in the sight tube. If the water level exceeds this threshold, the device fails the test. The allowable rise is specified in each ASSE standard.
Backsiphonage is just one aspect of comprehensive backflow prevention, but it’s often misunderstood. Understanding how ASSE rigorously tests devices for this risk helps underscore the importance of maintaining certified, properly functioning backflow preventers.
To have your product certified to any ASSE standard, please contact ASSE International at listingcoordinator@asse-plumbing.org. To purchase a copy of any ASSE standard, please visit the ASSE International Webstore at www.assewebstore.com.
