What Happens During a Backflow Test: Step by Step
Most property owners schedule backflow tests because their water utility requires it—not because they know what the process actually involves. You get a reminder notice, you call a certified tester, and someone shows up with a bag of equipment. But what exactly are they doing, and how do you know it was done right?
Understanding the process helps you verify that work was completed properly, prepare your property to avoid delays, and feel confident that your drinking water is protected.
A certified backflow tester in safety vest kneeling beside an outdoor backflow preventer assembly, connecting red and blue gauge hoses to the test cocks
What a Backflow Test Is Actually Checking
Before diving into the steps, it helps to understand the goal. A backflow preventer is a mechanical device installed on your water supply line to stop contaminated water from flowing backward into the public water system. Backflow happens when pressure drops or reverses in the supply line—during a water main break, heavy fire suppression activity, or a sudden pressure surge.
The test verifies that the internal check valves and relief mechanisms inside your backflow preventer are still working within acceptable pressure differentials. It does not test the water itself—it tests the device protecting the water.
Most properties use one of two assembly types: a Reduced Pressure Zone (RPZ) assembly or a Double Check Valve (DC) assembly. The test procedure is similar for both, though the pass/fail thresholds differ.
Step 1: Visual Inspection Before Any Gauges Go On
A certified tester's first action is always a visual inspection. They walk around the assembly looking for obvious problems: physical damage, corrosion, missing covers, improper installation orientation, or signs of recent tampering. They verify the assembly is the correct type for your hazard classification, and that the model and serial number match what's on file with your water utility.
If the assembly is installed in a vault, pit, or mechanical room, they check that access is safe and that the device hasn't been bypassed. Bypassed assemblies—where a property owner has routed water around the device to restore flow after a failure—are a code violation and will be flagged immediately.
This visual step takes anywhere from two to ten minutes depending on how accessible the assembly is and whether anything looks off.
Step 2: Shutting Off Downstream Flow
Before attaching any test equipment, the tester must eliminate all downstream water demand. If water is actively flowing through the assembly—someone is irrigating, running a hose, or filling a tank—the test readings will be inaccurate.
The tester will ask you or your facility manager to shut off any irrigation zones, close downstream valves, and stop any equipment that draws water. In commercial settings, this may require coordinating with kitchen staff, janitorial crews, or building maintenance. On residential properties, it usually just means shutting off the irrigation controller.
This is one of the most common reasons tests get delayed: the property isn't ready when the tester arrives. A quick walk-through before their visit will save everyone time.
Step 3: Attaching the Test Kit
With downstream flow stopped, the tester attaches a calibrated differential pressure gauge kit to the test cocks on the assembly. These are the small ports you'll see on the body of the device. Different test cocks measure pressure at different points inside the assembly—before the first check valve, between the checks, and after the second check.
The tester opens each test cock in a specific sequence and records the pressure readings. The sequence matters: opening them out of order can damage the assembly or produce false readings. A properly calibrated test kit is required by most state regulations, and testers must have their equipment calibrated on a regular schedule—typically annually.
Close-up of a differential pressure gauge test kit connected to the test cocks of a bronze backflow preventer assembly, showing pressure readings on dual gauges
Step 4: Testing Each Check Valve
The core of the test is verifying that each internal check valve holds against back-pressure. The tester manipulates the shutoff valves and test cocks to create a closed system, then measures how much pressure the check valve can hold before it leaks.
For a Double Check assembly, both check valves must hold at least 1.0 psi of differential pressure. For a Reduced Pressure Zone assembly, the relief valve must open before the differential drops to zero, and each check valve must meet its threshold. The numbers seem small, but the physics matters: a check valve that fails to hold even 1 psi will allow backflow under normal pressure reversal conditions.
If a check valve fails, the tester records the actual reading and marks the test as failed. They may also attempt a field repair on the spot—replacing an internal rubber disc or spring—and retest. Not all testers offer field repairs, so it's worth asking when you schedule the appointment.
Step 5: Testing the Relief Valve (RPZ Assemblies Only)
If your property has an RPZ assembly, there's an additional test: the relief valve. This valve is designed to open and discharge water to the atmosphere if the pressure zone between the two checks drops too low. It's the assembly's last line of defense.
The tester verifies the relief valve opens at the correct differential and closes cleanly when pressure is restored. A relief valve that is stuck open will drain water continuously; one that is stuck closed provides no protection. Both are failing conditions.
Step 6: Documenting and Submitting the Report
After the physical test is complete, the tester fills out a test report form—either on paper or through a digital reporting platform. This form includes the property address, assembly location, make, model, serial number, the date, the tester's certification number, and the actual pressure readings for each component tested.
A passing test report is submitted directly to your water utility, either by the tester or by you, depending on your jurisdiction's process. Keep a copy for your records. Most utilities want reports within 30 days of the test date.
If the assembly failed and was repaired on-site, the tester should perform a second test and document the passing results on the same or a companion report. If repair wasn't possible, you'll receive a failure notice and a deadline to replace the assembly.
Backflow tester writing test results on a clipboard form while standing next to a mechanical room wall-mounted backflow preventer assembly
What to Do After the Test
If your test passed, your obligation is fulfilled until the next required cycle—typically annual. File the report confirmation from your utility and set a reminder for next year.
If your test failed and the tester couldn't complete an on-site repair, you'll need to schedule a repair or replacement with a licensed plumber and arrange a retest. Don't ignore a failure notice: continued non-compliance can result in fines or, in some jurisdictions, a water service interruption.
Sources
U.S. Environmental Protection Agency — Cross-Connection Control Manual. EPA 816-R-03-002. Office of Water. epa.gov
American Water Works Association (AWWA) — Manual M14: Recommended Practice for Backflow Prevention and Cross-Connection Control, 4th Edition. AWWA, Denver, CO.
USC Foundation for Cross-Connection Control and Hydraulic Research — Manual of Cross-Connection Control, 10th Edition. University of Southern California, Los Angeles, CA.