Procedure to test a double-check backflow preventer assembly using a differential pressure gauge
Short answer
Testing a double-check backflow preventer assembly involves connecting a calibrated differential pressure gauge across test cocks to ensure both internal check valves maintain at least 1.0 pound per square inch differential. Technicians must first flush the test cocks and confirm the downstream shut-off valve seals tightly. Each check valve is then isolated and measured sequentially to ensure differential pressure remains at or above the 1.0 psid standard.
Testing a double-check backflow preventer assembly requires a calibrated differential pressure gauge to confirm that both internal check valves hold tight against a minimum threshold of 1.0 pound per square inch differential across each seat.
Municipal plumbing codes and cross-connection control programs require annual certification of all containment assemblies under ASSE Standard 1015 to prevent non-potable water from siphoning into public mains. A failure rate of roughly 5% across field-installed backflow preventers during annual testing makes precise field diagnostics critical before issuing a certified test report.
If you only do one thing: Verify that the downstream shut-off valve holds bubble-tight under system pressure before recording differential values, because downstream seat leakage yields false check-valve failure readings below 1.0 psid.
- Flush test cocks: Notify the facility owner of supply interruption, then open and flush test cocks 1, 2, 3, and 4 to clear scale and debris before connecting the differential gauge adapters.
- Mount the gauge and bleed air: Close shut-off valve 2 while leaving shut-off valve 1 open, then connect the high-pressure hose to test cock 2 and the low-pressure hose to test cock 3 before opening the bleed valves to vent trapped air.
- Test check valve 1: Close the gauge high- and low-bleed needle valves, observe the differential pressure reading across the first check, and verify the needle stabilizes at or above the 1.0 psid pass threshold.
- Test check valve 2: Move the high-pressure hose to test cock 3 and the low-pressure hose to test cock 4, bleed air through the low vent valve, close all gauge valves, and confirm the second check valve holds a minimum differential of 1.0 psid.
- Check shut-off valve 2 tightness: With test cocks 3 and 4 pressurized, open the gauge high bleed while keeping shut-off valve 2 closed to verify no continuous flow or pressure drop occurs, confirming downstream isolation integrity.
- Restore service: Close all four test cocks, remove gauge hoses, fully open shut-off valve 2 slowly to avoid water hammer, and document the psid values on the certified inspection report.
- Watch out for: Debris lodging under check valve disc elastomers, which causes immediate pressure drops below 1.0 psid and requires disassembling the housing to clean or replace internal disc retainers.
- Watch out for: Closing upstream isolation valves too rapidly on lines 2 inches and larger, which creates hydraulic shock and damages diaphragm gauges or downstream piping joints.
- Watch out for: Uncalibrated test kits; gauges must undergo certified recalibration every 12 months to satisfy local administrative water authority standards.
To schedule annual backflow certification or complete mechanical repairs on non-compliant double-check assemblies, contact ARS/Rescue Rooter at 866-399-2885 for professional plumbing service backed by the Exceptional Service Guarantee.