8.4 Backflow Assembly Field Testing Procedures & Gauge Use

8.4 Backflow Assembly Field Testing Procedures & Gauge Use

Quick Answer: Under the USC Foundation's Manual of Cross-Connection Control, 10th Edition, a double check valve assembly (DCVA) passes when each check valve holds 1.0 psid or greater; a reduced pressure principle assembly (RP) passes when the relief valve opens at 2.0 psid or greater, check valve No. 1 reads 5.0 psid or greater and above the relief valve opening point, and check valve No. 2 holds tight; and a pressure vacuum breaker (PVB) passes when the air inlet opens at 1.0 psid or greater and the check valve holds 1.0 psid or greater.

Maryland's Journey Plumber/Gas Fitter exam devotes 10 of 100 items to Backflow Prevention, and every applicant must already hold a non-expired 32-hour Backflow Prevention Device Testing certificate. That means PSI can write questions at the level of the field test itself, not just device selection. "Any standard backflow text" is an allowed open-book reference, so bring the manual you were trained from and tab the field test procedures.

Reading the Field Test Kit

A backflow test kit is built around a differential pressure gauge — a gauge that senses two pressure points and displays the single difference between them. When you use two hoses, the reading is in psid (pounds per square inch differential). When you use only the high-side hose and let the low side sense the atmosphere (the normal PVB setup), the same gauge is functionally reading psig.

The needle valves do the work. A five-valve kit has a high control, a low control and a bypass control needle valve plus a high bleed and low bleed; a three-valve kit combines the control and bleed functions. The bleed valves purge air; the control valves let you raise or lower pressure on one side of the gauge slowly and deliberately. On an RP relief-valve test you crack the high-side bleed no more than about a quarter turn so the opening point is captured, not overshot.

Kit accuracy is not optional. The USC 10th Edition standard for a field test kit calls for accuracy within plus or minus 0.2 psid on decreasing differential readings, verified against a reference traceable to the National Institute of Standards and Technology (NIST) at least once a year. A kit is USC-approved only with the manufacturer's original hoses and needle valves — swapping in generic hoses voids the approval. Most Maryland purveyors require the gauge's last calibration date on the test report.

Test Cock Identification and Numbering

Test cocks are numbered in the direction of flow, and the numbering is the same on a double check and on a reduced pressure principle assembly.

A PVB has only two test cocks: No. 1 upstream of the check valve, and No. 2 opening into the body downstream of the check valve, beneath the air inlet.

Flush every test cock before you connect anything. Debris under a test cock seat leaks, and grit carried into the gauge ruins readings. The 10th Edition RP flushing sequence is deliberate: open No. 4, then No. 3, then No. 2, then No. 1, and only then close them in the order 1, 2, 3, 4. Keeping water moving through the assembly is what prevents the differential relief valve from discharging before you have recorded its opening point.

Double Check Valve Assembly: The Standard Sequence

The DCVA test asks one question of each check: is it tight against reverse flow? The accepted logic is that a check holding 1.0 psid or greater in the normal direction of flow, with atmosphere downstream, will also hold tight in the reverse direction.

1. Notify the customer, identify the assembly, inspect it and observe the direction of flow.
2. Flush the test cocks and install adapters.
3. Set the gauge level with the centerline of the assembly, or level with the top of the water in the sight tube when test cock No. 3 sits below the check valve body.
4. Connect the high-side hose to the test cock upstream of the check being tested; open it and bleed the hose free of air.
5. Close shutoff No. 2, then shutoff No. 1.
6. Open the test cock downstream of that check to drop that side to atmospheric pressure.
7. When flow stops and the needle settles, record the static pressure drop.

Pass criterion: 1.0 psid or greater, needle steady. A needle that falls to 0.0 means the check leaked. Repeat for the second check, then restore service by opening shutoff No. 1 slowly, then shutoff No. 2.

One ordering rule matters: if check No. 1 reads less than 1.0 psid and shutoff No. 1 is leaking, repair check No. 1 and start over. A first check that will not hold lets the trapped zone water bleed backward out the open test cock, so the second-check reading you get is fiction.

Reduced Pressure Principle Assembly: Tests 1, 2 and 3

Test No. 1 — Relief valve opening point

The differential relief valve must hold the zone at least 2 psi below supply pressure, so the practical pass criterion is an opening point of 2.0 psid or greater. Record it before you exercise the relief valve. Popping a sticky relief valve first loosens it and produces an artificially high reading — and real backflow never exercises the relief valve as a courtesy first.

Test No. 2 — Check valve No. 1 tightness

This is the one criterion that genuinely varies, so know both versions:

Worked example. The relief valve opens at 2.4 psid and check No. 1 reads 5.2 psid.

• 9th Edition: required minimum = 2.4 + 3.0 = 5.4 psid. 5.2 < 5.4, so it fails.
• 10th Edition: required minimum = 5.0 psid and greater than 2.4 psid. 5.2 satisfies both, so it passes.

Same assembly, same readings, opposite verdicts. Answer exam questions using the criterion the question names; where none is named, the 10th Edition 5.0 psid rule is the current national standard.

There is no published maximum for check No. 1, but a reading in the mid-teens on a large assembly is a red flag: add the second check's minimum 1.0 psid plus the assembly's other losses and total pressure loss can exceed what that size assembly is rated for, which points to a binding or wrongly sprung first check.

Test No. 3 — Check valve No. 2 tightness

Check No. 2 must simply hold tight. Using a direction-of-flow test, it must hold 1.0 psid or greater with the downstream side dropped to atmosphere. A gauge reading that climbs instead of holding indicates backpressure past a leaking No. 2 shutoff.

Pressure Vacuum Breaker and Spill-Resistant Vacuum Breaker

Remove the canopy, flush both test cocks, and set the gauge at the elevation of the assembly.

• Air inlet opening point: high hose to test cock No. 2, close shutoff No. 2 then shutoff No. 1, then crack the high-side bleed no more than a quarter turn. Record the reading the moment the air inlet pops — it must be 1.0 psid or greater. Then pull the hose, let the body drain, and confirm visually that the air inlet opens fully.
• Check valve: high hose to test cock No. 1, close shutoff No. 1, open test cock No. 2 to drop the downstream side to atmosphere. When flow stops or is no more than a drip, record the reading — it must be 1.0 psid or greater.

An SVB uses the same two numbers: check valve drip-tight at 1.0 psid or greater, air inlet opening at 1.0 psid or greater and opening fully. If the gauge simply will not drop and the air inlet never opens, suspect a leaking No. 1 shutoff. If it drops straight to 0.0 with no pop, suspect a disc stuck to its seat, a broken or missing air-inlet spring, or an old non-loaded device that is not a true PVB.

Diagnosing Failures Before You Condemn the Assembly

Two errors invalidate more field tests than worn rubber does: trapped air and gauge elevation. Water weighs 0.433 psi per foot of head, so a gauge held below the water level it is sensing reads high by 0.433 psi for every foot of error. Hold the gauge 2.3 feet too low and you add 2.3 x 0.433 = 1.0 psi of pure fiction — exactly enough to turn a 0.6 psid failing check into a 1.6 psid "pass." Always hold the gauge level with the centerline of the assembly, or level with the top of the water in the sight tube.

Failure, Repair, Retest and Reporting

When an assembly fails, the sequence is repair, retest, report — never "note it and move on." Clean or replace discs, seats, springs and rubber, then retest the entire assembly starting from Test No. 1, because a repair to one component changes the pressures the others see. Record both the initial readings and the after-repair readings; purveyor report forms have separate fields for each. A high-hazard connection that cannot be repaired the same day must not be left in service.

A complete test report carries the assembly make, model, size and serial number; its location; the initial reading for every component; repairs performed; the after-repair readings; line pressure; the gauge make, serial number and last calibration or accuracy-check date; and the tester's name, license number, certification number and signature. Give one copy to the owner, submit one to the water purveyor under its cross-connection control program, and keep one for your own records.

Frequency comes from the code, not from the customer. IPC 2018 Section 312.10.2 requires reduced pressure principle, double check, pressure vacuum breaker, reduced pressure detector fire protection, double check detector fire protection and spill-resistant vacuum breaker assemblies to be tested at the time of installation, immediately after repairs or relocation, and at least annually. Section 312.10.1 requires annual inspections of assemblies and air gaps to confirm they are operable and that the air gaps still exist.

Exam Trap: 2.0 psid belongs to the relief valve, not to any check valve. Distractors offer "2.0 psid" as the double check criterion (it is 1.0 psid) or as the RP first-check criterion (it is 5.0 psid under the 10th Edition). A second favorite distractor is "0.5 psid," which is below every published minimum, and a third is applying the RP's 5.0 psid figure to a PVB check valve, which is judged at 1.0 psid.