Pressure Vacuum
Breaker Assembly.

Construction

A Pressure Vacuum Breaker contains a single spring-loaded check on the main flow path and a separately-spring-loaded air-inlet poppet that vents the downstream side to atmosphere when supply pressure drops within about 1 psi of zero. Two test cocks let the tester verify (a) that the air inlet opens above 1.0 psid below the check, and (b) that the check itself holds at ≥ 1.0 psid. The assembly is sized for back-siphonage only; it cannot stop back-pressure because the air inlet vents downward when supply pressure exceeds atmospheric.¹

Why a PVB is never for back-pressure

This is the question we are asked most often by customers: "If the air vent is on top of the assembly, why can't it stop back-pressure from below?" The answer is that the air vent is held closed by the spring whenever supply pressure exceeds about 1 psi gauge. If the downstream side becomes pressurized above the supply, the supply pressure no longer drops, the air vent never opens, and water can be pushed back through a leaking check. The assembly's geometry only protects against the case where supply pressure falls toward zero, which is back-siphonage. The most common error we correct in the field is a PVB installed downstream of an irrigation booster pump, where back-pressure from the booster makes the entire installation non-compliant. The fix is to relocate the PVB upstream of the booster, or to replace it with an RP. The USC FCCCHR Manual §3.4 is explicit about this; we cite it on the rejection notice.

Test sequence (ASSE 5110)

Step	Action	Read	Acceptance
1	Close SOV-2. Bleed supply slowly via TC#1.	p1 falling	air inlet opens at ≥ 1.0 psid
2	Read CK differential at TC#2 vs TC#1.	p1 − p2	≥ 1.0 psid, tight

Milwaukee-area irrigation rules

Commercial irrigation in Milwaukee follows three local rules in addition to SPS 382:

• Drain-down by 1 December.

  A PVB freeze-cracks at 28 °F if left wet. We schedule winterization between October 15 and November 30 for accounts on our books.

• Fertigation = RP, not PVB.

  If the irrigation system has any chemical injection — fertilizer, surfactant, pesticide — the protection is high-hazard and a PVB is non-compliant. RP only.

• Identification tag visible from grade.

  MWW's inspectors expect to see the tag without unscrewing the insulation cover. We re-tag during the spring start-up if last year's tag has weathered.

USC FCCCHR-approved PVB models we stock

Mfr / Model	Sizes	USC list #
Febco 765	1/2″ – 2″	20-021
Watts 800M4 / 800M4QT	1/2″ – 2″	20-033
Apollo 4SG-200	3/4″ – 2″	20-046
Wilkins 720A	3/4″ – 2″	20-018
Conbraco 40-700	3/4″ – 2″	20-012

Failure modes

From our 2025 books (n = 412 PVB tests):

• Air-inlet poppet stuck closed · 34%

  UV-degraded EPDM gasket. Replace poppet kit ($28 part).

• Check fouled · 24%

  Sediment from irrigation pond or grit from a sprinkler-head replacement.

• Body cracked from freeze · 18%

  Replacement. Customer skipped winterization.

• Wrong assembly (PVB where RP required) · 12%

  Fertigation added without re-evaluating protection.

• SOV worn · 8%

  Brass ball valve seat.

• Mount height non-compliant · 4%

  Sprinkler head installed higher than originally drawn; PVB now < 12″ above. Re-mount or replace.

References