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BLD-PLUM06-08 · Lesson 5 · 14 clock hours · lab

Atmospheric Vacuum Breakers and Pressure Vacuum Breakers: Installation Lab

Objective. The learner installs an atmospheric vacuum breaker and a pressure vacuum breaker on a training rig in compliance with IPC/UPC minimum air gap and elevation requirements, verified by instructor inspection.

Atmospheric Vacuum Breakers and Pressure Vacuum Breakers: Installation Lab

Why Vacuum Breakers Matter

A cross-connection is any physical link between a potable water supply and a non-potable source. When supply pressure drops—during a water-main break, for example—backflow can pull contaminated water backward into the drinking supply. Vacuum breakers interrupt that reverse flow by admitting air into the pipe, breaking the siphon before contamination can travel upstream. Two of the most common field-installed devices are the atmospheric vacuum breaker (AVB) and the pressure vacuum breaker (PVB).

Atmospheric Vacuum Breaker (AVB)

An AVB is the simpler of the two. It contains a float check that rides up when water flows forward, sealing the air port. When flow stops and pressure drops, the float falls, opening the air port and venting the downstream side to atmosphere. This breaks any siphon immediately.

Critical installation rules for an AVB:

Pressure Vacuum Breaker (PVB)

A PVB is a more robust device suited for systems that may see continuous supply pressure on the downstream side (such as irrigation systems with zone valves). It contains a spring-loaded check valve and an independently operating air-inlet valve. If inlet pressure drops, the air-inlet opens regardless of downstream conditions.

Critical installation rules for a PVB:

Thread Sealant Application

Both devices connect via threaded fittings. Apply PTFE (polytetrafluoroethylene) tape or pipe-thread compound (pipe dope) correctly:

1. Clean and dry male threads before applying sealant.

2. Wrap PTFE tape clockwise (when facing the male end) so the tape tightens as the fitting is turned in.

3. Apply two to three wraps, pulling the tape snugly into the thread form.

4. Pipe dope may be used instead of or in addition to tape; apply a thin, even coat to male threads only, avoiding the first one or two threads to keep compound out of the waterway.

5. Hand-tighten first, then snug with a pipe wrench or strap wrench—do not over-torque plastic bodies.

Pressure Testing

After installation, perform a low-pressure leak check before putting the system into service:

1. Close all downstream outlets.

2. Slowly open the supply shut-off to fill the assembly.

3. Observe the pressure gauge and all joints for at least two minutes.

4. Any visible drip, seep, or gauge drop indicates a leak; depressurize, correct the fitting, and retest.

Safety During This Lab

Inspection Simulation

When you swap stations and inspect your partner's work, use the written checklist systematically. For each item, record the observed condition and cite the applicable code section in IPC or UPC. Common violations to look for include:

Documenting violations precisely—with the code reference—mirrors exactly what a licensed inspector does on a jobsite and prepares you for the ASSE backflow certification field evaluation.

Key terms9

Atmospheric Vacuum Breaker (AVB)A backflow prevention device that uses a floating disc to seal during forward flow and opens an air port to break siphon when pressure drops; must be installed at least six inches above the flood-level rim of the highest outlet served and never downstream of a shut-off valve.
Pressure Vacuum Breaker (PVB)A backflow prevention device with a spring-loaded check valve and an independent air-inlet valve, rated for continuous supply pressure and for use with downstream shut-off valves; must be installed at least twelve inches above the highest point of the downstream system.
Flood-level rimThe top edge of a fixture or receptor over which water would overflow if the drain were blocked; used as the reference point for calculating required elevation of vacuum breakers.
Cross-connectionAny physical link or arrangement between a potable water supply and a source of contamination that could allow non-potable substances to enter the drinking water system.
BackflowThe undesirable reversal of water flow in a plumbing system, moving from a non-potable source back toward the potable supply; caused by back-siphonage or back-pressure.
Back-siphonageA form of backflow caused by negative pressure (partial vacuum) in the supply piping that draws downstream water backward into the potable supply.
PTFE tapePolytetrafluoroethylene tape used as a thread sealant on male pipe threads; applied clockwise (facing the male end) in two to three snug wraps before assembly.
Test cockA small valve port built into a backflow preventer assembly that allows a tester to connect a differential pressure gauge kit and verify the device is operating within specification.
Authority Having Jurisdiction (AHJ)The organization, office, or individual responsible for enforcing a code or standard in a given location; local AHJ requirements may be more stringent than the base code.

Worked examples3

1. You are installing an AVB on a hose-bib supply that serves a utility sink. The flood-level rim of the sink is 36 inches above the finished floor. What is the minimum height above the finished floor at which the AVB body must be installed?
  1. Identify the reference point: the flood-level rim of the highest outlet served = 36 inches above finished floor.
  2. Recall the minimum required elevation above the flood-level rim for an AVB = 6 inches.
  3. Add the two values: 36 inches + 6 inches = 42 inches above finished floor.
Answer: The AVB body must be installed at a minimum of 42 inches above the finished floor.
2. You are installing a PVB to protect an irrigation system. The highest sprinkler head in the zone is located 18 inches above the finished grade at the meter. What is the minimum height above finished grade at which the PVB must be installed?
  1. Identify the reference point: the highest point in the downstream system = 18 inches above finished grade.
  2. Recall the minimum required elevation above the highest downstream point for a PVB = 12 inches.
  3. Add the two values: 18 inches + 12 inches = 30 inches above finished grade.
Answer: The PVB must be installed at a minimum of 30 inches above finished grade.
3. During a pressure test you observe the gauge reads 45 psi at the start of a 2-minute hold. At the end of 2 minutes the gauge reads 43 psi. The joints appear dry and no dripping is visible. What should you conclude and what is your next step?
  1. A 2 psi drop in 2 minutes on a static low-pressure test indicates the system is not holding pressure—this is a failed test even without visible dripping, because a small internal leak or unseated check may not produce visible water at the exterior joints.
  2. Depressurize the assembly by slowly opening a downstream outlet or the test cock until pressure reaches zero.
  3. Inspect each joint and the device body for the source: apply soapy water to joints and re-pressurize briefly to locate bubbles, or disassemble and inspect thread engagement and sealant.
  4. Correct the defect (re-tape, re-tighten, or replace a damaged component) and repeat the full pressure test.
Answer: The test fails; depressurize, locate the leak source, correct it, and re-test until the gauge holds steady for the full 2-minute period.
In class: Instructor demo: install an AVB on a hose-bib supply, explaining the minimum required elevation above the flood-level rim and the prohibition on installation downstream of a shut-off valve · Learner bench exercise: each learner installs an AVB and a PVB on a pre-plumbed training rig, applies thread sealant correctly, and pressure-tests the assembly at low pressure to check for leaks · Inspection simulation: learners swap stations and complete a written inspection checklist on their partner's installation, noting any code violations and referencing the applicable IPC or UPC section
Materials: Training rigs with 3/4-inch and 1-inch supply lines; AVB and PVB assemblies (multiple sizes); pipe wrenches, strap wrenches; PTFE tape, pipe dope; pressure gauge; IPC and UPC code books; inspection checklist form
Check: Instructor inspection checklist: installation meets all code clearance and orientation requirements with zero critical defects; pressure test shows no leaks at working pressure; learner scores 80% or higher on written checklist review

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Backflow prevention device installation and code compliance
1. What is the minimum required elevation of an AVB body above the flood-level rim of the highest outlet it serves?
Backflow prevention principles and device limitations
2. Why is it a code violation to install a shut-off valve downstream of an atmospheric vacuum breaker?
Backflow prevention device installation and code compliance
3. An irrigation PVB must be installed at a minimum of how many inches above the highest point of the downstream piping system it protects?
Pipe joining materials and methods
4. When applying PTFE tape to a male threaded fitting, in which direction should the tape be wrapped?
Backflow prevention device installation and code compliance
5. During the inspection simulation, you observe your partner's AVB is installed horizontally rather than vertically. Which consequence is most directly associated with this error?
Occupational safety in plumbing operations
6. Which of the following best describes the correct minimum personal protective equipment (PPE) for a learner pressure-testing a training rig assembly?
Backflow prevention principles and device selection
7. A pressure vacuum breaker (PVB) differs from an atmospheric vacuum breaker (AVB) primarily because a PVB:
Backflow prevention device installation and code compliance
8. During an inspection simulation, you identify that your partner installed the AVB with its body only 4 inches above the flood-level rim of the sink it serves. On the inspection checklist, this finding is best classified as:
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