Mountain Man Plumbing / Field guide Understand your water protection
Good water. One direction.
Backflow prevention.
See what protects your water.
Backflow is water moving backward toward the drinking-water supply. A drop in supply pressure can pull water back from a connected hose or system. Higher pressure on the equipment side can push it back. Backflow protection interrupts these paths before used water reaches the drinking supply.
01 / Explore a mechanical backflow device
Two checks.
One intended direction.
A check valve is an automatic one-way valve. A dual-check device has two in a row. Rotate this 3D cutaway, change the flow condition, and inspect the parts.
The 3D cutaway loads here. The part explanations below also work without 3D.
Drag to rotate · Select a part below · Teal arrows show forward flow; orange shows attempted return flow.
Forward pressure opens both checks.
Forward water pressure pushes each disc away from the ring it seals against, called the seat. The springs compress and water passes through both checks.
The body holds the working parts.
The housing contains the water passage and both checks. Open the cutaway to see how the discs, seats and springs fit inside it.
The first check is nearest the supply.
It opens toward the equipment when forward pressure is sufficient. Its spring returns the sealing disc to its seat when forward flow stops.
The second check adds another closure.
The second check has its own spring and sealing surface. It closes independently, providing another barrier to backward flow.
The seal meets the seat.
The dark sealing disc closes against a fixed ring called the seat. Debris or wear can keep that contact from sealing correctly, even when the outside looks fine.
Springs return the checks to closed.
Forward flow compresses each spring as the disc opens. When flow stops, the spring moves the disc back onto its seat. Pressure trying to push water backward also presses the disc against its seat.
This compact dual check is for non-health-hazard applications: return water could affect taste or appearance but would not introduce a health hazard. A connection exposed to chemicals or contaminated water needs protection rated for that hazard. Watts LF7 reference ↗
02 / Learn the device names
Same purpose.
Different mechanisms.
Vented dual checks
A vented dual check adds an atmospheric opening between its two checks. The Watts 9D is a non-health-hazard device used on suitable boiler fill connections. The boiler’s fluid, including any additives, affects the protection that connection needs.
See the device & its limits ↗RP / RPZ assemblies
These names refer to reduced-pressure backflow assemblies. They combine two checks with a pressure-operated relief valve between them. They have test connections and protect against both a backward push and a backward pull when correctly selected and installed.
See an RP assembly ↗Vacuum breakers
These admit air to interrupt a backward pull, called backsiphonage. Atmospheric and pressure vacuum breakers do not protect against backpressure—a push from the other side. The exact type determines its placement, pressure, and testing requirements.
See a pressure vacuum breaker ↗An air gap protects a supply through physical separation. Mechanical devices use internal components to control reverse flow. Identify the installed protection and the hazard it serves before selecting parts or a replacement.
03 / Find service or a certified tester
Device problem?
Tell us what you have.
We work on non-testable backflow devices and their plumbing connections. Send the make, model and a photo of the connection when arranging service. Testable reduced-pressure assemblies (RP/RPZ) and vacuum-breaker assemblies go to a backflow specialist.
Need a certified test?
We do not perform certified backflow assembly testing. Your water provider can tell you when your assembly needs a test, including after repairs, and how the results must be submitted. Use a currently certified Utah Backflow Assembly Tester for that work.
Further reading: Utah Backflow 101 · EPA cross-connection manual · All plumbing services.