A 1 1/2 inch backflow preventer is a plumbing assembly with 1 1/2-inch inlet and outlet connections that stops contaminated water from siphoning backward into the potable supply. Double-check and reduced-pressure-zone types are the common designs.
For the full breakdown, see our best 1 1/2 Backflow Preventer guide.
One failed check valve can pull irrigation runoff, boiler water, or fire-sprinkler stagnation into a building’s drinking lines. The two families solve the same problem differently, and picking the wrong one is a code problem, not just a preference.
Double-Check vs. Reduced-Pressure Zone: What Decides The Choice?
The hazard level downstream decides which assembly you install. A double-check valve suits non-health-hazard applications — backflow would be unpleasant but not dangerous — while a reduced-pressure-zone assembly handles higher-risk jobs.
Watts describes the 1 1/2-inch double-check assembly as preventing reverse flow of contaminated non-potable water from backsiphonage or backpressure. A double-check relies on two spring-loaded check valves with no atmospheric vent, so it’s simpler with fewer parts to service.
Match the assembly to the hazard classification your local authority assigns, not to price or convenience. Size alone — 1 1/2 inch in and out — tells you nothing about which type you need.
| Assembly Type | How It Protects | Matches Systems Where |
|---|---|---|
| Double-check (Watts LF719-QT, 1 1/2 in) | Blocks reverse flow from backsiphonage or backpressure | Non-health-hazard service line entrances |
| Reduced-pressure zone (Watts LF909M1-QT-FS, 1 1/2 in) | Air-in, water-out relief action under emergency conditions | Higher-hazard lines needing atmospheric venting |
| Relief valve discharge | Dumps water instead of letting it cross over | Only RPZ designs — never plumbed solid to a drain |
| Strainer upstream | Keeps debris from fouling the check seats | Lines with grit, scale, or new-pipe debris |
| Elevation above flood level | Prevents submersion and contamination of the device | At least 12 inches above surrounding flood level |
How Is a 1 1/2 Inch Backflow Preventer Installed and Commissioned?
A qualified, licensed installer handles the job, and sequence matters as much as parts.
Commissioning follows a fixed order. Start with both shutoff valves closed, then slowly open the inlet shutoff valve until fully pressurized. The relief valve may discharge briefly — normal, and it should stop once pressurized. If it keeps running, the relief or checks need service before use.
Uneven torque causes weeping gaskets and callbacks.
What Goes Wrong Most Often?
Drainage errors and elevation mistakes cause most failures. The Watts manual warns against solid-piping the relief valve to a floor drain, sewer, or sump — that defeats the relief port and can let contamination travel backward through the drain.
- No air gap under the relief valve, so discharge can’t be seen or safely received.
- Skipped flushing, leaving debris to chew up check seats and cause nuisance discharge.
- Wrong environment — submerged, buried, or below flood level, all ruled out by the manual.
- Mismatched specs — pressure, size, and connection details must match exactly, even when nominal size is right.
Installation should be performed by qualified, licensed personnel per the same documentation. No verified manufacturer price was available, so budget by quoting your supplier rather than trusting a scraped listing number.
FAQs
Does a backflow preventer need annual testing?
Your local water authority sets the interval and keeps records. The test confirms checks and relief valve still hold; a failed test typically triggers a repair or replacement order with a deadline.
Can a double-check assembly be swapped for an RPZ?
Only if the hazard classification allows it — and usually it doesn’t work in that direction. RPZ assemblies serve higher-hazard lines, so replacing one with a double-check lowers protection below code. Going from double-check to RPZ is sometimes permitted but adds discharge routing and elevation requirements.
What size line does a 1 1/2 inch assembly fit?
That doesn’t guarantee it suits the system — flow rate, pressure, and thread type must match before ordering.
References & Sources
- Watts. “LF719-QT 1 1/2 Double Check Valve Assembly” Confirms 1 1/2 IN inlet and outlet connections and reverse-flow protection detail.
- Watts. “LF909M1-QT-FS 1 1/2 Reduced Pressure Zone Assembly” Source for the air-in, water-out emergency protection principle.
- Watts. “Series 909 / LF909 Reduced Pressure Zone Assembly Installation Instructions” Installation, flushing, elevation, drainage, and commissioning steps.