Usually, on a domestic line. Watts recommends a strainer ahead of its reduced pressure assemblies to keep debris off the check valves, a common reason assemblies drip or fail their tests. Watts also says not to add one on seldom-used emergency lines, where a clogged screen could block flow. The screen needs regular cleaning, or it costs pressure.
What the manufacturer recommends
Watts' installation manual for its Series 009 reduced pressure zone assemblies is direct: "Watts recommends that a strainer be installed ahead of the assembly to protect the internal components from unnecessary fouling." The manual for its LF909 assemblies says the same: "Install a strainer ahead of the backflow preventer to protect all internal components from unnecessary fouling."
Watts sells many of its assemblies with the strainer already attached; on the LF009, it's the S suffix in the model number.
Why debris matters so much
Look at a backflow troubleshooting table and debris shows up everywhere. In the Watts Series 009 manual, the first cause listed when a "Check valve fails to hold 1.0 PSID minimum" is "Debris on check disc sealing surface," and the fix is to disassemble and clean. In the LF909 manual, the first cause listed for a valve that "drips continually from the vent" is a "Fouled first check."
New work is a common source. The 009 manual says that after first installation, "a discharge from the relief valve opening may occur due to inadequate initial flushing of pipe lines to eliminate dirt and pipe compounds." FEBCO's pressure vacuum breaker manual warns that failing to flush the lines "may cause the check members to become fouled and require disassembly and cleaning." A strainer doesn't replace flushing, but it keeps catching what flushing missed.
When not to add one
Both Watts manuals carve out the same exception. The LF909 manual says not to install a strainer ahead of a backflow preventer on "seldom-used, emergency water lines," because "The strainer mesh could potentially become clogged with debris present in the water and cause water blockage during an emergency." The 009 manual describes these as lines "called upon only during emergencies."
That reasoning is worth keeping in mind for any line where flow matters more than a clean check valve. On an everyday domestic service that runs all day, the trade-off favors the strainer.
The cost: a strainer needs care
A strainer only helps while it's clean. The LF909 troubleshooting table lists a "Fouled strainer" as a cause when a valve "exhibits high pressure drop," and the fix is "Clean strainer element or replace." In a building with flush valves, that lost pressure shows up at the fixtures first; our page on backflow preventers and low water pressure covers how to measure it.
The same table covers a valve that keeps fouling right after it's been serviced. The cause it gives is "Debris in pipeline is too fine to be trapped by strainer," and the fix is "Install finer mesh strainer element in the strainer."
The plumbing code doesn't require a strainer at a backflow assembly, but it shows how it expects strainers to be built where it does require one. Section 604.8 calls for a pressure-reducing valve "with strainer" where static pressure is over 80 psi, and Section 604.8.2 says these valves and strainers must allow "repair or removal of parts without breaking a pipeline or removing the valve and strainer from the pipeline."
Local rules
We checked Phoenix's 2024 IPC and UPC amendments (Ordinance G-7397), the backflow sections of Phoenix City Code Chapter 37 we cite on this site, and Mesa's backflow chapter. None of them requires or bans a strainer ahead of a domestic backflow assembly, so the manufacturer's instructions and your water provider are the guide. Phoenix does require a permit "for the installation of any backflow prevention assembly or for the modification of any plumbing system" (City Code 37-143), so ask before adding a strainer to an existing assembly's piping.
Not every failed test is debris
A strainer won't fix everything. The LF909 table lists other reasons a "Valve does not test properly," including "Manufacturer’s test procedure not followed" and a "Leaky downstream gate valve." It also lists excessive backpressure, freezing and water hammer as causes of a distorted second check. If an assembly fails year after year, look at the system around it as well as the parts inside.
A practical routine for a commercial building:
- 1Flush new or repaired lines before the assembly goes back in service.
- 2Add a strainer ahead of domestic assemblies where the manufacturer recommends one and your water provider allows it.
- 3Open and clean the strainer on a schedule, and at least at each annual test.
- 4Read pressure on both sides of the strainer and assembly if fixtures upstairs get weak.
HQ tests, repairs and replaces domestic backflow assemblies and can check the strainer on the same visit.
If your assembly already failed, see what happens after a backflow assembly fails its test. For a drip from the relief port, read why an RPZ backflow preventer spits water, and for the numbers on the test form, see how to read a backflow test report.