Backflow is when used or non-potable water reverses direction and is pulled back into the clean water supply through a 'cross-connection' — a contamination risk that backflow preventers exist to stop. Here is how it works.
Written bySteven Lockhart·Reviewed byMiguel Lopez, Owner & Licensed Plumber (CSLB #974600)·Last reviewedJune 2026 · 4 min read
Quick Answer
The backflow problem is contamination: backflow is the unwanted reversal of water flow, which can draw non-potable water (from irrigation, boilers, or other sources) back into the clean drinking-water supply through a cross-connection — any point where the two could meet. Backflow preventers are the devices that stop this, and they require periodic testing to confirm they still work. This is part of our complete guide to backflow testing for West Los Angeles businesses.
A cross-connection in plumbing is any actual or potential link between the potable (drinking) water supply and a source of contamination — an irrigation system, a boiler, a commercial dishwasher, a chemical feed, or a hose left in a bucket. Normally water flows one way, out to fixtures. But if pressure drops or reverses (backsiphonage or backpressure), water can flow backward through that cross-connection, pulling contaminants into the clean supply. That reversal is backflow. The U.S. EPA maintains guidance on cross-connection control. This guide is part of our complete resource on backflow testing for West Los Angeles businesses.
A cross-connection is any point where the potable water supply could connect to a non-potable source, creating the path through which backflow can contaminate clean water.
Backflow contamination prevention relies on backflow preventer assemblies installed at the cross-connection — devices (such as reduced-pressure or double-check assemblies) that allow water to flow only one way and block any reversal. The right type depends on the hazard level. The table summarizes common sources and the safeguard.
| Cross-connection source | Risk | Safeguard |
|---|---|---|
| Irrigation/sprinklers | Fertilizer, soil contaminants | Backflow preventer |
| Boilers/heating | Treated, non-potable water | Backflow preventer |
| Commercial equipment | Chemicals, process water | Assembly + testing |
| Fire suppression | Stagnant system water | Backflow preventer |
A backflow preventer leaking is a common backflow issue, and the fix depends on the cause. Some discharge from a relief valve is normal on certain assemblies; continuous leaking, however, usually means worn internal parts (check valves, relief valve, seals) that need rebuilding or replacement, or pressure conditions that need addressing. Because a failed assembly leaves the cross-connection unprotected, a backflow issue fix is not just about stopping a drip — it is about restoring the contamination barrier. We test the assembly, identify the fault, and repair or rebuild it so it protects the supply again.
A property’s backflow assembly is dripping continuously and the owner is unsure if it still protects the water.
New Pro tests the assembly, finds worn check valves, and rebuilds it with the correct repair kit.
The leak stops, the assembly passes testing, and the cross-connection is protected again — with documentation for the water purveyor.
The Bottom Line
Backflow is a contamination risk wherever a cross-connection links clean water to a non-potable source, and backflow preventers are the barrier that stops it. A leaking or failed assembly leaves that barrier down, so it needs prompt, certified testing and repair. We handle both. Call any time. More on our backflow testing page.
Backflow is the unwanted reversal of water flow, where used or non-potable water is pulled back into the clean drinking-water supply. It happens through a cross-connection when pressure drops (backsiphonage) or reverses (backpressure), and it is a contamination risk.
Any actual or potential link between the potable water supply and a source of contamination — irrigation, a boiler, commercial equipment, a chemical feed, even a hose in a bucket. It is the path through which backflow can contaminate clean water.
With backflow preventer assemblies installed at the cross-connection — devices that allow water to flow only one way and block any reversal. The required type depends on the hazard level, and the assemblies must be tested periodically to confirm they work.
Some assemblies discharge briefly from a relief valve normally, but continuous leaking usually means worn internal parts (check valves, relief valve, seals) needing a rebuild, or pressure conditions to address. A failed assembly leaves the cross-connection unprotected, so it should be checked.
It can be — it risks pulling contaminants (fertilizers, chemicals, stagnant water) into the drinking supply. That is why backflow preventers are required at cross-connections and why they must be tested, to keep the contamination barrier working.
Properties with cross-connection hazards — commonly commercial and industrial sites, irrigation systems, fire-suppression systems, and similar. Your water purveyor identifies which connections require an assembly and testing.
Written by the New Pro Plumbing content team and reviewed for technical accuracy by Miguel Lopez, owner and California-licensed plumber (CSLB #974600). Pipe lifespans are presented as typical ranges, not guarantees, and any pricing is confirmed in writing before work begins.
What's new: June 2026 — first publication.
We inspect, test, and repair backflow assemblies and cross-connection hazards — certified. Call any time.
