A backflow preventer is working properly when water flows freely in the intended direction and no water or contamination can flow backward into your potable supply under any pressure condition. The challenge is that backflow preventers don’t fail loudly — when they stop doing their job, you usually won’t notice until a test fails or, in a worst-case scenario, contaminated water enters your supply. That’s why annual testing by a certified backflow tester is required by code in Colorado for most commercial and irrigation applications, and it’s also why knowing the observable warning signs matters.

What a Backflow Preventer Does

Backflow occurs when water flows in the wrong direction through a cross-connection — a point where potable water meets a potentially contaminated source. This can happen during a pressure drop or back-siphonage event: if the city main pressure drops suddenly while your irrigation system has fertilizer injected into its lines, that contaminated water can be drawn backward. A backflow preventer — whether a pressure vacuum breaker (PVB), reduced pressure zone (RPZ) assembly, or double check valve — uses internal check valves and, in some cases, a relief chamber to ensure water only flows one direction.

In Colorado, irrigation systems connected to the potable water supply are legally required to have approved backflow prevention. The type required depends on the degree of hazard at the cross-connection. A PVB is common for residential irrigation with no fertilizer injection; an RPZ is required for higher-hazard situations, including systems with chemical injection or submerged outlets.

Observable Signs of a Failing Backflow Preventer

  • Dripping or continuous water discharge from the relief valve port: On a PVB, a small weep hole on top of the device should be dry during normal operation. If water is dripping from it continuously when the system is pressurized, the internal seal is failing. On an RPZ, water discharging from the relief valve port between the two check valves indicates the second check valve has failed or the differential pressure relief valve is malfunctioning.
  • Reduced water pressure at downstream outlets: Significant debris or scale buildup inside the device can restrict flow. This is especially common on older devices in Denver’s hard-water environment.
  • Visible corrosion, cracks, or physical damage to the housing: Colorado’s freeze-thaw cycles are hard on outdoor plumbing. A PVB installed above grade is particularly vulnerable to freeze damage if it wasn’t properly drained or insulated before winter. A cracked housing means the device is compromised.
  • Failed annual test: In Colorado, certified backflow testers use a differential pressure gauge to verify that the device’s internal check valves are holding within required tolerances. A failed test report is the definitive answer — the device needs repair or replacement.

Why Annual Testing Is Required — and What It Involves

Colorado Regulation 11 (the state’s cross-connection control regulation) requires that testable backflow preventers be tested annually by a certified backflow prevention assembly tester. The test is not complex — a trained tester connects a differential pressure gauge kit to test cocks on the device and cycles through a series of valve positions to verify that each check valve holds its rated pressure differential and that relief valves open and close correctly.

The results are reported to the local water utility. If a device fails, the property owner is typically notified and required to repair or replace it within a specified timeframe. Failing to maintain a tested and approved backflow preventer can result in your irrigation system being disconnected from the water supply.

What Makes Backflow Preventers Fail

The most common failure modes are worn or deformed internal seals and check valve discs — these are rubber components that degrade over time from water chemistry, temperature cycling, and normal wear. In Denver, hard water mineral scale can deposit on check valve seats and prevent them from sealing fully. Debris from the water supply — sand, pipe scale, or sediment — can also lodge in the valve seats and hold them open. Freeze damage to the brass housing or internal components is another common failure point on devices that weren’t winterized.

Repair vs. Replace

Many backflow preventers can be repaired by replacing the internal rubber components — check discs, O-rings, and relief valve seats. Rebuild kits are available for most common device models and are significantly less expensive than full replacement. However, if the device is old, the brass body is corroded or cracked, or the device has failed multiple consecutive tests, replacement is the better long-term investment. A licensed plumber can assess whether a repair or replacement makes more sense for your specific device.

Done’s plumbing team tests, repairs, and replaces backflow preventers across the Denver metro. Learn more on our pressure and backflow prevention page, or visit our plumbing services page for the full range of what we do for Front Range homeowners.