Quick Answer:
If a backflow preventer is failing you’ll usually see an unexpected pressure change on the downstream side, water discharging from the relief vent, reduced flow from fixtures, or water quality problems. A quick check of the relief vent behavior and a comparison of upstream and downstream pressure will tell you most of what you need to know. If you see steady discharge or big pressure differences, stop and call a professional.
Why This Happens
Backflow preventers are mechanical devices with springs and seals. Over time they can wear, get stuck by debris, or be damaged by freezing. If internal parts wear or seats fail, the device can allow water to move the wrong way or leak through a relief port. If it has been exposed to very low temperatures it may have froze and cracked, which quickly causes loss of function. Debris or worn springs can cause partial seating and create a situation of uncontrolled or reverse flow.
Step-by-Step What to Do
1. Observe the relief vent behavior
- Locate the relief vent or relief port on the backflow device. It normally stays dry.
- Watch it for several minutes while running a few fixtures downstream (e.g., a hose or indoor tap). Brief, occasional drips can be normal during pressure changes; steady flow, spurting, or constant discharge indicates a problem.
- Note the timing: if the vent discharges only when water is running elsewhere, that suggests internal leakage or pressure imbalance.
2. Compare upstream and downstream pressure
- Turn off nearby valves so one supply side is isolated, then attach a pressure gauge to the supply (upstream) side and another to the protected (downstream) side if you have two gauges or can measure separately.
- With normal operation the downstream pressure should closely follow the upstream pressure with small, predictable drops under flow. A large persistent pressure difference or downstream pressure higher than upstream indicates failure.
- If you don’t have gauges, you can still note obvious pressure loss at faucets: sudden low flow downstream while upstream feed remains strong is a warning.
- Disconnecting or attaching gauges should be done with the system depressurized and following the device maker’s directions; if unsure, skip this and call a pro.
3. Check for leaks and physical damage
- Look for water on the ground, rust, or cracked housings. A cracked body or frozen damage can let water bypass internal parts.
- Listen for continuous water noise when fixtures are off. That can indicate internal leakage.
4. Isolate and test fixtures
- Close the downstream shutoff and see if the relief vent stops discharging. If it does, the issue may be with downstream piping; if not, the device is likely at fault.
- Test multiple fixtures to rule out a single clogged line or fixture causing pressure oddities.
5. Document what you find
- Note times, pressures, and behaviors (steady drain, spurting, smell, taste changes). This helps a technician diagnose the cause faster.
What Not to Do
- Do not rely on visual only — some failures are internal and won’t show visible leaks.
- Do not remove covers or internal parts while the line is pressurized.
- Do not bypass or install a temporary jumper around the device to restore flow; that defeats the protection it offers and can contaminate your potable water.
- Do not use makeshift repairs with tape or sealants on a device body or ports.
When to Call a Professional
- Call a licensed plumber if you see steady discharge from the relief vent, large pressure differences, or suspected cracks. Those are signs the unit needs repair or replacement.
- Call immediately if the device was frozen, physically cracked, or if there are signs of contaminated backflow (dirty water entering potable lines).
- Ask a pro to test and certify the device if your area requires annual backflow testing — they have proper gauges and test procedures.
Safety Notes
- Turn off water at the nearest isolation valve before attaching gauges or opening any access panels. Depressurize the line first.
- Wear eye protection and gloves when working near leaking or contaminated water.
- If you suspect contamination of drinking water, stop using the supply and contact a professional and local health authority for guidance.
- If you’re uncomfortable with any step, stop and call a licensed plumber. Avoid DIY fixes that risk contamination or injury.
Common Homeowner Questions
- How quickly does a failed unit show signs? Often within minutes to hours: pressure loss or relief vent discharge are early signs.
- Can I temporarily fix it myself? You can isolate the downstream supply to stop damage, but don’t bypass the device; call a plumber for repairs.
- Is a failing backflow preventer dangerous? Yes — it can allow contaminated water into your potable supply, so treat suspected failures seriously.