Quick Answer:

If you suspect a cross-connection because water is moving the wrong way through a backflow preventer, stop use and shut off the supply immediately. Perform basic checks you can do safely, then arrange for a certified tester to inspect and repair the device.

Why This Happens

Backflow devices are meant to stop contaminated water from flowing back into your clean supply. Reverse flow can happen when a check valve or relief valve fails, when isolation valves are left open during pressure changes, or when a cross-connection exists elsewhere on the system. Pressure shifts from hydrants, pumps, or nearby plumbing work can expose weaknesses in the preventer and let contaminated water move backward.

Step-by-Step What to Do

1. Shut the supply

• Close the isolation valve feeding the affected line or the main shutoff for the property. This stops any more contaminated water entering the system.
• Do this calmly and quickly. If you can’t find the valve, turn off water at the street if possible and safe.

2. Check visible valves and settings

• Look for obvious signs of failure: broken parts, heavy leaks, or debris around the preventer housing.
• Do not dismantle the device. Note any unusual sounds or water movement to tell the tester later.

3. Test check valves (basic observation)

• If the system has accessible test cocks and you are comfortable, close the upstream and downstream isolation valves and open the test cocks to see if pressure holds. If pressure drops or flow reverses, a check valve may be failing.
• Only perform this if you are confident and have basic tools. Otherwise, skip to step 4.

4. Isolate and protect connected fixtures

• Shut off any downstream equipment or fixtures that could draw contaminated water into appliances, irrigation, or domestic plumbing.
• Label or tag the shut valves so others don’t turn them back on by mistake.

5. Arrange professional testing

• Contact a certified backflow tester to perform an authorized test and repair. A professional will do a correct bench or field check and provide certification.
• Make notes of what you observed and share them with the tester. If you need guidance on symptoms, see How to tell if backflow preventer failed.

What Not to Do

• Do not continue using water. Continuing to use faucets, irrigation, or appliances can pull contaminated water into the safe supply.
• Do not try to bypass or jury-rig the preventer. Temporary fixes can make contamination worse and may be illegal in many areas.
• Do not attempt major repairs or disassembly unless you are trained and certified to work on backflow devices.

When to Call a Professional

Call a certified backflow tester if you’ve shut the supply and basic observations suggest the device isn’t holding pressure, if there is visible damage, or if you have any doubt about contamination. If the preventer appears to be stuck or cycling incorrectly, get help right away—especially if you see signs that the backflow preventer won’t reset.

Safety Notes

• Stop water use immediately and minimize any flow until the device is tested and cleared.
• Wear gloves and eye protection if you must inspect near leaks. Avoid contact with standing water that could be contaminated.
• Keep a clear path to your main shutoff and label valves for emergency responders or household members.
• Follow local regulations for reporting suspected contamination events—your water provider may need to know.

Common Homeowner Questions

• How long can I wait for a tester? Arrange a tester as soon as possible; don’t wait more than a day if contamination is likely.
• Can I flush the lines myself? No—flushing can spread contamination. Wait for professional instructions.
• Will the tester provide certification? Yes, a certified tester will provide test results and any required paperwork for compliance.