Back Flow Preventer Constantly Dumping Water

[SelfServe24/7]

Hi everyone,

I recently purchased a self-serve car wash location, and the previous owner had a new backflow preventer installed in October 2025. A few weeks ago, the seal failed and was replaced by a plumber.

Since operating the location over the past few weeks, I have noticed that the backflow preventer is consistently discharging water onto the floor. The busier the wash is, the more water seems to be dumping. I would expect an occasional discharge, but I would not think it should be happening constantly or leaving water on the floor every day.

After looking into it further, I noticed that when the main water tank float oscillates, the backflow preventer appears to discharge water in the same pattern. The float itself seems to be working, but I am trying to determine whether this behavior is normal or if not what could be the issue? How would you track down the problem?

Has anyone seen this before at a self-serve wash? Any advice on what I should have checked first would be greatly appreciated!!!

 

[soapy]

Backflow preventers do this at all washes. I have a small bucket wired under all of mine and a small drain line installed to route the water to a floor drain.

 

[Greg Pack]

Mine are usually dry during normal operation. Any idea what your incoming pressure is before and after the PRV?

 

[Eric H]

does the discharge only occur when the main float closes? I had to partially close the ball valves that feed the main tanks for my 2 razors at one location. It worked fine for years and then one day the Backflow preventer started leaking when the fill valve closed. I suspect something changed with the city water pressure that was suddenly causing some water hammer.
You could install some “hammer arrestors” to reduce the hammer effect. Throttling back the ball valves solved my problem and didn’t cause the tanks to fill too slowly so I just did that and took no further action

 

[Randy]

Back in the day when I was a young man I was a state certified Backflow tester. It shouldn't be leaking. What brand is the Backflow. The one I've found that is the best is the Zurn Wilkins 975, easy to work on. When was the last time you had it inspected and tested? What you can do is open up the check valves and inspect the plunger for wear and debris, on most Backflows you can flip over the check valve diaphragm. Remove the large diaphragm inspect it for debris, tears, inspect the surface of the seating surface.

 

[WashSingh]

Thank you all for the insight and suggestions. I was able to gather a bit more information today that may help narrow down the issue.

The RP currently installed is a Wilkins 1-1/4" Model 975XL2. It was installed in October 2025, and the internal seals were replaced approximately 6 months ago.

In addition to the consistent discharge of water from the RP onto the floor, I observed a very concerning condition today. As the float valve in my main water tank approached the fully closed position, the RP began dumping water and the piping throughout the machine room started vibrating heavily. The pipes were shaking and knocking quite aggressively.

The only way to stop the vibration was to either:

1. Activate a function that consumed water, or
2. Manually open a hose bib in the machine room.

Once I opened the hose bib, the vibration immediately stopped and the RP stopped discharging. However, once I closed the hose bib, it was only a matter of time before the vibration, knocking, and RP discharge started all over again.

I also tried throttling several ball valves to approximately 50% open in hopes of reducing the issue, but it did not seem to make much difference.

A few additional notes:

• I do not currently know the incoming city water pressure.
• There is no pressure reducing valve (PRV) installed between the incoming water service and the main water tank.
• There is a PRV installed downstream for the lower-pressure functions (presoak, engine cleaner, etc.).
• The issue seems to be most noticeable when the tank float is nearly closed and water demand is low.

At this point, I'm starting to wonder if I'm dealing with excessive incoming pressure, water hammer, a failing float valve, an RP issue, or some combination of the three.

Any thoughts or recommendations would be greatly appreciated. Thank you all again for your help and support!