Problem with Backflow preventer

[slash007]

At one of my washes, my back flow preventer keeps randomly running water and I haven't been able to figure out why. I was wondering if anyone here had a similar experience. Over the years it would randomly run small amounts of water whenever my hot water tank was in operation. Whenever I had the tank off and would use cold water, it wouldn't run any water at all. I did have an issue a few weeks ago where it was constantly running water, but I installed a complete repair kit and that fixed the constant flow. Now it is back to just running some water randomly.

I did install a new water tank and put in an expansion tank. The water tank is set at 100 degrees and the expansion tank was filled to match the incoming city pressure.

Any ideas what might be causing my water flow?

Thanks.

 

[Kevin James]

When was the last time you had it tested? What kind/brand of back flow preventer is it?

 

[slash007]

I have it tested yearly. It's a Watts 975XL.

 

[soapy]

They are supposed to periodically flow water like that. When it does this it is equalizing pressure between the 2 sides.

 

[slash007]

I thought so, but I have a backflow device at 3 other places and they have never let out a drop. Maybe the pressure at this location is different?

 

[MEP001]

It should let water run whenever the pressure is lower on the incoming line than the outlet. It might be the supply pressure dropping.

 

[Stuart]

It should leak on reduced pressure on the inlet side - however the check valves inside should close and at least the second check should close and not let any more water out other than what is between the checks.
If it is constantly leaking there is either a minor issue with the checks or the inlet pressure is constantly fluctuating causing the constant leakage. I have had 6 locations for years with only one that had leakage intermittently and a new kit fixed it.

 

[slash007]

I am guessing that it has to do with something causing a pressure variation. It is not constantly running, but enough to keep the floor wet. What could be causing the variation on the outlet side?

 

[MEP001]

Put gauges on it and see if the variation is on the outlet or inlet side.

 

[Stuart]

There are many reasons for a variation---upline or downline of your wash maybe some business, or development, opened which requires a large amount of water at various times, remember that even fast food locations can use as much or more water than we use daily. With a business a fluction may coincide with breakfast, lunch, dinner and at break times.

There could be on the upline in the main, or in your line to the wash, a slow buildup of corrosion in the line which may hinder the flow. It developed so slowly that it was not noticeable. I really do not think the problem is after the RPZ. It is the RPZ or before. Could also be the meter itself causing the problem but I doubt it. These are just examples of a reduced pressure zone to cause the water drainage.