So the issue of "forward-flow testing" of the backflow preventers has come up a lot as of late, at least here in Massachusetts. More and more design drawings are calling for "provisions" to forward-flow test the backflow preventer. I've seen multiple ways of doing this and I'm curious as to which method is best.
One method I see is adding a bypass around the FDC check with a normally-closed butterfly valve in it. This allows you to open the butterfly valve when you want to test the backflow preventer and flush through the FDC piping.
The other method I see is to extend the manifold and install a couple hose valves to use in testing.
My personal opinion is that if you have the ability to run the hoses outside, use the second method. The reason for this is that if each zone has a control valve, you can shut off the sprinkler system while flushing and not introduce any of the debris in the underground into your system. By doing it the other way with the bypass, you have to mess with the FDC, and if the FDC is fed from anywhere on the system other than the end of the manifold, you can introduce debris into your system.
Is this a better approach?
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