Mailport: June 2012
In reference to your “Avoid Plugging with Proper Vent Installation” sidebar and photos in the March 2012 issue: If you’re going to backflush the vent, I think it will take more than just a shutoff valve on the filter outlet, because shutting this valve will not allow the backflush water to return to the tank. It would take a filter bypass arrangement, also with a valve, for this to work. In normal operation, the bypass valve would be shut and the filter valve open. For backflushing, the bypass would be opened and the filter shut.
In the installation described and pictured, there is a bypass loop, incorporating a water-filled trap as an odor seal. A shutoff valve on the filter outlet (the end connected to the through hull) prevents water from entering the filter, forcing it through the bypass, just as you suggest. We didn’t feel the design required valves on both ends of the filter, since with one end sealed, trapped air prevents water from entering the filter; however, placing a valve at both ends would be a safe and conservative approach. In our design, the bypass is always open, sealed only by a water-filled trap.
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