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Unread 02/03/2008, 05:48 PM   #14
BeanAnimal
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Join Date: Feb 2003
Location: Pittsburgh
Posts: 20,772
Ahh good question!

The down turned elbows create a weir (exactly the same as your toilet trap).

When the return pump is turned on, the water in the box begins to rise. At some point water begins to flow over BOTH weirs. As long as the flow stays low it would continue to flow like this. The Open Channel standpipe works like a durso. It allows air to enter and prevents a siphon from forming. Simply put, it can not alone keep up with the return pump due to the aspiration. The water level rises even higher and the siphon begins to purge air and create suction.

It is certainly possible that, at lower flow levels, the siphon would never kick in. The overflow would still be silent and failsafe. The same as when your toilet flapper leaks water slowly into the tank.

The siphon will by nature handle MUCH more flow due to the suction created by the falling water. It "robs" this flow from the other standpipes. Remember, the return pump is providing a constant flow. That flow will take the path of least resistance to the sump.

So no, the open channel standpipe does not need to be at an intermediate level.

I am sure I did not do a good job explaining!


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