If you have U tubes, you have nothing to worry about *if* it is setup proper. You can of course test by turning off you return pump and then restarting it. The magic behind the U tube is simple. Once the water drops to a certain level, it no longer flows into the overflow box. This is turn causes the sending side of your overflow setup to stabilize as well. The water is in a state of equilibrium. Again testing should show if you are setup proper. Raising or lowering your over flow will raise or lower the level in your display tank. (It will also determine how high or low the water is in your sump) If you have it plumbed properly, you can get the same results by throttling back on your return pumps ball valve. Whatever the case may be, test it. I have a pair of U tubes (Yes it increases potential GPH
) and I am about to add a 3rd as I have a dual overflow which can outrun the 2 U tubes. (Causes a nasty 'sucking' noise) As a result I have to throttle up the return pump (Via ball valve) to get the water flowing back to over come the overflow just enough to keep it quiet. That said, I have tested and have witnessed power outages many times and evrything works as designed. Gotchas would be anything you innocently stick in the tank that drains in to the sump as well. (I put a ph
in the tank to feed my phosban reactor which emptied into my sump. When the power went out it still siphoned water, so becareful not to circumvent your failsafes! ; ) Also, make sure your resturns are drilled just below the water line to prevent back siphoning.