About 3 years ago, I added a sump to my existing system by adding a CPR overflow. Since the tank was already running, it would have been really painful to drain it in order to drill it. That's why I chose the overflow.
First off...the CPR doesn't NEED the powerhead in order to function or restart after a power failure. That seems to be a common misconception. If the siphon is completely broken, the powerhead will pull all the air out of the siphon and get things started again. The powerhead is basically there to ensure that the siphon just CAN'T accumulate any air leading to a siphon failure.
For nearly 2 years, I ran the powerhead connected to my evaporative control circuit. Whenever the float switch triggered the evaporative pump, it would also trigger the CPR power head. This was a way that I created random currents. Worked fine and the siphon never failed.
Nowadays, I'm running that powerhead all the time. Every few months, I think..."I haven't seen any bubbles shoot out of the powerhead in some time..." That leads me to check the airline and it sure enough there is something blocking it. No telling how long it had been blocked.
If the powerhead failed, I would notice LOOOONNNNGG before the siphon had any kind of problem.
Anyway, the only problem I've had is when I've foolishly jammed my arm into the tank while it was running and caused the water level in the tank to jump up and down wildly. That has messed up the siphon enough to cause overflow on occasion. Lesson learned...don't do that.
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