1. How do you prime these overflows?
you stick airline tubing up into the top of the U-tube and suck the air out.
2. Does the siphon ever fail?
Yup, if the waterlevel falls below the u-tube, it will fail just like any other siphon. Mines has a small skimmer/overflow box inside the tank. When the water level falls in the tank, ie a water change the inside the tank box stays filled with water. The one problem I had once, was that there are 2 holes in the box whereas 2 suction cups plugs into to hold the box to the glass. One of the suction cups pulled out and the water flowed out as I did a water change, hence dropping the level in the box and breaking the siphon.
3. Do you need a minimum amount of flow rate to keep the siphon from failing?
nope, it stops during a waterchange, whereas there is no flow.
4. How is their noise level?
Better than a inside the tank overflow, as the water typically falls equivalent to the depth of the tank. With tweaking, mine only falls less than an inch.
5. Is there any problems with siphon resuming after a power outage? I am thinking of running mine only at night to a refugium if the noise is too much.
I'd suggesting running it 7/24. Seal the suction cup holes, and mount the inside box in a different manner. I use liverock to prop it up and wedge it against the back.
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