Quote Originally Posted by lampro View Post
i am more worried about the "hang on back overflow U tubing section might have air gap that blocked the drainage flow, then it would be a disaster. or maybe the drainage suddenly stop and it wouldn't flow again unless i manually siphon the air out from the u tubing.Attachment 57621
The final baffle in the sump - the one separating the return pump from the rest of the sump - will prevent flooding should something happen to the drain system. If the water stops draining into the sump then the water stops filling that final return pump chamber. If the final chamber only holds 1/4 gallon of water, that's the most that the pump will be able to return to the display tank - everything else in the sump is hald back by that final baffle. Unless you have a display that is a very small nano setup like 10gal, 1/4 gallon of water or even a full gallon will not raise the water level up enough to overfill the display tank, so it is really a non-issue. The biggest pice of advice I can give you when setting up an overflow and return is to test failures. Get the whole thing running, then do things like unplug the return pump to simulate loss of power to ensure your return nozzle is high enough to break siphon before the sump overfills from backflow. Adjust your overflow box height so that the water level in the display tank gives you wiggle room in the event that something happens to the drainage system and you have those extra inches of room so that your sump can empty that final chamber without overfilling the display tank.

In my experience, you shouldn't have to worry about the overflow U-tube failing. The water is pulled through the tube very quickly and any little tiny air bubbles that form in the overflow box from splashing or whatever are sucked right through the tube as opposed to accumulating in the bend. If your water flow through the U-tube is slow it is because the end of it is too close to the bottom of the overflow box and you'll see a huge increase in flow if you simply raise the tube 1/2".

Just be smart about how you set it up and it will be very reliable and if you have your return pump baffle, return nozzle, and overflow box all placed correctly then you will have nothing to worry about should either the drain or the return systems fail for whatever reason.