HOB Basic Overflow Design

Since I've seen the question many times, and didn't really understand this myself for more than a year after getting into the hobby, I thought a graphic would be helpful.

A properly constructed overflow will not lose siphon during a power outage. When fashioned correctly (as is the case with major manufacturers), there is some means to achieve standing water in the overflow box for the sole purpose of keeping the U-Tube submerged. This way, when the power comes back on, the siphon continues as is it were never disturbed. As a disclaimer, I should say that there is one instance where siphon is lost, and that is when microbubbles accumulate in the tank-side box, and eventually cause a hollow at the apex of the U-Tube, breaking the siphon. It's probably best discussed in a different thread, but that should not happen if your set up is properly executed.

Most boxes utilize some sort of baffle, shown in the picture as the vertical section of acrylic right in the middle of the box. This maintains water level in case of a power outage.

Here are the pictures of the external box portion of a standard overflow. Your mileage may vary, but they're basically some version of this. The U-Tube is descending into the box on the left, the bulkhead/drain hole is in the right side, bottom.

NOTE: For some reason the bottom of these drawings are cut off in this view, but click on the pictures for a full, unobstructed view.

When the system runs as it should, the external box looks like this:



After a power outage, the right side drains, but the left side stays filled.

 

good article sparky

Plus 1 on this, could never understand why so many have problems with this, i have found after over 25 years of using these things the biggest reason for the accumulation of bubbles in the u tube is simply not enough flow.