How Does an Overflow Box Work?

Just how does an overflow box work, and why doesn't it overflow on my carpet? Well here I will explain the general principals of how an overflow works, and how it controls itself. After all, if they were that unreliable, nobody would use them!

Here is a typical overflow device for a "non-drilled tank". When you buy one of these, this is basically what you get. They are made of either some kind of Plexiglas or acrylic variant, and are usually clear.


The tankside box goes into the tank. When in the tank, water will flow through the teeth, filling the box. There will be a bracket that holds this all together, connecting the outside box to the tankside box. It will usually be a good stable bracket. The tankside box should be adjustable (with an up and down slide and a tightening screw) to set your desired water level for your tank. You will have to play with this until you get the right look. I like to make my water level just at or barely above the bottom of the top molding of the tank, so you can barely see the waterline.

The hardest part (other than the basic plumbing) in starting up the overflow is priming the "J" tube (filling it with water). This can be done by sticking a piece of aquarium air tubing up inside the tube, to the top of the bend, and sucking out the air (and usually a bit of tank water, spit spit...). You'll want to put some water in the outside box first, so that you can maintain a siphon between the two boxes. While running, both ends of the "J" tube will be under water. They should remain that way even if the pumps are off.


The main things to see here are first, the water level, which is controlled by the divider in the middle of the box. This keeps the outside part of the "J" tube under water at all times. The second is the drain to the sump. This can be connected to either PVC plumbing or flexible tubing. You can use just about anything that will keep a good tight seal and not leak. A leak at this spot would be really really bad!

Here is how it is all kept under control. The teeth of the overflow. As the level of the tank rises, more water flows through the teeth into the tankside box. As the tankside box's water level rises, more water rushes through the "J" tube into the outside box. As the water rises in the outside box, it flows faster over the divider, and down the drain to the sump. Be sure to check the GPH (gallons per hour) rating of your overflow box. It will tell you (or should!) and the GPH of your return pump. Return pump is the term used for the pump in the sump that brings water back up into the tank.
Here is the back view of the setup. Notice the Ball Valves on each pipe, these are not a total necessity, but can be handy in emergencies. When you are fiddling around and water starts pouring out your tank, some times the quickest thing to do is close the valves, or turn off the power to figure out what you did. If it is the sump overflowing, turning off the power won't help...I always use a ball valve on the line draining down from the sump. They are most handy while you are figuring out how high a water level to keep in your sump.

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How Does an Overflow Box Work?