I've brought this up in some other threads but it sounds like some people are confused about the concept so I thought I'd draw up a design to try to clarify things.
The problem with traditional overflow boxes is that you're only collecting water from the very top of the tank which can make it harder to pick up debris from lower down in the water column. Here's an overflow design that will allow you to draw water from several points in the water column. It also eliminates surface skimming which can be useful if you have floating plants, or a CO2 supplemented tank and want to minimize surface agitation and degassing in the overflow. Another nice thing about the design is that it takes up zero real-estate inside the tank and looks very sleek - all you see are the bulkheads.
Here's how it would work: bulkheads are drilled at several different depths in the tank and are plumbed outside the tank into vertical pipes that rise up to a horizontal tube. The horizontal pipe is then plumbed into a "T", the top of which leads to an air vent to prevent siphoning and the bottom leads down to the sump.
By plumbing all the bulkheads into the same horizontal pipe, the height of the horizontal pipe determines the height of the water in the tank. If your pump fails, water will drop to below the level of the T on the horizontal pipe and the overflow will stop draining the tank.
I didn't draw it this way but it would probably be better for the horizontal pipe and the final down pipe that leads to the sump to have a wider diameter than the vertical pipes to accommodate the flow from the multiple bulkheads. You should make sure that this diameter is more than enough to handle the flow from your return pumps.
A possible modification of the design is to have the bulkheads exit the tank with a T, instead of an elbow, and add a ball valve on the bottom of the T. You can then open the ball valve for easy tank draining and water changes (water will drain down to the level of the bulkhead).
Comments/criticisms/suggestions welcome.
The problem with traditional overflow boxes is that you're only collecting water from the very top of the tank which can make it harder to pick up debris from lower down in the water column. Here's an overflow design that will allow you to draw water from several points in the water column. It also eliminates surface skimming which can be useful if you have floating plants, or a CO2 supplemented tank and want to minimize surface agitation and degassing in the overflow. Another nice thing about the design is that it takes up zero real-estate inside the tank and looks very sleek - all you see are the bulkheads.
Here's how it would work: bulkheads are drilled at several different depths in the tank and are plumbed outside the tank into vertical pipes that rise up to a horizontal tube. The horizontal pipe is then plumbed into a "T", the top of which leads to an air vent to prevent siphoning and the bottom leads down to the sump.
By plumbing all the bulkheads into the same horizontal pipe, the height of the horizontal pipe determines the height of the water in the tank. If your pump fails, water will drop to below the level of the T on the horizontal pipe and the overflow will stop draining the tank.
I didn't draw it this way but it would probably be better for the horizontal pipe and the final down pipe that leads to the sump to have a wider diameter than the vertical pipes to accommodate the flow from the multiple bulkheads. You should make sure that this diameter is more than enough to handle the flow from your return pumps.
A possible modification of the design is to have the bulkheads exit the tank with a T, instead of an elbow, and add a ball valve on the bottom of the T. You can then open the ball valve for easy tank draining and water changes (water will drain down to the level of the bulkhead).
Comments/criticisms/suggestions welcome.
I run the type I posted as my tank is a plywood build and I don't want any holes through the walls which could be a potential fail point should it leak.