# Overflow box? Why does it work?



## jake106 (Nov 5, 2006)

Okay, I'm building my own sump filter, it's too much fun to not do it. I have read, re-read, and read again the various different plans on building these things and one common theme is to put an overflow box over the side of the tank and draw your output water from this.


I have no better way to phrase this question, so I'll just put it out there Barney style and hope someone can answer for me:

I understand the overflow box is for use in case your power fails. If the input to the overflow box is at the water level you want the sump to stop pulling water from, why can't I just have my input pipe come down into the tank and then end at exactly that level? Isn't it the same thing? Once the water gets down to the point, the sump starts sucking air and just shuts off. Am I missing something important? 

I KNOW that if I just say hell yeah and go ahead with that plan, then I will find the critical piece of information once my entire set up has had a critical failure. That is how I usually learn these lessons, but my wife would be rather angry at me if I spilled 10 gallons of water onto the floor.

So can someone tell me the real benefit here? Thanks!

Jake


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## Acep38 (Nov 11, 2006)

Overflow boxes that are not properly designed can have one fatal flaw. If your power goes out whiule you are not home, the syphon breaks and the overflow box stops drawing water. When your power comes back on, your sump pump starts pumping water from your sump back up into the tank, but now there is no where for that water to go. Eventually the tank overfills, and you have a very unhappy floor when you return home.

If possible, a drilled overflow inside the tank is always safer. those realy are not feesable unless you have an acrylic tank or access to glass drilling tools. 

If a drilled tank is not in the picture, You will want to be very careful when chooseing your overflow box. I personally would avoid ones with long continuous syphon portions like these: Overflow Box - Marine Depot - Marine and Reef Aquarium Super Store as they tend to lose syphon easier than the "U tube" designs. Those models do flow more water than the U tube models though, so if you need the additional flow and are willing to take the risk they can work well. 

If you do get an external overflow box, I would highly reccomend getting a float switch for your sump. Position it so that if the upper aquarium gets too much water in it, it will cut power to the sump pump. 

You can also use a small powerhead to re-prime the syphon after a power failure automatically. You put it in the wet side, and have it push water up one of the U tubes as soon as the power comes back on.

Hope that helps!


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## jake106 (Nov 5, 2006)

*Powerhead*

Okay, so how about this: my input to the sump is through a powerhead, and then I use an actual pump to get the water back up to the tank. If power goes out, they both shut off and the flow is stopped. When power comes back on, they both start at the same time. Obviously I will have to buy a pump and a powerhead that have very similar gph characteristics or one will outpace the other.

To be on the safe side, I should drill some holes in the pipe going from the powerhead to the sump, that way if a suction is created that continues after the power shuts off, it will be killed by the air it draws in once the water level goes below those holes.

I'm sorry if I am repeating material that has been posted in stickies or anywhere else, but I'm currently building the whole set-up in my garage and the back and forth of ideas is helping to solidify the whole concept in my mind.


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## Acep38 (Nov 11, 2006)

The main issue you I think are going to run into with that approach is that it will be very hard to keep the pumps balanced, even if they are rated equilly. If one gets some algae buildup, or a wayward snail stuck int he intake, you could end up with a dry pump or a flood very easily. Plus, pumps can fail. If your outflow pump stops up but the other keeps pumping, you have instant problems. Over the back overflows can have issues, but i think they would still be easier to mitigate than two pumps. Interesting concept though.


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## crazy loaches (Sep 29, 2006)

Mathcing the pumps will be impossible. That is the advantage of the overflow that as long as the oveflow has a flow rate esxceeding that of the return pump you'll never have a problem with one flow outpacing another. If you return pump stops then the tank level wont be pushing any more water over the overflow. As long as the overflow is rated high enough the pump can pump as fast as it can and that will just mean more water is pushed through the overflow, always balanced as long as its designed accordingly. BTW, the overflow box is the same thing as in-tank overflows, just that you use the HOB boxes when your tank isnt drilled. The 'continous siphon' part means that even with no flow (power outage) water will still be in the unit and a siphon will be maintained. This is not necesary in an in-tank overflow since no part of the overflow has to suck water above the tank level (like it has to in the HOB box to 'get over' the tank wall). There really isnt any other good way to do it. Either an in-tank overflow or a HOB overflow box with continous siphon. People have come up with all sorts of alternative ways to do it but they simply wont work good or at all.


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## BlueRam (Sep 21, 2004)

jake106 said:


> I understand the overflow box is for use in case your power fails. If the input to the overflow box is at the water level you want the sump to stop pulling water from, why can't I just have my input pipe come down into the tank and then end at exactly that level? Isn't it the same thing? Once the water gets down to the point, the sump starts sucking air and just shuts off. Am I missing something important?


As posted already, your idea of the "input pipe" at water level will be fine for a drilled tank with a slow to very slow flow rate (to reduce noise). Otherwise you will want an overflow section so that you can use a "Durso" type standpipe. Search the internets for "reef ready" and "Durso" for examples.

If you do not have a drilled tank an overflow box is a good alternative. I use and recommend the "U tube" designs as they are generally the most inexpensive and can be tuned with flow rate so that bubbles are passed. Again a durso can be used to cut the noise.

The other issue you post alludes to is the return forming a reverse siphon when the pump is off. This can be defeated with technology (a check valve) or simply by drilling a hole ~water line. I recommend the cheaper and simpler drilled hole.


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## jake106 (Nov 5, 2006)

BlueRam,
Thanks! I found that Durso standpipe earlier and have started incorporating it into my design. I'll post pics when it's all done. Just a note to anyone reading through this:

When measuring the length of your baffles, use the inside length of the tank, NOT the outside. I don't know why, exactly, I used the outside length, but it didn't work so well. I was able to insert the baffles, with some heavy force, and apply the silicon, but eventually they put so much pressure on the tank walls that when I went out to check on their progress they had shattered the walls of the tank. GAH!! There goes 12 hours down the drain.

SO I had to buy a new tank, remove the baffles from the wreckage of the old one, cut them down to proper size and then start over. Fortunately, this whole process has given me time to figure out how to do the overflow tank properly and everything has now been assembled. I am just waiting for the adhesive to finish curing and I can test the whole set up with a bucket. Wish me luck!


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## Naja002 (Oct 12, 2005)

The problem with 2 pumps is: if one fails for any reason--the other is going to keep on pumping. This also applies to any flow-restrictions, clogs, etc.

If you just set the overflow pipe at the minimum water level then the siphon will break once it starts sucking air. When the power comes back on--the pump will start pumping and you will not have an overflow out of the tank--except straight onto the floor.

Overflows are design to stop the flow, BUT maintain the siphon. Without that--you are in trouble.

My advice is: Do NOT do this until you fully understand how, what and why. Otherwise you are just asking for trouble.

HTH


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