Why do I need an Overflow?

[Dewman]

I have seen them work, and I know they skim the surface of the tank.
I have had my sump connected to a syphon tube in my tank for a while. Someone asked me why I dont have an overflow box.
So please tell me, what purpose they serve and why it is Good/ Bad to have them.

Thanks

 

[2poor2reef]

If they are combined with a bulkhead then they don't require maintaining a siphon. That leaves one less point of failure and risk of flood. Your mentioned point of surface skimming is another major plus.

On the down side, they take up some room in the tank. In my small systems I often do not use an overflow box. I still want the benefits, so I drill a large hole and mount a bulkhead about three inches below the surface. The I use a 90 degree elbow or a street el point up with the open end horizontal at the water surface. This gives me decent surface skimming, no siphon requirement, with very little internal space lost.

 

[Dewman]

I don't think I understand. Explain to me how it works. I have seen them work, but I guess I don't understand the mechanics of them.

Do they maintain the same level of water as is in the tank? How does it keep the syphon from flooding your sump when the power goes out?

Could you give me a play by play on the things?

Thanks

 

[2poor2reef]

When an overflow box is used, teeth are cut into the box and the water level in the main tank fills until it flows through the teeth that are cut into the top of the overflow box. That is what controls the water level in the main tank. The water level within the overflow box is controlled by the height of the standpipe that drains the overflow into the sump. The water flows into the standpipe via gravity, so no siphon is required.

 

[2poor2reef]

Hey dewman, maybe we're talking about two different things. I'm talking about an overflow box permanently built into a drilled tank. Are you perhaps talking about a hang-on overflow? That's a different story. They do require maintaining a siphon since they are lifting the water over the edge of the tank on the way to the sump. The box itself still typically has teeth cut into it which still controls the water level in the main tank. But the rest uses a siphon similar to the U tube you described that you are currently using.

 

[Dewman]

How does the water get from the surface skim box (with the slots), to the stand pipe box? Is it a syphon between the two boxes? How do you maintain the syphon when the waterlevel goes below the slits in the box?

 

[Dewman]

I guess I need to ask you to describe the waters journey from the time it drops over theedge of the skimmer box.
Sorry I am not very bright I guess. Treat me like I am five I guess...

 

[2poor2reef]

It's just one box. The teeth are cut into the top of one edge of the box. When the water flows through the teeth it falls into the overflow box. Within that box is a standpipe that is shorter than the height of the bottom of the teeth. That determines the level of the water inside the overflow box. Initially, the water level rises in the overflow box until it reaches the top of the standpipe, at which point it falls via gravity into the sump.

In the case of a hang-on overflow box, instead of a standpipe you would have one end of a U tube. A siphon would (hopefully) draw water from the overflow box into the sump.

 

[Anonymous]

Dewman-if you're talking about the 'two box' overflow, here's the deal:
one box is in the tank.it's teeth prevent the water from the tank from draining below the tooth level.a second box is connected to it outside the tank.
the outside box has two compartments-a vertical wall divides the two, and is shorter than the walls of that outside box.one side has a hole in the bottom. to which a drain pipe is attached.the other side allways holds water.
a 1"(usually) u-siphon is placed connecting the inner box with the chamber in the outer box that is allways full(you gotta fill it first), and emptied of air, creating a flow from the toothed box to that chamber.
water flows from the tank to the inner box,to the chamber in the outer box, and spills into the second drain chamber, via a drain hose into your sump.
unless air bubbles find their way into the u-tube,siphon is not broken even if flow to tank from sump stops, and tank won't drain past teeth on inner box.HTH

 

[SPC]

2Poor, do you get any slurping sounds with the method you described (no box)?
Steve

 

[Dewman]

Excellent Guys

So, what you're saying is that even if the flow of water into the pump stops, the syphon is still active. So, when the power from the sump goes out (elec storm), what happens to the pump? Can it just run dry like that?

I just dont want to burn up a pump and overflow the tank. ??

 

[danmhippo]

I am not following, Dewman. When there is no power, the pump stops. How can the pump run dry?

 

[2poor2reef]

Right. When the power goes out and the pump stops, no additional water is being moved from the sump to the main tank. As a result, no additional water is flowing into the overflow box or through the siphon or down the standpipe, depending on the type of overflow. So your sump level will not fall and the pump will still be under water when the power comes back on.

The only problem is when you are relying on siphon as in the case of a hang-on overflow. If the siphon breaks for any reason, then when the pump restarts the main tank can overflow. If you go with a hang-on overflow get one that's reliable in terms of maintaining siphon and maintain it well is my advice. I prefer drilled tanks that rely on gravity. Gravity has never failed me.

 

[Dewman]

So, basically, the overflow is just to PREVENT the pump from running dry when the syphon is broken, but also, to stop the sump from flooding when the pump stops as well.

I see, I am clear now.

So , here's my second question,
Now that I know I need an overflow, half of my SUMP building problems are over.

I know it would be easier with a tank, but I can't afford the cost of a 30 gallon tank right now ($65), so I will be using a TUFF container from Home Depot. I'll get more water volume anyway.
I need to think of a way to fasten a partition into the container. I want to be able to use my hang-on skimmer, and separate the skimmed water from the unskimmed.
Has anyone had any luck doing this yet?

 

[Anonymous]

you should be able to silicone in place either a sheet of thick plastic, glass, or acrylic - but don't forget to allow for the extra volume of water that will drain into the sump until the tank level reaches the teeth level of the skimmer box.
In a 75 gal. for example, this can mean from 2-5 gallons into your sump, depending on how low you set the inside skimmer box.
a 30 gal container should be adequate for what you're looking to do.
also, don't use thin material for the 'wall' of your sump, as it may bend from the pressure of the higher level side.HTH

 

[Dewman]

Vitz,
When you say thick, is 1/4 inch O.K.?
I must be honest, I wouldn't think silicon would hold a piece of acrylic in place against a slick surface, with no support. Especially with the way the walls bow out when the containers are filled with water.

 

[Anonymous]

not sure about that, but one easy way is to use a small i.d. pvc pipe-prop from the 'wall' to the sump side- what about a 29 gal. tank, and siliconing in a glass divider?...

 

[danmhippo]

I'd suggest you take a look at the reef ready tanks in most LFS. It would be a lot easier to see what's involved when you are looking at a real thing.

 

[Dewman]

Dan, i know I have the overflow question solved now.
I just need to figure out the sump design now.

Vitz,
That's how this whole thing started! :lol:
I leaned on the edge of my 29 gallon that was already setup to be my sump. I cracked the bottom of it across one corner.

I am still investigating the possibility of using the 29 gal, even though it's cracked. Seeing as how it will only be filled 1/3 of the way full, and will have equipment in it. I think I might try to spread a bead of liquid nails or some permanent cement along the crack, and cement a piece of acrylic to the bottom of the tank.from the inside. Since it will never be used for an aquarium again, I might as well get some use out of it.

 

[Rothie]

Dewman-Thank you so much for asking that question. I have been reading about overflow boxes for 2 weeks and seemed to be missing the point.I now have a much better understanding of why the siphon is kept during a power outage.

Vitz-that was a wonderful explanation of how the double box works.

Now I have a question. What determines the flow rate through the box and into the sump. I have a 15G reef, and I only want to have between 100 and 200 GPH flow.

Thanks for all the good info.You guys are great!