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View Full Version : Size of overflow box?


ashton1881
05/17/2011, 03:18 PM
So, I know that an overflow box that says it is rated for "up to 600gph" means that it can support that much, but depending on the size of return pump and if you have a ball valve on the return it doesn't mean it has to push 600gph, it can do less if thats what you want. But, i don't know how to tell how big of an overflow box I need. I am doing a 75 gallon build and that appears to be on the borderline of which overflow box to use. Some people recommend using an overflow box with one drain and a return pump that will push its max gph through it. while others say use a overflow box with two drains that would do say 1200gph max and just use a valve on the return line to make the pump only push say 8-900gph. I would rather get an overflow box with the one drain, but I am concerned that wont be enough. I know on skimmers that even though the specs say "for use on tanks up to 75 gallons" that doesn't mean I should use it...75 gallon is the max size, but something bigger is recommended to handle the bio load. Are overflows the same? I would assume that as long as my turn over on my tank is good then i could go with a single drain. i will have three power heads moving water in the tank itself already which with those added to the 600gph return from the sump I should have plenty of movement. Am I over thinking a bit?

Dyzio545
05/17/2011, 04:16 PM
What kind of return pump will you be using?

I'll give you my plan for a sump, just to get an idea. I'm using a MAG 7, that is 700 gph. You want the overflow box to be much larger. So, I will need an overflow box that will do lets say 900 gph. Just like you said, it's like the skimmer. Even though it might say up to 75 gallons, you want a skimmer that will handle up to 120 gallons or more IMO. It shouldn't be something you should stress about. :thumbsup:

greech
05/17/2011, 06:27 PM
Assume we are talking about HOB overflows?

Regardless, its kind of hard to have too big of a drain and 2 drains are better than one for safety reasons. It really depends on what kind of flow you want running through your sump. If you think you want 600gph through the sump and your return is capable of that then get one that is rated at 800+gph. Give yourself some wiggle room. As you said overflow boxes tend to handle less than the box says. Also make sure your return is rated for 600gph at the height it needs to push water too (not just out of the output).

The other thing with U-tube overflows is you need to move enough water through it to keep the u-tube free of bubbles. If you don't push enough through them a large bubble can form and your siphon can break as a result.