sump pump vs. overflow box vs. tank size

[dax29]

Is there a formula or rule of thumb that helps one understand the amount an overflow box can handle vs. the output of the sump pump in a given tank size?

Let's say you have a 180 gallon tank and you want to use an overflow box rated for 500 gph. If you go with a 1000 gph sump pump in a 29 gallon sump tank, why does the sump pump not run dry as the overflow box should not be able to keep it primed?

I've been told that the relationship between the overflow and the sump pump depends on the size of the tank and it is not directly related to flow in/flow out. I need to understand this.

Is there a standard way of balancing flow on a wet/dry sump between the overflow box and the sump pump?

 

[fishfreek]

Take a look at this article as it should help explain alot of your general questions.

http://www.aquariumadvice.com/showquestion.php?faq=2&fldAuto=12

As far as a rule of thumb the only one I know if is that you should always size the overflow to be able to process more water per hour than the pump putting water into the tank. If the overflow is undersized comapired to the sump pump the sump will run dry as the main tank overflows.

 

[dax29]

So the best thing to do is to get an overflow that is greater than my pump capacity? Say, a 1200 gph overflow for a 1000 gph pump?

 

[fishfreek]

Yep. As the concept is the sump pump is always pumping water into the tank and the overflow is always draining water out of the tank (from a specific level). If you put more water in the tank than can flow out it will clearly overflow. Since the overflow is designed for water to spill into it at a specified level it will never drain the tank below that level. This means the display tank will always be at a specific water level just above or right at the overflow height.

If you vastly oversize the overflow you can have airbubble problems that can cause the overflow to malfunction so you dont want to get crazy with the size of the overflow vs the pump speed.

In addition you want to know the pump's speed at a specific head height vs the advertised speed at 0' head. As pumps have to push the water up they will flow less and most manufacturers will have a chart on the pump box showing approximate flow rates at specific head heights. The most common head height for tanks is between 3' and 4' and quite often a pump rated at 1000gph at 0' head will do between 800 and 750 gph at 4' head. If your head height is 4' and your overflow is rated at 700gph then you want to find a pump that does approxmatly 700gph or a little less at 4' head.

Head height is the distance from the sump to the top of the tank.