I need help determining pump size for overflow/sump filter

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PoopSmart

PoopSmart

Fire Eel
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Jun 26, 2007
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I understand how the overflow works (pretty much entirely due to lurking the helpful posts on here), but there's one thing I'm still quite puzzled about. How do you determine the pump size needed for returning the filtered water from the sump?

My caveman-esque calculations and deductions have left me with this:
1.) Fill the tank up, and mark where it's been filled to
2.) Let the overflow run for a few minutes (record how many), and then stop it (somehow)
3.) Measure how much water it takes to refill it back to the line in gallons, then figure out the gallons per hour the overflow flows at using "higher math" (Gallons overflowed/number of minutes x 60 minutes/1 hour = GPH).
4.) Buy a pump rated somewhere near that GPH flow rate

I feel like without figuring it out won't the filtration system slack on one side? (ie. either the return will be too fast, and the pump will choke or the tank will overflow in the bad way, OR the overflow is too fast and can become unprimed.)

I'm obviously missing something very important here, but I can't figure it out. Please! Any help would be appreciated!
 
The overflow will not become unprimed if built correctly. I based mine on how much I wanted to filter. In my case I wanted to turn 1000gph. So I bought a pump capable of that. I then built my overflows to handle roughly 800gph EACH (a total of 1600gph). It runs perfect, even though I'm probably doing closer to 800gph.
 
Ok that makes sense, the overflows determine the rate is what it sounds like, but how did you determine the GPH flow rate of your overflows?
 
The overflow always has to flow more than the pump.

If you go through your calculations and you buy a pump that matches the flow of your overflow the pump will not actually pump that many gallons per hour because of head pressure.

If the pump will pump half the gph of your overflow you are fine. With a sump the excess (in system) water is stored in the sump. You pump it from the sump into the tank and it falls back into the sump as fast as you pump it into the tank.

Another way to tackle your problem is determine the pump you want to use then make an overflow that will flow more than the pump. I finally gave up on my DIY overflow pipe and drilled my tank and I am much happier. If nothing else it looks a whole lot nicer now!
 
Oughtsix, predrilling an overflow changes the flow rate you say? I'm calling what-I'm-thinking-of an "overflow", but I may be wrong. Basically I want to build a plywood aquarium, and I want to drill holes in the upper back corners, direct the water into the sump and pump the water back into the aquarium. I'm not talking about an overflow box or the PVC overflow thing. I'm sorry, I suspected they all worked based on the same principle! I'm glad I'm hashing all this out before I build the tank though!

OK! So knowing that the overflow rate needs to be greater than the pump flow, now I just need to calculate the overflow rate! And here's the hard part I gained from Gill Blue's link:

Q=[(2/3)h^(3/2)]*W*√(2g)

First off, what is g?! Acceleration due to gravity? (9.8 m/s^2 right? Which for an easier end result units should be converted to obtain 980 cm/s^2)

So I'm guessing this whole thing will have to be calculated in metric units and will spit out cm^3 (mL) per second then right? I can do the unit conversion after that then to achieve GPH.

This only works for holes that are square in shape though right? If you have a circular bulkhead then you wouldn't be able to use the simple height/width equation and variables above, right?
 
Step 1: Determine desired GPH. Example: If you want 10x turnover per hour on a 300G tank, then you want 3,00GPH.

Step 2: Determine head height (how high does the pump need to push the water?).

Step 3: Determine which pump(s) you would like to use that push 3,000GPH @ 6' of head. Choosing the right pump is important because buying an inefficient pump can cost you a lot of money in the long run, since pumps are running 24/7.

Step 4: Determine what size plumbing you will need to handle that type of flow.**

** Oversizing your plumbing (especially drains/overflows) is ALWAYS better than undersizing. You can always restrict flow with a ball valve or with a reducer fitting, but you cannot easily increase your drains/overflows. If you decide you want more flow and you are stuck with 1" drains it's going to be a real headache to install larger drains to accommodate more flow.

Check this calculator out: http://reefcentral.com/index.php/drainoverflow-size-calc

Enter your desired GPH and it will provide the minimum drain size required to achieve that GPH.

What is the size of the tank and how many GPH do you want?
 
Ok ok, I haven't built anything yet. I'm in the planning stages, but I just wanted to figure out how to determine this before I start building. Most likely I'm just going to go as big as possible (ie. as big as filtration and space will allow), but that link helped along with a sticky I just read on here in determining the pipe width.

Thanks everyone for the extra insight and help!
 
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