Overflow vs. Return Pump GPH

If I buy an Eshopps Pf-300 overflow box that rates at a max flow of 300gph, what size (gph-wise) ViaAqua return pump in the sump to the aquarium would I need? (I'm confused what pump I need because of "max head" and whatnot; I'm trying to avoid having a water mess...)

 

as long as the overflow can drain more than the pump can produce it wont matter. the system is determined by the pumps turnover rate. if your overflow can handle 300 gph. get a pump that an push under 300 and the height you have. for example. i have a mag 7 that produces 700 gph at 0ft. at 6 ft of piping from the pump to the return point in the tank. at that distance the pump is only pushing 400 gal at the max. so my overflow only needs to handle 400gph or so. say 450 to play it safe.

 

yes and no... while i will agree that the overflow should handle more than the pump can deliever up to the tank after figuring the head loss and stuff, you dont want to be too far below..

meaning that if your overflow is good for 300GPH, then you dont want to be delievering only 100gph, see thing is that you need enough flow velocity to push any trapped air bubbles out of the u-tube... if you go too slow, then air will build up and eventually break the syphon...

my suggestion is if you want the 300gph overflow box, then find a pump that can deliever just a tad bit more than that at whatever lift height your tank is... and then put a valve between the pump and the tank, and you can trim down the flow so that its not too much for the overflow, but your not comming up short either...

i had to learn this one the hard way... which invloves a shopvac, wet carpet, and saltwater....

 

i dissagree with the air build up idea. less water = less air. as long as the overflow box keeps the syphon. it could be pushing one gph and the syphon would keep or even 0gph. if the design is right you wont have a prob. add a little water/air pump on the u-tube and then you can forget about a syphon break ever happening in the first place.

 

Okay. How do you calculate head loss?

 

that pretty much has to be done by the manufacturer. or you could get a gallon jug and a stop watch and have at it

 

Okay. Thanks guys.

 

if it is your only option i would set the system up and get a gallon or 2 gallon bucket hold it up to the return and get somone to turn the pump on and off for you. how ever long it takes for one gallon to fill up divide it Hrs and then it will give you a rough estimate as to how much you are loosing.

 

feel free to dissagree, but i'm telling you i know it happens... yes adding a small suction or dosing pump to the u-bend can help reduce and or eliminate the possibility of it happening, but i'm telling you that below a certain flow rate, air will accumulate in the top of the tube, and over time, will cause the syphon to fail...

in theory, your right, it shouldnt matter, but this is the real world...i've seen it happen, iave had it happen.

as for calculating headloss, you can generally just look at the specs on the pump and it will tell you that at 0 feet it can move ie. 500GPH and at 10 feet it does 250GPH, so, if the top of your tank is 5 feet, then you can pretty safely bet that it will do close to 375 give or take(it'll be a lil less but close enough)

and actually, if you really wanna get into it, there are tables that give the specific losses for each bend, size of tubing, and soforth, but i havent found a real need to get that accurate yet...

 

I have a 700GPH overflow with a 1.5" bulk head.

The pump is 700gph at zero head.

By the time the pipes rise 4 feet, that pump is down to about 350-400gph due to head pressure.

I'd like to get a 900GPH pump. Should be down to 600gph+/- after 4 feet of head

Most pump manufacturers will list specs - head pressure losses - max head etc... Just make sure you size the overflow and drain lines so they can flow at least 25% more than the pump can push with head pressure. I like a little safety margin.

 

Just an FYI in case your in a pinch and have limited options. You can modify the flow of the overflow box to get a little more than 300gph by putting in a bigger u-tube or adding a second u-tube (if the box is big enough). The u-tube comes in something like 1.25" and 1.5". But messing with this stuff can require more modifications to ensure you don't get air bubbles in the u-tube.

 

i use the largest u-tube you can and get a small pump take the air off the top. easy to do takes like 5 min and maybe $5 from a hardware store plus a small pump. and you never have to worry about it