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Water pumps: How much is enough, and is there ever too much?

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Inactive Moderator
Jan 4, 2001
Northern VA
I have some question concerning the right water pump for a system.

First off, what is the ideal GPH flow rate for a waterblock? I'm assuming that too low a flow will not take heat away, and too fast a flow might not allow the water to absorb and remove the heat...is this correct? If so, is there an ideal flow rate through a waterblock?

Second, if your running a resevior outside of the case, what size pump would be ideal?

I know the GPH rate drops due to hose height. What if your pump was mounted high though, would that still happen even iwth a few feet of tubing running down?

Thanks for the advice....I need to pick a pump by the way :) Using my big resevior I found for free, want to put a submersible pump in it, just need ot know whats enough and whats too much.

Use a radiator. I'm no using one and my temps are 40C... I'm using 25liter reservoir.

I have a Sicce Nova pump, it pumps about 800 liters per hour. Seems good.
I was wondering about that pump being higher thing too...

I'd say anything around 300gph<Gallons per hour> would be good and make sure it will put that out up too 3-5 feet of hosing. I have the Eheim 1250 and I shouldn't need another pump for a while. Great 315gph output and quiet as hell. I love it. Go Eheim if you can.
First off, a resevoir only delays the inevitable. If you are only running for an hour or two at a time, a resevoir is probably fine. If you want to run 24x7, a resevoir will not help. Why? Because if your radiator isn't enough to cool the water to an acceptable level, the only thing a resevoir is doing is storing mass to absorb more heat instead of exhausting the heat via radiator to the surrounding air. When you try to run for a long time this additional mass will slowly rise in temp until the Delta T (the difference in temperature) between core and water is not high enough for the core to dump wattage to the water. AMD says their CPU's will operate up to 95C.


I like my eggs soft boiled. Preheat the resevoir by leaving the computer on over night and stick 'em in there for exactly 4.5 mins and they're great. Good for hangovers too. MMMMMM.
Actually I was planning on a radiator for starters, while I work on my bong ofcourse, cause they just look cool!

(lol sorry, orb humor there :) actually its b/c of the performance and the conversation peice, plus something else to build...lol)

Yeah, bongs are cool, economical, fun to put together, and can perform just as well or better than radiators -but are noisy and require regular water changes and top-ups. My pump spews 350gal/hr (pump is rated at 400gph @ 0 head) and based on some rough calcs using some basic flat plate heat transfer equation, about 80L/hr (21 gph) should be enough for my homemade (converted old pentium heatsink) waterblock to suck 50watts. Imagine what 350 gph can do! Now the upper limit, I think, is my bong itself.
Breadfan (Jun 18, 2001 11:55 a.m.):

I know the GPH rate drops due to hose height. What if your pump was mounted high though, would that still happen even iwth a few feet of tubing running down?


You need the remember, what goes down must come up. Sure, you pump the coolant down into the block... but then what? It's a circuit, right? The water needs to 'lift' back to the pump...
Flow rates decrease with the use of 3/8” tubing and every turn the water makes. In a Maze 2.2 and Cooling Cube system I had the best results with a Danner Mag 7. Restricting the flow causes the temps to go up. The same system with a 69” bong cooler produces an actual flow rate of 36 gallons per hour! There should be an article by Rockhammer on flow rates somewhere on the front page.