Hi, I was pretty bored today so I went and done some water flow test with different pump setups. You may or may not know that pump specs. is very misleading data, and that every water cooling setup is more or less restricting, and all the restrictions(hoses, blocks, bends, radiator,valves ...) are causing preasure drops,and all that preasure drops summs up at the end.Every pump has characteristic of curve in x,y diagram, where x is water flow in (l/h) and y is preasure (bar) or water column in (m) .
This is the only diagram I found on internet: It shows three different pums, and the curve should meet x and y axis
You can see that when we are at the top of the curve the flow drops to 0, and the pump basicaly just maintain that preasure, and that preasure is the one from all the preasure drops in your setup.
So for very restrictant sytem like water cooling setup in computer you need as much preasure as possible to maintain water flow as high as possible. You can do that in two ways, take bigger pump with bigger flow with higher water column, or you can take two smaller pums and run them in serial, which means that you just double the water column while water flows remain the same. If you run them in parallel, water column is the same while water flows doubles, but we don't want that, we need bigger preasure. For instance my Ehem model 1250 pumps 1200l/h at 0m and 0 l/h at 2m while pumping out 28W, a lot better solution would be using two smaller pumps which would probably pump less heat in the water combined.
Ok, so here is the test. I used two small Renna Flow pums 580 l/h at 0m and 0 l/h at 1.1m (5W) and Eheim 1250 (28W).
First I tested with one Renna flow using only my water block(avatar) and 0.7 m silicon hoses 12mm ID , real water flow numbers were 276 l/h, than I add my heater core and another 0.5m of hoses which is the setup I use in my computer, water flow dropped to 257 l/h , here I was surprised how my heater core didn't cause a lot of drop, Next I used Eheim,block and rad. water flow was 370 l/h , pretty dissapointing considering the size and power. Then I hooked two Rena Flows in serial with block and rad. and the flow was 325 l/h that good and at 10W combined still a lot less than Eheim. And for final test I used Eheim + Rena in serial + block+rad. the water flow was 461 l/h wow huge increase just by adding one small 5W pump.
Conlusion, using two or even more pump in serial connection is realy helpful to increase water flow in restrictant system like computer water cooling, I need to test two Maxy-Jet 1100 l/h at 0m and 0 l/h at 1.48m (14W each) I bet it would increase my water flow a lot + two of this pumps is still a lot cheaper than on Eheim 1250 and one more thing if on pump should somehow fail, you still have one for backup.