Keep in mind that its not a "rule" only a possibility. A set of unrestrictive blocks would probobly fair well in a series system that wasn't getting close to the max head. However a look at the most modern blocks show a tendency to go toward a high flow rate at the cooling point. This means narrowing the path to increase flow however this also increases resistance.
Its important to either A) do the math on the best setup for your system or B) experiment in your sink/tub to figure what method will work best for you.
My method of testing head resistance is imperfect but its costs nothing and is fast. I hook up my pump(inline or submerged doesn't matter) outside. I make marks on the wall every cm. I connect about 10 feet of hose. Then I raise the hose until the water stalls. I mark this point as Max. Head. Then cut the hose at about a foot and connect that to my test piece and the other 9ft to the outlet. Then I raise it to max head. Take the difference and Bam.
I know this is imperfect because since the flow decreases so does the resistance so then flow increases(makes sense if you think about it). So my number is a little low for each block. I guess I could meause the height at which the pump puts out a gallon per minut(60gallons per hour) which would be closer to my normal flow rate, and hence give me a more accurate number but this would also take alot more time and I'm just to eager to build my sytem and not to test it for hours.
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