wuzzapiman said:

**What is the differce between:**

lets say all pumps have a 4' head

and 6' of tubing in the system

2x 150Gph vs. 1x 150Gph

2x 150Gph vs. 1x 300Gph

First right off: I don't think that there is a substancial gain between running 1 pump and 2 identical pumps, except safety.

Maybe if you have a really restricted system a series connection would help somewhat.

Getting a NEW pump with higher head is the way to go, OR lowering the restrictions in your system.

I had a high restriction system due to my rads loooong series of tubes of only 1/4 inch inner diameter. I used a pond type centrifugal pump, like Ehiem with a max flow of 315 GPH, maxhead of 8 feet. I got like 24gph !! I talked to the distibutor of the pumps asking about a second pump. He claimed that it doesn't work that good, it isn't that simple. So I used 2 rads in parallell, and each rad was split in half, so the water had 4 paths to go. With that mod I got flow to 104 gph. All measurement via the "fill da bucket method" Just to be safe I bought another bigger pump aswell, with increased head.

Another reason for 2 pumps not working too good is all experiments with 2 fans stacked, that hasn't brought much good, I can't remember a single positive review. Maybe comparing air/water is a bad example.

On a more physical level:

You have 1 pump with a maxhead of 4' max flow 315 gph.

For ordinary centrifugal pumps the max flow and max head occures at different occasions in a nonlinear fashion.

You have 6' tubing, a block, a rad. It has a backpressure of x psi at a flow of 100 gph.

The pump max cap isn't reached because of the backpressure. It balances out at 100 gph through your system mentioned above.

Then you add a identical pump in parallell:

The flow will increase some due to the flow being halved through each pump. With halved flow the pumps head capability will increase. By how much? I don't know.

You add a pump in series:

Okay, theoretically this should mean that we have double max head at our disposal. As the pumps see only half of the resistance each, we see a increase of flowrate. As the flow through the pumps increases the head capability decreases. Since the head capability decreases, the ability to fight the higher backpressure(a result of forcing water quicker through the system) also decreases.

Again, a smaller result than expected.

I think this is nasty. But then again, this is just my theories. Maybe some ppl have have been successful in using 2 pumps.