Open VS closed loop

So, when my house is finished in about 6 months, there will be major watercooling going on! Got a couple of threads on it around here..

Basically, the WC'd pc is on the first floor, with the cooling loop in the crawlspace under the ground floor. So there is about 4m from the lowest point (the pump) to the highest point (the pc)

Now, some reading suggest that in a closed loop system, this height difference doesn't matter. The force needed to "push" the water up, will also "pull" it down.

But where is the line between open and closed loop? I'm thinking of a res up by the pc, so that mass of water should also "push" on the whole loop negating the height difference? Right? And a res is probably needed, there will be about 45 liters of coolant in that loop, so thermal expansion could be an issue if running closed loop..

My systems are much like yours, with my radiators going in the basement below my desk, and a reservior mounted on the wall next to the computer.

With the reservior opened, it didn't seam to make much difference in flow, but I had plenty enough water in it that inlet and outlet into the res were submerged. That is with my pump in that reservior though.

What I'm looking at doing with the new system is to have the pump in the basement as well.
The reservior will be airtight when closed, still a closed loop system, allowing suction from one side and pressure to the other.
If it were to be an open top res, then the suction/pressure effect would be gone and it would be up to gravity to do it all...not bad flow rates, but probably diminished somewhat.

If I beat you to completion (I only have to finish building the case, not the house) I'll be sure to let you know.

Bladerunner's Stealth project you may find interesting as well (Check out the below ground 'Bomb' he implemented.

http://www.dwpg.com/content.php?contid=3&artid=57

Here's more info on Blade's work, he's a hell of a craftsman and is a legend in his own time.
http://zfz.com/default.asp