[Skulemate]

I just wanted to add something about whether or not a system is closed. In any system, "closed" or not, you're going to have a point where there's zero pressure. The only difference between an "open" system and a "closed" one is the fact that in the open system you are forcing this point to occur at a specific point in your loop, due to the presence of the resevior, and the water level in it.

Oh, and about the res on the 50 ft. high roof thing... you can get a siphon effect in any system, whether it is "open" or "closed", provided that the system is primed first... I've played around with this in my system, which is a so called open system. All you'll do is cause a spot in the elevated location with lower pressure (i.e. when the tubing rises above the piezometric head line of your system... in a "real" watersystem this can potentially cause problems).