Will the siphoning effect help a system pump water upwards if the water's entry and exit points are at the same height? I am looking to pump water from a pool up to a roof for solar heating (black plastic tubing) and then back into the pool with the original source water. Does the gravitational force of the water flowing back down the pipe into the pool assist the pump and therefore decrease the pumps required strength?
I have been told that the strength of the pump needed would have to be the same regardless of the exit point's height. i.e. If the water was being pumped into a tank on the roof instead of flowing back to the pool.
 A: Short answer:
If you have a U-shaped tube full of water with both ends in the pool, and you lift the center of the tube up to roof height, you can pump water through that tube without regard to how high the top is.
The only resistance will be the resistance of water flowing through the tube (and solar collector).
The height will not matter.
(That's if the height is less than 10 meters. If it's more than 10 meters, air pressure will be insufficient to keep the water in the tube, and the water will evaporate, form bubbles, and separate at the top.)
You also have to make sure the tube and solar collector have no air leaks, because, depending on the height, the solar collector and its joints will be at negative pressure.
If enough air leaks into the tube, it could block the water from flowing.
ADDED: If you get bubbles in the tube or solar collector, the way to get rid of them is 1) block both lower ends of the tube so no water can flow out, 2) go to the top, fill up the tube and solar collector with water, to remove all bubbles, 3) seal it up again at the top so no air can leak in, 4) unblock the tube at the bottom ends.
A: "I have been told that the strength of the pump needed would have to be the same regardless of the exit point's height."
This is false. You would want to bring the pipe exit to the same inlet height as the pump or you are adding required head. As long as it's not over ~33 feet (from pump center line to the highest point in the pipe), you should be fine. That being said, the pipe, bends, fittings and solar collector will all add length in the form of equivalent length due to frictional losses. This equivalent length will also increase the required head. One other thing you may want to consider is the outlet size of the pump and the inlet size of the solar collector. 


*

*Don't go over ~33 feet 

*Outlet height = Inlet height 

