Most solar drainback systems that I know of are actually pressurized systems that would vent when they drained. They have a fill valve and a drain valve, but depended on water pressure to push the air out of the top of the collector. It would be possible to have a system with a powerful enough pump to push water to the top of the loop, but this pump would be oversized for normal circulation. The 007 circulator (as an example) only puts out 23 feet of head which might go up 3 floors, but probably won't have enough flow at that height to purge out the air. The pump won't lose its prime, but the siphon back down will be broken and require additional pressure to reestablish flow. Keeping both ends of the loop submerged will prevent drainback, but the upper parts will be in a vacuum and any leaks will cause a problem. Keeping the system pressurized solves a lot of these problems, but at the cost of a heat exchanger and pump.
Chris
Chris
