While it is true that a drainback system is "less" prone to transferring the "over heated" heat to the tank or tubing due to the lower thermal mass (the thermal mass would then just be the tubes, fins, headers and not the 1.5L of liquid, per panel, which would be there in a glycol system), the first bit of water that exits the collector could be superheated steam, then very hot water. Depending on the flow rate of the pump, the very hot temps could continue for some time. But, many drainback systems need high head pumps that also are at a higher flow rate than a glycol system so the temps will be a bit lower.
BTW, don't put an auto air vent at the top of the system. If you are having a closed loop system with a big coil in the tank, you can use an iron body pump in the system (cheaper than bronze) and the O2 in the loop will never be replenished to rot out the pump. However, if you are pumping and dumping into the big tank, the bronze pump is needed.
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