Lance, that type of system is called "drain down". Just make sure you have very good motorized or solenoid valves. The problem with these system in the past was that the valves would fail to open when the system shut off and the panels or piping would split. The other issue with them is that sometimes a LOT of water was wasted draining out. I have made a couple of these systems where I put a "low water cutoff" from a boiler high enough that the water only drained to remain in the heated area. That said, all this was 20 years ago and you might have an easier time doing a "drain back" than a "drain down" system. You would only need a heat exchanger by the looks of it.
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