It's a system where a solenoid opens from the tank and potable water is filled up to the panels when the control calls for heat. The pump then starts and circulates the water directly from the tank through the panels and back to the tank. When it is too cold out, some solenoids close and some others open and the potable water drains out and down the drain. It is a bit wasteful of water but it was very efficient because there is no HX and only one pump but sometimes the solenoids would fail and the panels would freeze.
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