Hmmmm. It would seem the only way know if it is the CCU is to swap out for new/rebuilt. That's a pricey part but for a daily driver in the Northeast in Winter floor heat is pretty nice. I just want to be sure I rule out everything else first.

Do the switchover valves ever fail? Is there a way to test by sending voltage (12V?) to a swicthover with vacuum applied to see if it opens?

Someone needs to figure out how hack the climate control system with an Arduino. What are the control voltages for all of the various sensors and actuators? I suppose if I quit my job and left my family I would have the time needed to dedicate to such an endeavor.

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