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as that the only wire in question? or is the white from thermostat also open?
Regarding to "fan only" function.
Do you ever use fan on....or do you leave in auto?
put the white on equipment to O
Put white at thermostat to O
just keep me posted with any issues
If O is reverse operation put on B.
with the new panel you do not need a master /slave thermostat set up.
Strip back thermostat wire cover at slave stat and at board. Put red to r, yellow to y green to g etc. Put white on W at thermostat and on B in zone control..
Do this on both thermostats.
At equipment terminals....put the white wire on B
those are not needed as that was what determined master and slave.
Now we have direct input by both.
The o and b just gave the master thermostat power after it was done.
Just tape the 3 off at panel.