Most heat/cool tstat have a build in jumper. so that when the stat is turning on the ac, the furnace fan turn on.
On a heat/cool stat.
RH is for heating, the high limit control the fan in the furnace.
RC is for cooling, the stat turn on the furnace fan and the ac.
You need to add a relay to prevent the ac turning on when you run the HRV.
If your HRV ducts are connected to return air of the furnace then the furnace fan should run when the HRV is on.
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