Any HVAC experts?

[weyburnt]

That is really odd. Everything sounds correct. Somehow the circuit is connected from the r to y via your hrv. Try disconnecting the t stat wires and swing if the problem still occurs. If so it is in the furnace wiring if not the problem is in the t stat wiring or the t stat itself. The wire from the hrv to the furnace is to make a contact from r to g to force the furnace fan to run when the hrv is on. Write down the wire colors to terminals so you sent chasing your tail when you re connect everything. So you know what your dealing with, the r is the positive feed off the furnace, when it is connected( via tstat) to w furnace calls for heat, g furnace calls for fan y furnace calls for fan and Ac contractor is energized