I'm in the geothermal and directional drilling side of hvac not so much the furnace side of things. What I am having trouble with is my blower fan on the dc90 in my home. What I am trying to do is get the blower fan to run on call from a separate thermostat without the gas heat exchanger. I have the second thermostat hooked into the 24vac and the fan call on a 2 wire and the old t-stat works fine. I cannot however get the fan to run alone no matter what I do. The fan also does not run if I call for "fan on" or auto. When the t-stat calls for heat the fan will turn on no matter what way the switch is and turn off after the heat cycle. Is this a problem with the fan control board, or something that can be changed? Either way I'd like to be able to control the blower fan only and with the second thermostat wired on a 2 wire only calling for the fan relay it still does nothing. I'm stumped because this is how we run an outside wood burner/ac unit or geothermal ac unit. On my dc90 I can't figure out for the life of me why the fan well not run unless heat is called for.