I have three wires running from the furnace to the electric air cleaner, black, red, and white. I want to run the fan on low, and high when there's a call for heat. Black and red wires have 120 volts. The red is connected to the black wire from the air cleaner and black is not connected. Where do I hook the black wire? This is an olsen gas furnace 25 years old. I can't get the furnce to run on low when switched at the thermastate to on. Its running on high when there's a call for heat. I want the fan to run on low and when there's a call for heat switch to high?