Most (95%) of all the new generation furnaces have fan/ignition modules (computer boards) which monitor and run a homeowners furnace. This Module also has the capability of running your furnace fan on continues (low usually). On this module you will find the word "cont" or something related to the meaning continuous.
This 120 volt terminal is used...running the appropriate guage wire to a switch...then from the other side of the switch to the lowest speed of the motor. If you are not careful..and this is done incorrectly...you could have 2 speeds running at the same time... motors generally dont like to run 2 speeds...
I am going to assume that you don't have air conditioning. If you did you would have a fan switch on the thermostat that when set to "on", would enable constant fan. Your model furnace does not have a "constant" terminal on the circuit board. If wire is already run to the furnace connect one wire from the circuit board "R" terminal to the switch, from the switch connect the other wire to the "G" terminal on the circuit board. This will turn on constant fan and ensure that 2 speeds will not operate at the same time........Gasman
IF YOU WIRE THE R AND G TERMINAL TO A SWITCH OR TO THE G ON THE T-STAT THE FAN WILL RUN ON HIGH SPEED AS THIS WILL BE THE COOLING SPEED