How To Turn On Furnace Blower. When increasing the blower speed, loosen the setscrew a little bit. If the furnace starts, the problem is either with the thermostat wire or thermostat.
Start by turning the furnace off, turn your thermostat to “fan only,” and wait a few minutes. In normal operation, your furnace blower fan should come on 1.5 to 3 minutes after your thermostat calls for heat and should turn off several minutes after the heating cycle ends. If any of these are are off, turn them on once (and only once, for safety) and allow your system to run.
If you don't know where to find the limit control switch on.
Turn the power to the furnace off and disconnect the wires from the thermostat. To continue testing the fan switch, replace the wires and turn on the furnace. This is the part that holds the pulley to the furnace’s driveshaft. If you don't know where to find the limit control switch on.