Some furnace motors have the capability to reverse the direction of rotation. When replacing a reversible motor, it is important to get the motor turning in the proper direction or the air will not blow. You can reverse the direction in two ways, either with internal wiring or external wiring. Either way of reversing direction is easy to do with minimum tools and a little know-how. The wiring schematic that comes with the motor is helpful. If you don't have a schematic, directions for reversing the direction of the motor should be printed on the side of the motor.
Things You'll Need
Move the motor to an area that is well lit and easy to work in. If the motor is direct drive (no belt or pulley), you may have to remove the entire blower assembly from its position in the blower compartment of the furnace. This can be done by removing the screws that hold the blower in place and sliding it out along its rails. This may mean disconnecting wires or moving other parts of the furnace out of the way. Pay special attention to where everything is before you disconnect anything. It is often helpful to take a picture and write down the color of any wires you have to disconnect.
Make sure that the motor you have is reversible. The nameplate on the side of the motor should tell you.
Read the nameplate on the side on the motor and follow the directions for either clockwise or counterclockwise rotation. All motors come already set to either CW or CCW rotation, as you are looking at the shaft of the motor. Changing rotation involves switching the position of two wires. On externally wired motors, two short wire (usually a yellow and an orange) are connected with spade clips. By pulling these apart and connecting one to the other (for instance, orange to yellow), you can reverse the rotation. Internally wired motors have a black and a red lead wire connected to a spade end that can be switched to change rotation.
Put the motor back in the proper location and refer to any photos or drawings you made to reconnect the wiring.