Many modern air-conditioning air handlers use multi-speed blower motors. Switching fan speeds can change an air handler's seasonal energy efficiency ratio (SEER). Units with a higher SEER rating use less electricity while operating. Changing an air handler's fan speed will affect humidity levels in the home and the refrigerant's heat exchange with the outside ambient air. The positive effects of increasing fan speed, like increased air flow to duct vents, can sometimes outweigh the negative aspects, like the reduced temperature difference between the air entering and leaving the air handler.
Disconnect the power to the air handler at its electrical disconnect box. Follow the wires from the air handler to the metal or plastic box located near the unit. Open the disconnect box and either pull out the buss bar or flip the switch to the "Off" position.
Unscrew the air handler's cover with a nut driver, either 1/4 or 5/16 inch.
Pull the cover off the air handler.
Inspect the wires coming from the fan motor. Two will go to a capacitor located on the air handler's blower housing. The rest will lead to the electrical control box. In the box, one wire will go to a terminal block labeled "Line 2" or "Neutral." The wire that controls fan speed will go to a fan relay. The other speed wires will have wire nuts capping them.
Inspect the fan motor's identification tag, located on the fan. The tag will identify which color wire controls which speed. Each manufacturer uses its own wire colors. Use this information to identify the current speed and the desired fan speed wire colors.
Remove the current fan speed wire from the fan relay's electrical terminal. Grab the wire's terminal connector with pliers and pull it straight out. In some cases a short jumper wire connects the fan speed wire to the fan relay. If so, untwist the wire nut that holds them together and discard the wire nut.
Cut the wire's terminal connector off with wire cutters.
Twist a wire nut over the end of the old fan speed wire.
Remove and discard the wire nut from the desired fan speed wire.
Strip 1/2 inch from the end of the desired fan speed wire with wire strippers.
Squeeze a wire terminal connector to the stripped part of the wire with pliers. The wire will slip into the round end of the connector. Skip this step when using a jumper wire.
Push the rectangular end of the electrical connector onto the fan relay's electrical terminal. When using a jumper wire, twist the two wires together and secure them with a new wire nut.
Close the air handler's cover and secure it with the removed screws.
Turn the power on and test the air handler's fan.