How to Adjust the Blower Speed on an HVAC

Shut off the furnace and blower motor at the circuit breaker and wait for the unit to cool completely.

Remove the service panel plate from the side of the furnace, using a nut driver to take out the bolts that secure the panel. Locate the blower circuit board with the black relay switch on the right side. A colored wire connects to the switch at the top with a modular plug. There may be one or two wires plugged into holding ports above the relay switch. On a three-speed motor, the wires are red, black and blue. On a two-speed model, the wires are black and red.

Unplug the wire in the relay switch by pulling on the plug itself. On a three-speed motor, the blue wire delivers the medium speed. Black is high and red is low.

Plug in the wire to the relay switch that corresponds to the speed you want for the blower motor. For example, plug in the black wire for the high speed. Plug the other wire or wires into the holding ports above the relay switch. The ports simply hold the wires out of the way and do not connect to any circuit.