We will be discussing changing a direct drive blower motor and capacitor on a traditional up flow gas furnace as would be found in a basement of a home as these types of furnaces are by far the most common. Changing motors in horizontal units in an attic or a crawl space are very similar but will vary with the equipment. If you are not comfortable with working on 120 volt equipment please consult a professional Heating and Air Conditioning company to do this work.
Before we begin, we go shut off the main 120 volt power to the furnace. This may be able to be accomplished by turning off the switch at the furnace where the main power comes into the furnace. Sometimes this switch is mounted on the floor joists above the furnace, or on the side of the furnace itself. In the interest of safety, we also shut the breaker off at the main house panel. We use of a volt meter to be sure the power is really off.
The next step is to remove the screws or bolts that hold the blower assembly up inside the furnace. Some blowers are on a sliding track while others are simply held in by these bolts or screws so it is a good idea to use a small 2"x4" piece of lumber to lower the blower out of the furnace as the holding screws are removed. If there is a control board attached to the blower assembly all wires should be labeled so that if any wire is knocked off it can be re installed on the correct terminals that it came from. Hopefully the blower assembly will come down with all the wires in place. If there is a control board mounted on the blower we will have to remove it with the wires all attached and tie it up out of harms way. The same is true of any other component mounted on the assembly.
Once all this is done we should be able to remove the wires from the motor and capacitor, but first the capacitor wires must be shorted out with an insulated screwdriver by touching the screwdriver to both terminals of the capacitor at the same time. Not doing this will result in us getting a nasty shock if we touch those terminals with bare hands! Now we remove all the motor wires from where they are attached to the furnace control board or fan center (we have labeled all these wires before this).
Once the blower assembly is on the floor, we turn it on it's side and remove the screws or bolts that hold the mounting bracket to the assembly. We then turn the blower over again and remove the set screw or bolt that holds the motor shaft onto the wheel at the hub. With this done, we should be able to turn the assembly on it's side and the motor should drop out of the assembly. We say "should" because 7 out of 10 times it will, the other 3 times we will have to gently tap on the end of the shaft with a brass drift and hammer to drive it out of the wheel. Often we will have to use WD40 on the shaft first and sometimes may need to use sand cloth on the shaft if there is any rust present. Beating on the shaft with a hammer will ruin the shaft and it will not come out of the wheel! A new wheel will be needed if this happens.
With the motor out we can now remove the mounting bracket and attach it to the new motor. Some motors have the bracket welded to the motor from the manufacturer so they will not require this step. Now everything is reversed for the re installation of the motor. We must be sure the wheel spins freely in the assembly without hitting anywhere before we raise it back with the 2"x4" lumber into the furnace. Once it is bolted back in place we reach in to the wheel area and spin it once again to be sure it is running free.
All that is left is to re install the wires (including the green ground wire that goes to a metal unpainted part of the blower), on the control board or fan center and mount the new capacitor or re use the old capacitor if it is the correct one for the new motor. We now go turn on any power source switches and or breakers that we previously had shut off. Turn the thermostat up to call for heating and our furnace should be running at this point. Be sure the rotation of the motor is actually blowing air through the registers, if not we may have to reverse the rotation wires on the motor. We will now check the amp draw of the motor with an amp meter and compare it to the amp rating that is on the motor or documentation that came with the motor to be sure the motor is operating as it should be. This amp draw check must be done with the blower door on however.