If you're currently lacking a stand alone fan and you've got a spare furnace blower fan lying around, you might be in luck. First, you'll need to know a little bit about furnace fans and how they operate in order to properly complete the conversion from furnace fan to stand alone fan.
Furnace fans are comprised of a few basic components: the impeller, the motor, and the plenum.
The impeller actually draws the air in using specifically designed blades, much like any fan. It relies on suction pressure to properly move air. This is one of the key aspects you'll need to understand when constructing your fan.
The motor of a furnace fan can come in several different horsepowers, ranging from about 1/6 to one. More important is the speed of the motor. Furnace fans can have anywhere from one to four speeds, depending on the make, but you'll want to operate at the lowest speed or speeds for your stand alone fan, unless you are in the mood to blow your pictures off the wall.
These speeds are represented by color-coded wires. White is the common or ground, then progressively from lower to higher speeds the colors normally go red, blue, yellow, and then black. Last, you'll want to make sure this motor can operate using 140 volts, which will be indicated on the motor. This will ensure that you can plug it into your standard outlet.
The plenum is really just the chamber that the air flows through. It's a pretty simple component and shouldn't need any more instruction other than remembering that nothing should be lodged inside the plenum, or it will prevent proper airflow.
Allowing Proper Suction Pressure
As noted earlier, proper suction pressure is key in the operation of a furnace fan. Secure some type of material such as a sheet of metal to the hole by the impeller, blocking about 90% of the hole. This will create the optimal atmosphere for suction pressure. If you want to make it aesthetic, use a skill-saw to cut a 3-inch hole in the center of the sheet metal and cover the hole with chicken wire, or some other caging to prevent body parts or debris from entering the impeller. If you do use sheet metal, make sure that the edges are ground down to prevent inadvertent cutting of people or furniture. You can secure it with screws, which will keep it adhered and prevent the metal from chattering.
Blocking/Modifying the Outflow Port
You will also want to use chicken wire or another type of caging to prevent body parts or debris from entering the outflow port. As it is, the fan will blow uni-directionally in a small area so you may also want to construct a cone to disperse the air in a broader pattern. One example would be to use a small cone and a common household vent port to disperse multi-directionally.
Electricity can be a dangerous element if you aren't careful. Before tampering with any electrical device, always ensure that there is no power being supplied to it. In order to divert the wall socket power supply to the motor you will need a wall plug. Cut the wall plug wire at the desired length. Now comes the hardest part of the whole project. You will need to decide whether you would like to run at the lowest speed or, optionally, run at two different speeds.
If you require two different speeds, you will need to purchase a switch and secure the white wire (ground) to the ground connector and then assign the red wire (low) to one connector and the blue wire (high, or higher), to the other connector. Additional speeds can be used but are not required for household applications. Connect the plug wiring to the switch, or if you are working with only one speed, splice the red and white wire to the plug wires, ensuring you are insulating properly with electrical tape.
Additionally, you might want to attach a handle to the top of the furnace blower fan or even build a casing for the fan for aesthetic purposes.