I decided to add a fan to my smoker in an effort to even out its temperature and smoke levels, but the fan kits which are available are pretty expensive, so I figured out how to add a fan for less money. I bought a replacement motor for a bathroom vent fan for $15 at Home Depot. Replacement motors are available for about the same cost from Lowes and Ace Hardware. And I ordered a 3.5" aluminum fan blade from Grainger for $4.66, plus $9.23 of shipping.

Before you buy a motor, measure the thickness of the back of your smoker and confirm that the shaft of the motor is long enough to hold the fan blade, with enough clearance between the fan and the back wall of the inside of the smoker to allow air to circulate. The fan blade I bought is supposed to move 100 cfm of air. A bigger and/or higher volume blade will fit in my smoker, but I decided not to go overboard. The fan blade blew air back at the motor when I tried it, so I dismantled the motor and reassembled it with the rotor facing the other direction. Note the difference in the location of the copper wire in the picture above and the picture below. Now the fan blade blows air away from the motor. Grainger shipped the fan blade to me in a padded envelope, and it got slightly squished in transit, so I had to realign the blades. If you order a fan blade, ask the supplier to ship it in a box.
Then I rooted around in my workshop and found a power cord, a power switch, and a galvanized framing angle, I picked up about $1.00 of miscellaneous hardware at a local hardware store, and I assembled the fan. The total cost of the parts was about $30.

Then I drilled holes in the back of my smoker for the shaft of the motor and two mounting screws, and mounted the motor on the back of my smoker.

The fan works pretty well, although it doesn't seem to blow as much air as it did before I mounted it, probably because the blade is pretty close to the back wall of the smoker.

I'm going to catch some Lake Trout this weekend and smoke them, so I can see if the fan improves the performance of my smoker.
