I decided that I needed some control over my fermentation temperatures. With 10 gallon batches, my fridge didn't have enough room, and besides, I didn't want all my refrigerated bottled beer to warm up each time I needed to ferment an ale. So I decided to build an extension box off of the freezer compartment and use a fan to circulate cold air from the freezer thru the extension box.
My freezer is a large side by side, with the freezer on the left side. The freezer actually has two doors, so don't let the pictures confuse you into thinking that it is an over/under refrigerator. Here are some pictures and descriptions of what I did.
First I built a box about 4' high by 4' wide by 18" deep. I made a basic frame of 1 x 2 lumber, then filled in the spaces with 3 layers of 1/2" thick polyisocyanurate foil faced foamboard, staggering the joints. I glued the layers together using liquid nails. I added another layer of foamboard on the outside that covered the stick frame. Then I faced it outside with 1/4" luan plywood, and inside with 1/8" tileboard, which is a hard masonite coated with a slick white tile-like surface. The door is built just like the box, and there is a magnetic gasket from an old refrigerator door fastened to it. The door opening is 1" smaller than the gasket on all sides, and I epoxyed metal strips to the face of the door opening for the magnetic gasket to stick to when closed, just like a fridge. The hinges are wooden blocks with 1/4" steel pivot pins inserted, with matching holes in the door frame. The bottom hinge is screwed & glued to the luan, the door was set on it, and the top hinge installed, using screws. I was sure to place a frame piece so that the hinges would be screwed into the frame, not just the 1/4" luan.
Overall view of extension box
Connection to the freezer. Duct is wrapped with insulation & duct tape (what else?)
View of the inside. You can see the duct fan at the top, and the baffle box underneath.
The extension box is connected to the freezer side of the fridge using two 6" pieces of 6" diameter galvanized duct; the supply duct is near the top of the chamber, and the return duct is near the bottom. The supply duct has a 120VAC fan in it, and was purchased as a unit (called a duct fan) at the local home supply store. I wanted to keep my refrigerator plugged into the wall so it would operate as a normal fridge, and the freezer would be around 0°F. This way I can keep bottled beers & serving kegs in the fridge while using the extension box to control fermentation temperatures. I did this by using a Johnson controller to switch the duct fan on/off as needed to circulate freezing air thru the extension box.
Upon the first use of the box, I found that the temperature got too low in the box. This was because when the fan is off there was nothing preventing natural convection to occur, which allowed cold air from the freezer to migrate thru the return duct into the extension box, warming & rising, and returning to the freezer thru the duct fan.
To correct this, I tried hinging a flap over the return duct inside the freezer, but the fan wasn't strong enough to push it open when it turned on. So I made a wooden baffle box that would force any air entering the box at the bottom to move up then down thru the baffle to get back into the freezer. This corrected the problem, and now I can set the temp anywhere between 65°F & 32°F and it will hold within about 3 degrees, even when outside temps are in the 90's. My brewery is in a basement garage, which is NOT air conditioned or heated.
Here is a drawing showing the baffle that I added.
The fan pulls air into the cold box at the top. When the fan was not running, cold air from the freezer would 'fall' into the cold box thru the bottom opening, keeping the temp too low. That's when I decided to make the baffle which effectively prevents this. The baffle is basically a box with a 1 x 2 frame covered with 1/4" plywood both sides. Halfway between these sides is another piece of 1/4" plywood, dividing the chamber into two 3/4" x 6" x 24" chambers. It seals against the bottom, but stops about an inch below the top. When the fan is off, this baffle prevents a natural convection from occurring. A cross-sectional drawing is at the right: