Spend some time with brewers, and eventually you'll hear the phrase, "Brewers make wort, but yeast make beer." If you want to make the best beer you can, you must give the yeast the proper environment. By controlling the temperature of the wort during the first 72 hours of fermenting, you can drastically reduce off-flavours. Most ales need a temperature of 20.0 to 22.2 degrees C to produce the proper flavours, while lagers need lower temperatures, from 7.22 to 12.8 degrees Celsius. You can make a fermentation chamber to keep your beer in the proper temperature range, using easy-to-find materials.
Determine how large the chamber needs to be to contain the fermenter, with several inches of extra space at the sides and top to allow cold air to circulate around the fermenter. Remember to leave room for the thermostat as well.
Plan two additional chambers on one end of the fermenter chamber. One will be a baffle for warm air returning to the cooling chamber, and one will be a cold chamber. The cold chamber needs to be large enough to hold two two-liter soda bottles.
Draw a scale plan of the layout to ensure you have enough room in the fermentation chamber and the cold box. See the References for plans for fermentation chambers.
Outline the pieces for the walls, floor, and lid on the insulation board with a marker and straightedge ruler.
Cut a piece of 1 1/2-inch thick extruded polystyrene foam with a keyhole saw or utility knife to make the floor of the chamber. If you can't find polystyrene foam in 1-1/2 inch thickness, layer the sheets together with construction adhesive to build up the proper thickness. Make the entire floor out of a single piece of foam to keep condensation from leaking.
Construct the exterior walls from single pieces of polystyrene foam, and glue them to the base with construction adhesive. Make the walls of the baffle and cold chamber in the same manner.
Cut 3-inch square holes at the bottom of the wall between the cold box and fermentation chamber, at the top of the wall between the baffle and the fermentation chamber, and at the bottom of the wall between the baffle and cold box. If the computer fan is a different size, scale the hole between the cold box and fermentation chamber accordingly.
Glue the computer fan in place over the hole between the cold box and fermentation chamber. Orient the fan so it pulls air from the cold box into the fermentation chamber.
Attach the thermostat to the wall of the fermentation chamber about halfway up the wall. If the thermostat has a liquid element, make sure the thermostat is level. Hook-and-loop tape works well for attaching the thermostat, since it allows you to level the thermostat after the chamber is in its final location.
Poke holes in the wall between the cold box and fermentation chamber, and in the exterior wall of the fermentation chamber. Measure appropriate lengths of wire to connect the fan to the thermostat and the power supply.
Connect the fan, power supply and thermostat in series using bell wire and wire nuts. Test the fan to ensure it blows air from the cold box to the fermentation chamber. If the fan runs the wrong direction, reverse the power leads. If the fan comes on when the temperature is below the setpoint, consult the thermostat instructions to determine which terminals should be connected.
Test the thermostat to make sure it switches the fan on when the temperature rises above the setpoint.
Cut a lid for the fermentation chamber out of the extruded polystyrene sheet.
Fill four empty two-liter soda bottles with water about 3/4 full and freeze them.
Place two of the frozen 2-liter bottles in the cold box and check the setpoint of the thermostat. Keep the other two bottles in the freezer for replenishing the cold box.
Place the fermenter in the chamber, and put the lid on.
Check the 2-liter bottles in the cold chamber every day or two. Replace any bottles that don't contain ice with the reserved frozen bottles. Re-freeze the thawed bottles so you can repeat the process as long as necessary.
The fan and thermostat won't work properly if connected to 110-volt AC power, and could create a fire hazard. Resist the temptation to use line-voltage power for this project. A mechanical thermostat with a bimetal strip or a liquid metal element works best for this project. Electronic thermostats usually won't work at all on 12-volt power. You can improve your fermentation chamber by adding an external LED or 12-volt lamp that will glow when the fan is operating. If condensation forms in the chambers, a container of absorbent from the hardware store will help control it.
Electricity can be dangerous, even if it's "only" 12-volt power. Be careful, and always unplug the power supply before working on the wiring.