Beer fermentation is a temperature-specific process. Ales ferment best around 16.7 to 22.2 degrees Celsius, while lagers tend to ferment best around 40 to 50 degrees Fahrenheit. Lagers should also be cold-stored at temperatures close to freezing. Depending on where in the world you live, you may find it difficult to maintain these temperatures for the time periods required to ferment and age your beer properly. You can, however, convert an old, working fridge into a home brew cooler with little more than a refrigerator thermostat control.
Purchase a refrigerator thermostat control. You can buy these units at many brewing supply and hardware stores, as well as many through many online dealers.
Remove any unneeded shelves from your old refrigerator. If you intend to ferment your beer inside this refrigerator, you will need at least enough room to store your entire fermentation vessel. If you can put the shelves back in easily, you may want to keep them and use them when cold-storing your beer.
Turn the refrigerator's internal thermostat to its lowest setting. Turning the setting down will allow the controller you bought to fully adjust the temperature inside the refrigerator.
Place the thermostat's temperature probe inside the refrigerator. The wires for these probes tend to be thin enough to allow the door to shut without any resistance.
Plug the refrigerator into the back side of the thermostat plug, then plug the thermostat into the wall.
Adjust the thermostat to the desired temperature. Most refrigerator thermostat control units can adjust the temperature inside the refrigerator from about -1.11 degrees Celsius to about 26.7 degrees Celsius.
Always follow the specific directions for the type of thermostat controller you buy. Most brewing thermostats work in the same basic way, but many have slightly different set-up and operational procedures.