Most ales can be fermented at room temperature, which is usually about 70-75 degrees F. But did you know that most breweries ferment their ales at no higher than 65 degrees F and some at 55-60 degrees F? Why? Because you get a smoother, cleaner tasting beer at lower temperatures. And lagers need to start out fermentation at 60 degrees F or so, but need to be slowly brought down to just above the freezing point. And if you would like to serve your kegged (or bottled) beer at "cellar" temperatures (50-55 degrees F), your standard refrigerator won't usually maintain a temperature this high.
How do we solve these temperature control problems at home? The best way is to use a separate refrigerator or freezer with an external temperature controller. This is a device that overides the internal thermostat in the refrigerator or freezer and allows us to set the temperature more precisely and in the range we need.
To understand how the external temperature controller works, you need to understand how
a refrigerator or freezer works. Basically there is a compressor that pumps cold
refrigerant through some cooling coils. These coils then cool the air inside the
refrigerator or freezer. Most people are not aware that a standard home refrigerator has
only one set of coils and these are located in the freezer compartment. A fan blows some
of the cold air from the freezer into the refrigerator portion to keep that section cool
but not frozen. A freezer, of course, just has one set of coils to cool the enitre unit.
Inside the refrigerator or freezer is a thermostat. When the temperature inside the
compartment is above the setting on the thermostat, a switch is closed which applies AC
power to the compressor, causing the air inside to cool down. When the temperature gets to
the thermostat's setting, the switch is opened shutting off the compressor. Since no more
cooling is taking place, eventually the temperature inside will rise again causing the
switch to be closed, turning on the compressor and so on.
An external temperature controller works in the identical fashion, in fact it is a thermostat. It has a plug which plugs into a wall socket, a socket where you plug in your refrigerator or freezer and a temperature sensor that goes inside the refrigerator or freezer. You turn the refrigerator or freezer's internal thermostat to its coldest setting (so it's basically always on), set the temperature that you really want using the dial on the external controller and it takes over, turning the power to the refrigerator on and off just like the internal thermostat would do. If you're concerned that this will be hard on your refrigerator or freezer, don't be. It is doing exactly the same thing as the internal thermostat would do - in fact it will run less.