Chapter 10 - What is Different for Brewing Lager Beer?
10.7 Maintaining Lager Temperature
Temperature controllers are very handy for use with a spare refrigerator to maintain a constant brewing temperature. They work by plugging into the wall outlet and plugging the fridge into it. A temperature probe is run inside the fridge and it governs the on/off cycling of the compressor to maintain a narrow temperature range. Here in Southern California, I use it to maintain 65°F in the summertime for brewing ales. Check your local homebrew supply shop or some of the larger mail order suppliers for one of the newer controllers. Some controllers will also operate a separate heating circuit (usually in conjunction with a heat lamp) for cold weather brewing conditions.
Meanwhile, my frozen Vienna lagered for 6 weeks at 34°F. I placed blocks of ice next to the carboy instead of relying on the refrigerator for temperature control. In fact, insulated Ice Boxes are a good way to control temperature for lagering. Because of the alcohol present, the beer actually freezes at several degrees below normal. Depending on the time of year and your ambient temperature, an insulated box is a very convenient way to lager. If it freezes, just warm it back up, swirl up the fermenter to rouse the yeast and let it continue lagering. My frozen lager went on to take first place in two separate contests in the Vienna/Oktoberfest category.