An old refrigerator, a controller and a one-degree band
The best piece of brewing equipment in the garage cost me nothing: a short refrigerator that a neighbor was throwing out, which now holds one small fermenter and nothing else. Its own thermostat is switched to its coldest setting and ignored. Instead, a plug-in temperature controller sits between the wall and the fridge, with a probe taped to the side of the fermenter under a square of foam insulation so that it reads the beer and not the air. The controller also has a heating outlet, into which I plugged a small heating pad wrapped around the base of the fermenter, because a garage in January drops to 40 °F and yeast does not care for that.
I set the target at 66 °F for most ales, with a 1 °F differential, so the fridge kicks in at 67 and the pad at 65. In practice the beer wanders within a two degree band and no more. Before this setup, with the fermenter sitting in a water bath on the garage floor, I logged swings of eleven degrees between a warm afternoon and a cold night, and I am sure that is where a couple of my harsher batches came from.
Two things I measured once the system was running. First, active fermentation makes heat: on day two of a 2.5 gallon ale, a probe taped to the side read 66 while a sanitized thermometer dropped into the beer read 68. For a batch this small that two degree gap is about as large as it gets, so I set the target two degrees low during the first three days and ignore it afterward. Second, the fridge ran for roughly four hours total per day in summer and almost not at all in winter, when the heating pad did the work instead.
The problem I had to solve was short cycling. The compressor on an old fridge does not like being switched off and back on within a minute, and with a tight differential it was happening. The controller has a compressor delay setting, which I pushed to ten minutes, and the clicking stopped.
The setup also made my first lager possible: three weeks at 50 °F, then a slow ramp to 62 over four days for the cleanup, then two weeks as close to 34 as the fridge could manage. The garage in March could never have held 50 on its own. The beer came out clean and a little too bitter, which for once was a recipe problem rather than a temperature one.