Fermentation Temperature Control

All the brewing resources out there put heavy emphasis on fermentation temperature, and for good reason.  Yeast are actually making your beer, after all, so it's important that the conditions of fermentation are optimal for the yeast to perform as we want them to: adequate attenuation without off flavors or aromas.  Many factors contribute to this, including pitching rate, oxygenation, wort composition, and water characteristics.  But by far the most important factor, and the one homebrewers should get control over first, is temperature.

There are a lot of products out there that are designed to help homebrewers maintain proper temperature.  The products range from heating pads to carboy parkas to full-blown glycol jacketed conical fermentors.  All of these are probably effective, but it's also possible to get a handle of fermentation temperature with just a few inexpensive items from your local hardware store:

A plastic tub:  $5-10.  Fill this about 1/3 full of water (I use leftover sanitizer from the end of brew day), and set the carboy inside.  Surrounding the carboy with water increases the thermal mass which has the effect of slowing temperature changes.  The water also serves as the medium for temperature variation by adding cold or hot objects.

A dozen water bottles: $5ish.  Store these in the freezer.  To cool the fermentor, add 4-6 frozen bottles and cycle them between the tub and freezer every 12 hours.  Even in the hottest summer months, you can keep your fermentation temps in the mid to low 60s using this method.

Submersible aquarium heater: $20.  Use this to warm up the fermentor if your ambient temps are cool or towards the end of fermentation if you want to ramp up the temperature to maximize attenuation.  These things aren't exactly accurate, but I can't think of an easier way to heat of a tub full of water (up to 80ºF).  

Infrared thermometer: $20.  Of course, you'll need a way to read the fermentation temperture.  This is a cheap and fairly accurate tool.  Use it to check the temperature of the tub water, adding a few degrees when fermentation is active.