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This Month's Question: With winter temperatures here, how do you keep your fermenting beer warm enough?

Answer: It seems like in the LA basin that we are always fighting against fermentation temperatures that are a little too warm. Fortunately, winter weather offers us the opportunity to make beer without a lot of temperature control. One thing that can happen, though, is that a cold spell can lead to a stuck fermentation. Around my place in West LA, the temperature hovers around 60 to 65°F throughout most of the winter. Unless you are making a Scottish Ale, that is a bit too cold. Some of my biggest beer failures came from stuck fermentations.
I found the simplest solution was to take my 10 gallon insulated mash tun and use it as a water bath to hold my fermenter. Either a carboy or a bucket fits nicely inside. If you then fill the mash tun with water, it adds a considerable thermal ballast. Usually, you will find that the carboy is floating in the water.
During the initial fermentation, the wort evolves heat. Quite often, this works will to keep the beer around 70°F. I worked out a few numbers for the heat generation and concluded that a 5 gallon carboy produces an average of about 15 watts of heat during primary fermentation. This was just about right to keep the temperature constant.
To fine tune the temperature, it is very easy to add either warm or cold water. Keep a thermometer in the water and check it a couple times of day. You'll probably find that the temperature creeps up quite a bit on brew day if you didn't chill your wort all the way to room temperature. Drain out a bit of the water and add some cold water. Or, you and add a bit of ice. It will also be on the warm side, particularly on the second day of fermentation, when heat generation is maximized.
As the fermentation level drops off, the heat generation drops off too. Starting about day 4, and continuing into secondary fermentation, you might have to add warm water to keep the temperature high enough. When you add warm water, watch out for stratification layers which will result in the top of the carboy being warmer than the bottom. You can get the temperature uniform by gently lifting the carboy up a few inches and then lowering it back down. The water bath will swirl around and get thoroughly mixed.
To take your temperature control to the next level, you can buy a small aquarium heater. Look for one with a 25 watts rating. This is about the smallest one the local fish store has for sale. For best results, get one that can be fully submerged and place it at the bottom of the water bath. That way, the thermal gradients will naturally dissipate. I use a couple of bricks to prop up the carboy about an inch above the heater. If you mount it at the top of the water bath, you'll need to mix it up a couple of times a day.
One thing you should do before dropping it into the water bath is to check the actual temperature vs. the set point temperature. I've found that the little number on the thermostat dial is usually off by several degrees. I ran it over the full range so now I can dial it in for Scottish Ale at 65°F, Pale Ale at 68°F, Wheat beer at 71°F and Belgian Ale at 73°F.