Originally Posted by beerbelay
It seems as though it would be hard to determine the temp. rise. How does one estimate the rise in temp. and know how much to lower?
I use a stick-on thermometer and just anticipate that the temperature will rise a bit. There are some things you can do to minimize the temperature change.
For one, put the entire fermenter into a water bath. That helps "insulate" the brew so that temperature swings are much more gradual and minimal. If the fermenter raises a couple degrees, drop a frozen water bottle into the water to keep it cooler. It takes a LONG time for the temperature of 4 gallons of water and 5.5 gallons of wort to change temperature, so that will minimize fluctuations.
Also, many people pitch too warm. If you pitch the yeast into 75 degree wort, and fermentation begins, it isn't going to cool to 65 degrees until after fermentation is over. By then, it's too late.
Also, a "slow and steady" fermentation will rise only a couple degrees, but a very vigorous fermentation will rise up to 10 degrees. Then, it's a double whammy because the warmer temperatures will encourage the beer to ferment faster and harder. If the beer starts fermenting more wildly, you can anticipate that the temperature will rise quickly.
Brewing in the summer is challenging- but it can be done with just a few easy modifications. Some people in warm climates have dedicated fermentation fridges, or fermentation set ups (like son of a fermentation chiller). I find that just keeping the fermenter in a water bath works for me.