Originally Posted by pjj2ba
I suspect there is some data out there somewhere. My gut feeling is that the amount of heat produced is fairly linear with respect to the amount of yeast. However, how this affects the temperature in the fermentor depends on a lot of other factors such a volume, fermentor geometry/surface area, and air flow, etc.
Some homebrewers report 10F over ambient and other only a couple degrees for a 5 gal batch. For the typical homebrewer, I suspect these differences are largely do to air circulation. If you ferment in a closed closet, the heat won't dissipate well. If you ferment in an open room, much of the heat can quickly be carried away
I think this is spot on.
I'm one of those who have personally seen 10F differences. But here's what else I have personally seen- in a cool room and a moderate fermentation, the fermenter might only be 2-3 degrees warmer than ambient. Say I pitch at 62. It seems like even in a 65 degree ambient room that it's more of a "slow and steady" fermentation and never gets much above 65. But in a warm pitch, and a warmer room, it seems like it gets hot fast. And then the hotter it gets, the more vigorous the fermentation which in turn makes it warmer inside.
I know that's not the least bit scientific, but that's been my experience. I've never had a lager go higher than a degree or two, even when fermentation has been active. But with a saison, look out! In both those cases, I'm certain it had to do with pitching and fermentation temperature, as the pitching rate would have been the same. Yeast go crazy at higher temperatures.