how many degrees F is your fermenting beer higher than its ambient temp?

ive heard 3, 5, and 10. i go by 5. back your opinions up with facts. this is very important and i think many brewers such as myself are a little unsure about this. do different yeast strains cause more of a rise in fermentation temp than others?? does pitching more yeast cause more of a temp rise as well? how about pitching healthier yeast as opposed to not as healthy yeast? does it differ from plastic, glass, and stainless?

 

Measuring the rise in temperature is determinant on many factors, like the size of your batch, the capability for the fermenting vessel to disseminate heat, whether the vessel is placed in water or is refrigerated, etc. I tend to see 3-5°F in a glass carboy at just under 5 gallons.

 

yes, this is for a 5 or 10 gallon batch.

 

+1

The starting gravity has a significant impact as well. Generally speaking, higher OGs result in higher fermentation temperatures.

 

...this is a question I too was wondering about and I'm now finding out about through experience. I've found that I need to keep my Rockstar cooler 5 degrees below my desired beer temp (using a Johnson controller). I recently put a Better Bottle with 5 gallons of wort in the cooler and set the controller for 58. After two days in there, the stick on thermometer on the bottle has consistently read 63F during full krausen. So I'm going to say that during fermentation you need the ambient temp at least 5 degrees cooler than your desired temp.

 

As to the question, I get about a 5 degree differential when the fermentation's really moving along, though that number can increase with a bigger beer.

 

Please keep questions technical and OT commentary out.

 

There's been a lot of threads and a lot of debate about this, because like you said in the OP, it's mostly about opinions.

I know Bobby_M (I think, at least) had some pictures showing his fermentation chamber with both the air temp of the chamber and the wort temp...

If I recall correctly it was ~7*F difference...

 

Not sure who I got the idea from (maybe jamil?) but...I use a mini fridge to ferment in and I put the temp controller probe on the side of my bucket, then a small piece of foam insulation, and then duct tape it. Seems like a pretty good way to get it as close as possible without putting the probe in the bucket...but take that with a grain of salt since I still consider myself a noob.

 

The temp. differential can vary depending on your set up. You'll have some people who will insist it is 5 F, others will say 10 F, and yet others will say no difference - and many will insist they are right - which might be the case for THEIR SYSTEM. One can even make it cooler than ambient air temperature. I personally strive to keep my fermenting beer at ambient air temperatures. This usually involves a large water bath and typically fans to keep the air moving so any generated heat is quickly carried away.

If you put your fermenter in a closed closet with no air circulation the temperature of the beer can easily be 10 F over the air temp. Your best bet is to go ahead and do your best to determine what the difference is for your system.

 

That's how mine is set up, too, but I also have a thermometer in the corner of the fridge, out of curiosity to see what the relationship between the ambient and fermentation temps are. The ambient temp is reading 5-6 degrees lower. For what it's worth, there's no fan inside this fridge, so air circulation is minimal, and I ferment in a Better Bottle. The fridge faces east.

 

East!! Man that beer is doomed. Everyone knows your fridge should face North to obtain optimal magnetic forces to ensure complete fermentation. Send it to me for disposal.

 

I've also found this to be a nonlinear problem. Higher fermentation temps mean more vigorous fermentation which releases more heat which increases temps. In other words, cooler fermentations have less temp increase than warmer fermentations.

 

This has been the case for me. If I can keep the ambient temp below 65 degrees my fermentation will be about 68. I've seen my fermentation temp rise more quickly than ambient temp when we have warm spells, and my highest temp was when I brewed a high gravity Russian Imperial Stout during the summer, it went off of my fermometer scale. I learned a good lesson then: Thou shalt only brew above 1.065 gravities during the winter.

 

As has been mentioned, the temperature difference depends on both your setup and, more importantly, how vigorous your fermentation is. The vigor of your fermentation depends on a number of factors including OG, pitching rate, yeast strain, aeration, ambient temp, use of yeast nutrient, etc. So each batch is probably going to be a little different. For example, a batch that I brewed last month was about 3-4 degrees warmer at the peak of fermentation whereas the batch that I brewed on thursday was about 8 degrees warmer than ambient temp last night. The main difference in the two were the yeast strain and the OG (1.049 compared to 1.065). Your best bet in controlling the temperature of fermentation is to do what has been suggested and use a probe attached to your fermenter.

 

Thermodynamically, fermenting 20 L of 15°P beer should release about 2600 kJ of energy as heat. If it's a steady-state release over 48 hours of actual active fermentation, that's 15 W. The temperature increase then depends on the surface area and the heat transfer coefficient. For a plastic bucket, the SA is ~0.55 m^2, and I'd assume an h value of 10 W/m^2-K (water -> plastic -> air). That gives a differential of 2.7°C, or 4.9°F. Which matches up pretty nicely with what I actually see for a fermenter just sitting in still air.