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Temperature: Wort vs Room

The expert homebrewers in the room can scroll down to the chart and explanation below. As background for the rest of us, it is said that “Brewers make wort, yeast make beer.” and that “Happy yeast make good beer”. In a quest to keep my yeast happy, I have read lots of posts, gained some sound advice via the NB Brewmaster’s chat, and read the book Yeast. I’ve learned that, for the home brewer, the three most important things are sanitation, sanitation and sanitation. The forth is temperature. After that comes pitch rate and oxygen. Those last two are easily handled by a yeast starter and oxygenation. Temperature can be handled too, but it takes a bit more thinking and planning. Depending on where you are, and the time of year, it may involve either heating or cooling. Various heating and cooling methods are available.

I’ve learned that:

  • fermentation produces heat, so wort temperature runs above room temp
  • the yeast care about the temperature of the wort, not the room
  • although the yeast’s optimal temperature range may be wide, say 55 to 75 F (for a particular ale yeast), the yeast will be most happy if temps don’t vary by more than 2 or 3 degrees during active fermentation.

I’ve seen a few posts offering estimates of how much hotter the wort is than the room. Some say 4 to 7 degrees (F). Others say it can be more, still others say it’s only 1 or 2 degrees. It seemed to me that the temperature difference would reach some maximum at the peak of active fermentation and taper down to no difference once fermentation completed. Since some fermentations are very quick and active while others might progress at a more steady pace, it would seem that the size of the maximum peak is would probably vary according to how active the fermentation is.

To begin to find out, I bought a temperature controller and connected a window air conditioner in a small room holding my just brewed wort. The chart below shows the results.

I had pre-cooled the room to 65 then set the ac to max cooling to let the controller take over based on wort temp. The temperature controller was set to turn the AC off when the wort temp fell below 67.0 and on when it rose above 68.0. The brew was NB’s Bourbon Barrel Porter Extract Kit in a 6 gallon glass carboy with no external wrapper or insulation.

Obviously, I should have cooled the wort further at the end of the brew before pitching the yeast. I’m still learning, but I think the large initial temperature difference is simply due to my failure to cool the wort enough. I was surprised that it then took nearly 12 hours to bring the wort down 10 degrees in the air conditioned room. At that point, the controller turned off the ac, and the room temp rose much faster than the wort temp. A few hours later, the wort reached 68 and the ac came back on, driving the room temp back down. About 20 hours in, I changed the ac unit from max cool to a minimum room temp of 63 so it would run any time the wort was above 68.0 but would not cool the room below 63. As the chart shows, after initial cooling, the wort runs about 4 degrees F above room temp at 24 hours after pitch to nearly room temp at less than 72 hours after pitch. The fermentation appeared to be most active at around 24 hours. It was active and steady with several inches of foam on the surface although it never did reach up into the blow-off tube. “Your results may vary.”

For my next brew, I’ll do a better job of initial cooling and report back. Comments, suggestions and feedback are strongly encouraged. I’m here to learn.

One to two degree temperature rise with a low OG beer and moderate ambient temperature. Two to four degree temperature rise with a low OG beer and high ambient temperature. Low OG beer less than 1.046 and moderate ambient temperature around 67°F.

Can have triple trouble with a high OG beer and high ambient temperature.

Interesting read. This is why I like the swamp cooler. The thermal mass of the surrounding water keeps the temp of the beer more stable than just an air mass. I’ve done experiments with my swamp cooler and have never measured more than a 1difference in the water and the fermenting wort. But I keep my water level almost equal to the level of the beer, and it can be a PITA when you’re trying to keep it in the low 60’s and it’s 98 outside and mid 70’s inside. Definitely not a “set it and forget it” method like the AC unit!



The temperature rise depends on the vigor of the fermentation. So higher gravity beers will generate more heat than lower gravity beer, and beers fermented at higher temperatures will also generate more heat. Higher pitch rates and healthier starters will likewise contribute towards more vigorous fermentation, though not to the same level as the first two factors.

The fact that higher temperature will cause the yeast to generate even higher temperatures is something that I don’t think enough brewers appreciate. If you let the fermentation get away from you, it can get sucked into a positive feedback loop that results in lower quality beer.

And by the way, yeast are very happy to have higher temperatures to operate in. This is an exception to the “happy yeast make good beer” rule.


Nice to read

I like your setup. I’m considering doing something similar in a large closet for a fermentation chamber. I currently use swamp coolers. As mentioned above the thermal mass of the water helps reduce temp swings but it sure is a PITA and requires constant monitoring.

You are correct that you should chill the wort to optimal pitching temperature. That will make for a cleaner fermentation as well as an easier to control fermentation temperature. The first few hours/days are the most critical in producing good beer. You’ll find that keeping your wort toward the lower end of the optimal range will produce the cleanest fermentations. Then you can let it rise to 8 degrees or so for a diacetyl rest after 3-4 days for an ale. In my set up I just take the fermenters out of the swamp cooler and let them rise to the 70ish ambient temp.

I had a similar setup in a rental house once. There was a small concrete walled room in the basement with a window air conditioner. I kept the AC set to 60 degrees and my wort generally stayed in teh 63-64 range which is perfect for most ale yeasts.

I did a similar thing years ago with one exception. I didn’t like the fact that the AC unit would not go below 62F. I brew 20+ gallons of beer and my fermenters hold 10-12 gallons with head space, so they heat up quickly and need a very low ambient to stay cool. To fix that, I took the temp probe out of the AC unit leaving it attached but attached the end to a night light bulb. Then I used a Johnson temp controller to set the desired temp, and turn the night light on when cooling was needed. I was able to get the room (under the stairs) close to 50F without freezing out the AC unit. Basically, the controller and nightlight fool the AC unit into staying on longer.

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Very well said.

I think what makes this non-obvious to many brewers is this misnomer of “Happy” yeast. The natural “Happy” state for yeast is closer to 90 degrees. Just ask bakers. That’s when they’re most active. As Brewers we actually want the yeast to be “controlled” not “happy” because controlled yeast make better tasting beer than really active yeast.

That positive feedback comes from the active yeast’s “body heat” making the wort warmer, which brings the temp closer to their “natural” happy temperature, which actually allows them to be more active…


I’m not sure how big your fermenter is but cooling a whole room to cool a small fermenter seems quite inefficient. The most efficient would be an immersion chiller but that has some sanitation issues. Next would be a cooling jacket. After that a small freezer. A cooling jacket in a freezer would be most efficient. I’m talking lagers. Ales probabl a swamp cooler is pretty efficient . Next would be the jacket/ freezer. You get my point ambient is fine if you don’t have to actively heat or cool a room. Just my opinion though, I just hate sending money to the power company.

Thank you all for the comments. Please keep them coming.

This started as an experiment in wort temp vs room temp but I’ve learned more already. The comments by flars, rebuiltcellars, and jmck that wort temp can run away since higher temps can make the yeast too happy, especially for higher gravity brews, is a key point. I’ll sure work to start cooler and stay cooler in the future.

As for setup, in reply to frenchie, rebuiltcellars, dannyboy58, and mullerbrau and brew_cat, my air conditioned room is more of an initial experiment setup so I could easily measure room and wort temps … and keep the brew at the desired temp of course. I like the simplicity and low cost of the swamp cooler but I need something more set and forget. For a more permanent setup, I was leaning toward finding a used refrigerator, hopefully with a bottom freezer. mullerbrau has now got me thinking about building a small, well insulated, fermentation chamber in my basement. I brew ale in 5 gallon batches. I’m thinking a freezer is colder than I need. My basement temps are fairly stable but a chamber would need a little cooling in the summer and maybe a little heating in the winter. Space for 2 or 3 brews in secondary and one primary would be good. The primary probably needs more cooling than the secondary, at least for a few days.

I like the idea. I do 10 gal batches and would brew more regularly if I could ferment for than 2 fermenters in swamp coolers. I’d like to use bigger fermenters like mullerbrau’s. I’d like the set and forget aspect of it as well as higher capacity.

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