how long do you need to control your fermentation temp?

I'm getting into using some more specialty yeast. I'm wondering if it's critical to control your fermentation temperature for the entire duration of the fermentation. My thinking is that off flavors from high fermentation temps must come from one of two sources:
1) reproduction of yeast
2) metabolism of yeast.

After a couple of days of active fermentation, the reproduction should have slowed, since the there are already so many yeast cells in the wort. So (1) should not be a problem. Also the yeast should have consumed a lot of sugars already, and so there should be less off flavors from metabolism.

My fermentor is next to the air conditioner. The sticky thermometer says the temperature is 20C. Fermentation temp for the yeast is 15C-24C. What I'd like to do is leave it next to the air conditioner for a couple of days and then put it under the kitchen table (this is where all the fermentors / carboys go). The temperature will probably creep to the top of the fermentation range and maybe a couple of degrees higher. If it gets hot out and the temperature starts heading towards 80F (26C) I'll put it back by the air conditioner. What do you think?

My personal observation is that if I pitch the yeast towards the lower side, the temperature usually is ok, but if I pitch on the high side above room temp, it's not coming down on it's own and will stay up until the fermentation slows.

Ideally you should control it the whole time. this allows you to be much more consistent. but you're right the first few days are absolutely crucial. Most of the esters or phenols would be created then, so if you pitch low and keep it there for a while you should be ok. This all really depends on the yeast you're using. I can be 100% certain from personal experience that if you ferment too warm over even the first day you can get some bad results with phenols and esters.

80F might be a bit hot. But I think it really depends on the yeast. For some beers you want to rise it up to 80F, like a saison. So it's not fair to say yes or no in a general sense.

Also, lagers have a few other considerations, but it sounds like you're making ales.

Some background. I've learned the hard way about fermentation temps. When I started brewing I used the cooper's kits and the yeast that came with it. I think I pitched my first batch at 93F, and the next couple fermented at 85F range. I didn't have any noticeable off flavors (it probably did cause some fruity tones but nothing bad). I figure that all yeast worked basically the same. I tried to ferment with a Trappist Ale Yeast at the same temp and got bubble gum. Since then I've really tried to use yeast that match the temperature range I can control. I can do a lager in the basement during the winter, but in the summer I stick with ales.

I've got a wheat beer and I'm using Safbrew WB-06. The temperature range is 15-24C. Which means a max of 75F. It's 70F in my kitchen right now. I'm figuring 80F would be worst case scenario. I'm also thinking that after the fermentation slows there will be less yeast activity to drive the temp up. How much do you think the worst case would affect the beer? I don't want bubble gum again.

Also I really wish someone would have told me that Cooper's ale yeast can ferment at 85+ with minimal off flavors, but other yeasts can't do that. People tell beginners about all kinds of cooling contraptions. Often times it's much easier to control the yeast. I'm not going to spend three times as much or more on yeast if I don't think I can hit the temperature range. The cheap yeast will give you a good beer with possibly less then perfect flavor overtones, the expensive yeast at the wrong temp will give you a crappy beer. I wish someone would have told me that.

In any case, it's been a mild summer so far, and I think with the help of the A/C I can stay below 75. I just need to put the couch back before my wife comes home next weekend and I have one more batch to brew.

I carefully control them for the first week, after that I usually move them out to make room for a new batch. I'm convinced that the off flavors caused by hot fermentation will happen during the lag phase and the most active part of fermentation.

Brian, where do you live? It is not technically summer yet. sure, here in central Texas it feels like August, but, that just makes me worry about my fermentation temps more. It's been 98-100 for the past week, and it doesn't look to get too much better anytime soon. I like your idea about using yeast for the temps, I think the brew after next will be based on fermentation temps. Right now though, I am just throwing in lots of frozen bottles in water baths for the three beers I have fermenting right now.

I think you are right about the slower fermentation after the first few days not affecting the temps as much, but I think you should use some temp regulation method.

One more little thing. I don't know much, but I think the fermentation could bump up the temp from the ambient 70 to 75 or higher. I would think if you are approaching 80F it is time to put it back by the AC.

I live up in Indiana and we have minimal air conditioning. (Two window air conditioners: one in bed room and one in the living room).

The sticky thermometer on the out side of the fermentor reads 20C (68F). I used a temperature probe on the inside of the fermentor, just now and got 67.8F. The air conditioner is set at 66F. I'm kind of surprised that there isn't more of a difference between the sticky thermometer and the inside of the fermentor.

Actually only the first few hours of initial fermentation is where temp is crucial....but since those few hours can occur anytime...factoring in lag time, I would say making sure the temps are cool for the first week is your best bet..But I wouldn't jump into panic mode, if lets say it's been four days, and you forgot to swap out the ice bottles in your swamp cooler....one of the things about having your fermentors in a water bath is that rapid temp swings are slowed down.

In the summer, I like to get my wort down to about 60F with my IC, then pitch (usually Notty or S-04 or S-05). I keep the fermenter in a swamp cooler for 3 or 4 days, just to be on the safe side, then I keep it at ambient temps. for another 3-4 weeks (no secondary).

I'd best not let my wife see this. When I'm at work and she's home, I've had her swapping ice bottles every 4 hours for the last month on my APA.