I finally hooked a PowerSwitch Tail II up to my Raspberry Pi and Fermwrap. The RPi has DS18B20 temperature probes directly measuring the temperature of the wort by going down through the airlock.

For this particular yeast strain, Northern Brewer recommends its optimal temperature range as 66-74°F (18.9-24.3°C).

What I did was subtract 66 from 74 to get 8. I rather arbitrarily chose that the fermwrap should be turned on when the temperature is lower than the bottom 30% of the range -- (8 * 0.3) + 66 = 68.4°F, and the Fermwrap should be turned off when the temperature is hotter than 60% of the range -- (8 * 0.6) + 66 = 70.8°F ---------- This is 20.2°C and 21.6°C respectively.

I left the house for several hours and came back to find the brew at 74.8°F -- 0.8°F over. The Fermwrap was turned off as it should have been (I am assuming it turned off exactly at 70.8°F). Yet the temperature increased way more than I had intended it.

I figured that by cutting the Fermwrap off at 70.8°F, and given that the ambient temperature was dropping as it was getting later into the night, that the carboy's temperature would not have kept increasing.

What point should I cut the Ferwrap off? Is 60% of the scale too high?

Could the fact that I am surrounding the carboy and the fermwrap with a towel (to block sunlight) have anything to do with it?

2 Answers 2


You have to base the choice on what temp you want to ferment at, not necessarily an analytical choice in the optimum range. Different points in that range will create certain flavor profiles, all temps will make beer. The profile you want comes from experience.

You need to remember that the fermentation itself generates heat. I am sure that your temp setup is working fine. The yeast simply started getting active and took it from there. I do the same thing with a towel and my fermwrap. But I normally start lower than I want to ferment at.

The towel helps hold heat in obviously, but inorder to hold a temp you need to let heat escape too. Ideally you want to match the rate of input and the output such that you maintain a certain temp. Very hard to do when you consider that at first the only heat in is from the fermwrap. But as time progresses the fermentation itself starts generating heat with its own increasing then decreasing rate. All in all its delicate balancing act if you don't employ both active heating AND cooling at the same time.

  • This particular batch has been going for 7 days, so the yeast had already done the majority of its exothermic effects. I meant to hook my system up much faster but have just been able to get to it. Incidentally, do you think if I screwed up the programming and left the fermwrap on 24/7, would that pose any fire hazard given it is wrapped in a towel? I posed this question here: homebrew.stackexchange.com/questions/11758/… Feb 18, 2014 at 5:11
  • Hopefully over the next few months I will get an extra freezer and do this the right way by employing active cooling. Feb 18, 2014 at 5:13

Yes, wrapping the carboy in a towel will act as insulation and retain some of the heat. Fermwrap's do require some amount of trial and error to find their sweet spots. Keep in mind that while the Fermwrap is powered on, it is actively generating heat. Immediately cut the power, and while it is no longer generating heat, the wrap itself is still warm, therefor it is still transferring heat to the carboy/bucket. If the recommended temperature range is between 66°F - 74°F, I would start by setting my temperature controller around 67°F (assuming room temperature is lower than 66°F). If it never goes about 67°F, I'll raise it a degree or two until I hit the exact degree I'm looking for. The expectation that I'll have is that even though I've set it to 67°F, it will likely rise up to around 71°F given the fluctuation in heat.

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