Home Brewer's Corner: 4 Tips to Keep Your Fermentation Temperature Under Control

"IT'S GONNA BE A SCORCHER!" - That's the way the weather update starts on the news every morning of late. It's easy to forget that despite school being in session again, it's still very much, hmm, Summer. By the way, what's up with the start date of school over the last decade? I digress.

Back to this heat wave we're grappling with. For the average person, heat waves are at the least water-cooler talk, and at most an inconvenience, but for home brewers, it's a terrifying forecast that will wreak havoc on a batch of beer. Every time I see the temps rising, I can't help but pity the folks with rudimentary fermentation equipment, and how their latest batch might be faring in a hot closet somewhere in home brew land.

In case you aren't a brewer, and hence aren't cognizant of this problem, allow me to give you a brief explanation as to why rising temps could be catastrophic. Yeast, like most other living things, is most comfortable in a temperate climate - somewhere in the 65-75 degree range. There are strains that make exceptions, such as cold-fermenting lager strains, or Belgian-style strains that appreciate a little more heat, upwards into the 80s even, but for the majority of common ale strains, they like their weather like Southern California residents do: 70 year-round. Even without the influence of external temperatures, a fermenting beer generates is own internal heat during fermentation and can reach 85+ degrees quickly, even if the ambient temperature is moderate. Exceeding temperatures north of 80 degrees can lead to unwanted off flavors or a finish often described as "unclean". It can also lead to a persistent, unwanted haze in the beer, and promote the growth of unwanted micro-competitors, like wild yeast and bacteria.

An example of an infected batch of fermenting beer - bacterial "pellicle"
An example of an infected batch of fermenting beer - bacterial "pellicle"

Basically, it can completely ruin your beer.

Unfortunately, not all home brewers have the luxury of controlling the ambient temperature of the space their fermentors live in. Often times a garage, closet, or cabinet houses their carboys/buckets, which can make for a rather warm habitat. There are some things you can do to mitigate the summer heat's effects on your brew though. Here are 4 of our favorites:

Knock Out Lower Than You Typically Would

In a commercial brewery "Knocking out" refers to chilling the wort at the end of the boil/whirlpool while pumping it from the brewhouse to the fermentor where it gets inoculated with yeast. At home, most brewers do everything in one or two vessels, but regardless of how you chill your wort - an immersion chiller, an ice bath, or a pump - make sure you get it cooler than usual during a heat wave. Not only will it give you several degrees of extra wiggle room to rise, but it'll typically take the yeast a bit longer to really get going, and a less aggressive fermentation is going to produce less heat. Obviously your knock out temp depends on the fermentation profile you are trying to achieve as well as the tolerance of the strain, but aim for about 5 degrees shy of your target. Note: Generally speaking, the colder the temperature of the wort, the longer it'll take the yeast to get started. For example, a neutral ale strain like White Labs Cal Ale 001 states a recommended temp range of 68-73 degrees, but you can knock out at 65 and lets it free rise to your target with few issues.

Pro tip: Always use a starter or double pitch when using the cooler knock out method to prevent a stall early on in fermentation.

Wrap You Fermentor with an Insulated Blanket or Neoprene

a neoprene blanket over a glass carboy jar
A neoprene cover offers the best insulation.

This probably seems like the most basic solution, and it is, although it's also the one with the most limited application. insulating your fermentor will help prevent rapid spikes, but sustained heat is going to equalize eventually and then you better have a plan B. Most fermentations take between 4 and 7 days so reserve this method as a supplement to the others or when you only need to worry about a short temp spike.

Pro-tip: Grab a beat-up wetsuit on Marketplace or OfferUp and cut it up to fit your vessel. Then use clothes pins or stitch some velcro onto it to keep it in place. If you don't want the hassle, just get one from More Beer for a measly $30.

Drop Your Fermentor in a Tub of Water

This is a pretty basic hack but it's effective. The water surrounding the carboy acts as a buffer and insulates the wort or beer from the ambient temperature, slowing the heating process. Pick up an inexpensive plastic container, like a rope-handled storage bin or a trash can. Make sure its got a bottom wide enough to comfortably fit your carboy while leaving enough space around the sides for a high volume of water. It doesn't have to be very deep but you also don't want it so shallow that you cant fill the water line up past the halfway mark. Drop the carboy into the bucket and fill it with cool water. You may have to add ice to get the temp down. Just make sure your ground water isn't above the wort/beer temp. If your ground water is 90, for example, you are only adding to the problem. It's one of the most rudimentary methods available to cool things down, but it definitely works.

Ferment with an Immersion Wort Chiller in Place


This might strike brewers as an unnecessary risk, for contamination reasons, but it might be the most effective method of the bunch (short of buying a temperature controlled fermentor). I'll start by saying you have to do this in a bucket, not a carboy, due to the size of the opening required to immerse the chiller. Essentially what you'll do here is leave a sanitized wort chiller in the bucket during fermentation that you can use as a heat exchanger when things start to heat up. Here's how it works, you'll be taking a source of cold water and running it through the coils to cool the wort/beer to your desired temp. This is one of the most advanced techniques, so here are a list of considerations before you get too far down the rabbit hole:

  • This presents a high risk of contamination. Take the time to thoroughly clean and HEAT sanitize all the parts of your fermentor and chiller before exposing them to fresh wort. You'll need to make sure the chiller is in place and sealed in without an exposure.

  • You'll need an internal thermometer to know when to stop chilling. Alternatively, although less desirable, is to measure the temp of the outlet water to get an idea of the wort/beer temp.

  • Make sure there are NO LEAKS that will allow ground water or chill water to leak out of the coils and enter the fermentor. You need a sealed system.

  • Since the body of wort/beer nearest the coils will chill long before the surrounding liquid, make sure to stop chilling early and get a reading that you feel represents the average internal temperature.

  • Chilling too rapidly can shock the yeast, especially if you drop too low. try taking it down a few degrees at a time and allowing it to acclimate to its new environment.

Here are some graphics that represent the basic flow in both scenarios:

Pro-tip: If you really want to step this method up, you can have a cooler with an ice water bath that you re-circulate through the immersion chiller. This allows you to have more control and not be as wasteful with water, but it means you need a pump. Plus, like I said before, if your ground water is too hot, the chiller method won't work anyway.

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