Fermenting the Worts of Summertime Brews


It’s hot this time of year — too hot for healthy fermentation. But summer’s heat also increases the thirst, inspiring brewers throughout the ages to find easy, sometimes ingenious solutions to the need for summertime cooling of worts. This review spans the range of low to high tech solutions that can be easily applied at home to keep your beer fresh all year.

Originally Published By Robert McIlvaine, Jr. (Brewing Techniques, Volume 2, Number 4)

Mowing lawns, tending gardens, watching baseball games, and attending barbeques; summer is a busy season. It’s also the season many home brewers hang up their mash tuns and wait for the cooler weather of fall before they begin brewing again.

If you had a particularly good winter and stockpiled enough homebrew to last about three months, you’re all set. If you’re like me, however, there is no such thing as stockpiling homebrew. The alternatives are to brew during the summer or to resort to buying beer from a local purveyor (that’s cheating).

One problem with summer brewing is heat — too much for fermenting. During the summer months, temperatures climb into the 70s and 80s Fahrenheit (20s to 30s centigrade) and even higher, into the 90s and 100s (30s to 40s centigrade), depending on where you live. These temperatures spell disaster for fermenting worts, which prefer much more temperate environments (60–70 °F [15–20 °C] for ales and 50–55 °F [10–13 °C] for lagers). What can a poor home brewer do?


Refrigerators: As usual, the problem can be approached in many ways. The most common solution is the spare refrigerator. Because fermentation requires more precise temperature control than most refrigerators can provide, you will likely have to replace or override the unit’s built-in thermostat (see reference 1 for more details). Electronic controller units designed for air conditioners and commercial refrigerators are available from local industrial suppliers. For the electronically inclined, thermostats can be built from kits or from designs found in periodicals such as Electronics Now and Popular Electronics. Whatever you use, be sure the design includes a time delay that forces an off time of about 4 min, which is necessary to protect the compressor. 

Going underground: Many home brewers have insufficient room for an extra refrigerator or may not want the costs associated with its operation. Fortunately, other options are available. Brewers were probably the first to discover what alternative energy pundits call subterranium geothermal stability. In other words, the temperature 72 in. below the surface of the earth is about 52–55 °F (11–13 °C) year round. This means you can bury pipe in a trench in your backyard and circulate air or some other coolant through the pipe and around your fermentor. The ground provides a thermal sink that will help cool your fermentor. The bigger the area and/or mass you want to cool, the more of your backyard you’ll have to dig up.

If your spouse doesn’t want you to dig up the yard, terra firma offers other ways to use its temperature stability. If your basement happens to be below grade, there is a good chance the basement temperature is below the ambient outdoor temperature and is relatively stable. Here in New Hampshire, my basement averages about 55–68 °F (13–20 °C). Although this temperature is still high for lagering, it is adequate for many ale yeasts.

Ice is nice: During the 1800s the grandfather of the modern refrigerator, the icebox, was used in industry and home to maintain cool temperatures. The icebox is a very simple device: you simply put a large block of ice in the top compartment and it cools the bottom compartment. It offers no real control of the temperature, however, and as the ice melts, it cools less, which means you have to replenish the ice regularly. Back then, the ice used to stock an icebox came from Mother Nature. During the cold winter months, ice was cut from lakes and stockpiled in caves or sawdust-filled ice houses. Large breweries of that period used this same stockpiling technique to create lagering caves. Similar techniques can be applied, on a smaller scale, at home.

Modern insulating materials can be used to make a brewer’s icebox. The carbon-insulated icebox that your grandparents had would be about the right size for a 5–7 gal carboy. Unfortunately, the only place you’re likely to find one is in the local antique store, and usually antique means expensive. A more affordable option would be to build an icebox that combines modern insulation with old designs. Plans for these units can often be found in publications like Mother Earth News and Homestead.

Evaporation: If you need to drop the temperature only a few degrees, try evaporation. Because water evaporating from the surface of an object tends to cool it, a carboy covered with wet cloth will be cooled by the evaporating water. The amount of cooling depends on the surface area and the humidity of the air around the carboy. The larger the wet area and the lower the humidity, the more cooling that occurs. Cooling can be accelerated by moving air over the wet surface. Using a fan controlled by a temperature sensor could actually provide a small range of temperature control.

Another tool that works on the basis of evaporation is the clay wine cooler. You’ve probably seen one in fancy wine shops — a clay crock that’s just big enough to fit the body of a wine bottle. The clay is soaked with water, and when the wine is opened it is inserted in the emptied crock. The evaporating water causes the crock to stay cool, which in turn keeps the wine cooler than room temperature. A cheap alternative is the pottery planter. They are made of the same orange-colored, nonglazed clay and come in sizes large enough to fit a carboy. This technique won’t cool to lagering temperatures but may decrease the temperature a few degrees, if that’s all you need.

Springhouses: Ice and evaporation can provide cooling with a minimum of cost and effort, but Mother Nature provides other options like spring or stream water. If you are lucky enough to have a natural spring on your property, a springhouse can provide a very stable cooling chamber. Springhouses are usually built into the hillside from which the spring originates, creating a small cave that uses both the earth’s temperature and the temperature of the spring water to maintain a cool environment. If your option is a small stream, then you will have to rig up some kind of heat exchanger and piping to carry the heat away from your fermentor and deposit it in the stream.

High tech: Being a technophile, I couldn’t cover this topic without glimpsing at some new technology. The Peltier diode is a semiconductor device that generates heat on one side while heat is removed on the other. Arrays of these devices will actually provide a significant amount of cooling. The new beverage coolers that plug into the cigarette lighter of your car use the Peltier diode.

It is possible to assemble the Peltier diode array as a pail of a carboy cooling chamber. The Peltier diodes should be able to lower the temperature of a small, well-insulated chamber into the lagering range (see reference 2 for ideas). Bear in mind that the Peltier diode is not very efficient; it dissipates lots of heat (read energy) to gain only a little cooling, so it is a poor choice when large thermal deltas are needed or cooling a large mass is required. Nevertheless, it should provide some interesting possibilities for home brewers, especially when a few degrees can make or break the fermentation.

Cool Fermentation Bag:

We offer a great solution to beating the heat with the Cool Fermentation Bag.  This is an insulated bag big enough to fit your fermenter and numerous ice packs, which can cool the carboy down as much as 30°F below ambient!


This quick overview of a large topic leaves much to the imagination. The accompanying box shows sources of a variety of things like pumps, insulation, tubing, heat exchangers, circuit boards, and lots of other stuff to fuel your imagination. So, have a good time making and enjoying that lawnmower-season beer.


(1)   K. O’Connor, “The In-Laws’ Refrigerator,” BrewingTechniques 1 (3), 45–46(1993).

(2)   “Build a Solid-State Beverage Cooler,” Popular Electronics, June 1993.

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