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Temperature controllers are used to control the temperature of a fridge or freezer for fermentation, or for converting a freezer into a keg freezer or 'keezer' to allow it to run at beer serving temperatures.
The Johnson Controls A421 (model A421ABG-02C shown in picture) is a popular industrial choice that has proven very reliable. It's what we use on our keezer. It includes features that avoid fridges or freezers from cycling too often (not good for the compressor). Over the years brewers have also seen less expensive consumer grade temperature controllers come to market, with the most popular models sold by Inkbird in China. We've used both and prefer the Johnson Controls models for their ruggedness and reliability.
To use simply set the fridge or freezer to the coldest setting, plug the fridge or freezer into the temperature controller, and the temperature controller into a wall outlet. Use the up/down buttons to set the temperature. The temperature probe placed inside the freezer or fridge now controls the temperature instead of the fridge or freezer thermostat.
More advanced temperature control units such as this Johnson Controls A421 allow for both heating and cooling control. Heating is used for fermenting above ambient room temperature.
Not suitable for controlling high powered heating elements.
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