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Johnson A421 Digital Temperature Controller

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The Digital Temp Controller operates by overriding the internal thermostat in a refrigerator or freezer and gives the brewer greater control over the fermentation. Precise control of fermentation temperature is one of the most beneficial things you can do to improve your home brew beer.

It is also ideal for controlling the serving temperature of your kegerator or keezer.

The digital controller has a digital display with adjustable differential. It can be used to heat or cool a fermentation environment. When hooked up to a refrigerator, it turns the compressor on when the ambient temperature rises above your programmed temperature; when hooked up to a heater, it activates when the temperature drops too low. A sensor probe monitors the temperature inside while the readout displays it in either Fahrenheit or Celsius. This is the ideal choice for lager brewers who include a diacetyl rest in their fermentations or for ales like altbier or Kolsch that require fermentation temperatures in the low end of the ale range.

External overriding thermostat with backlit digital display. High and Low Temperature Stop Values determine whether the unit is heating or cooling. There is no longer a need to open the unit to switch between heating and cooling modes.


  • Plug play - this controller is pre-wired, so you don need a degree in electrical engineering to get it up and running.
  • New! Backlit display for even easier-to-read display - this one speaks for itself, but when we say easy, we mean easy. 
  • New! Faster Programming with three-button interface
  • New! High and Low Temperature Stop Values determine whether the unit is heating or cooling.
  • New! Internal defrost settings
  • Fully detailed manual, excellent service and support
  • Features a wide setpoint temperature range (-40° F to 212° F). The temperature sensor is a 1.94 inch thermistor on an 8 foot cable.

Additional information

Support Documents Click here for documents
SKU 41944

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Johnson A421 Digital Temperature Controller

Johnson A421 Digital Temperature Controller