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Question: Johnson Control

I have my johnson control set to a setpoint of 64 degrees with a diff of 1 degree. I have read the manual correctly this means that the unit should cool to 63 degrees and then turn off. My problem is that i go look at the fridge and the temp is 61 or 62 degrees. I would think that is wouldn’t be seeing these temps. Help

If your getting lower temps you should be heating and not cooling. Hook the controller to a mild heat source like a light bulb but shield your fermenter from the light. Since your ambient temp is right on the border line you’ll have to monitor the situation or go to a two stage controller.

What they call a differential of 1 works like what you and I would call a 2.
It keeps the temperature within 1 degree of the setpoint,
63* is within 1* of 64*, relay stays powered until it gets to 62*
If there was a differential setpoint of 0*, it would switch as soon as it changed 1*
So you set your setpoint 1* higher or just don’t worry about the 1*

hmm, that sounds interesting, but does make sense based on what i have seen. I moved the temp to 65, which by what you have said, i should not see a temp below 63. That, i could live with. Let’s see if that is the result.

It also depends alot on how your probe is hooked into the system. Maybe your probe is reading a cooler temperature within the chest freezer that doesn’t necessarily correlate to your fermenting wort’s temp. You also have to remember that when your chest freezer kicks on, it immediately goes into freezer mode, even a fridge for that matter, it begins to circulate very cold air. A good analogy would be when you bring a pot of boiling water to a boil, you say to yourself, I only want this to boil for 2 seconds, the burner gets so hot trying to get it there that by the time it does, it boils for a good minute after being completely shut off. Chest freezers and fridges often do the same thing in reverse. Not uncommon to see them take you a degree or two past where you have them set once all that thermal mass makes its transfer to the fermenting wort, even after the fridge or freezer has been told to shut off by the controller. Make sense?

No, that makes sense and I figured that would happen. The reason why it bothered me is that I could watch it hit target temp and the Johnson control would not turn off the fridge for a couple minutes. I wonder if it is simply because of the rounding of the numbers on the display…

Either way it sounds like it is ok, but i need a self help book because i want to look at my carboys all day long. This is my second batch so hopefully this will wear off soon.

Nope. That never goes away!

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