Trouble with Johnson controller

Just got a new 5.0 cu chest freezer for a fermentation chamber, and I've got the analog johnson temp controller hooked up to it. The fridge keeps temps very constant, it was at 65 ambient for almost 2 days without turning on the compressor. However last night the controller turned on the fridge, and when I checked this morning, the temp was at 54. I have the controller set at 63.

Also, I've noticed that the fridge will get up to 68 before the compressor kicks in as well. What's going on here? I know directions tell me to turn the fridge to the coldest setting, but I'm wondering if that's just too cold. I have two thermometers to double check that I'm getting the right reading.

The problem is that there is no air circulation in a freezer like there normally is in a refrigerator. Freezers are designed to run at sub-zero temperatures continuously where this would be of no concern.

Here's what's happening. The compressor turns on at your 63 F setpoint and runs until the sensor detects a 3 degree drop and shuts off at just under 60 F. The cooling occurs in the walls of the freezer where the evaporator coils are located. These coils and the freezer walls cool to much below 60 F by the time that the air temp in the freezer falls to 60 F. The air continues to cool even though the compressor is not running because the walls are still very cold and will stay so for a considerable length of time. Eventually, the air will warm back up to 63 F and the cycle will be repeated. Also, the air temperature in the freezer will typically be much warmer near the top of the freezer than at the bottom. The difference can be as much as 15 F.

The solution is to install a muffin fan in the freezer to circulate the air. I have my fan mounted to the lid of the freezer blowing downward and the controller probe is mounted directly under it and in the air stream from the fan. This arrangement help maintain even temperature throughout the freezer compartment.

Oh yeah, do keep the freezer set to it's coldest setting. You want the controller to do all of the controlling on it's own without interference from the freezer internal thermostat.

Thanks for the info! This makes perfect sence. Do you keep your fan running constantly?

Yes, I run the fan continuously. The fan only consumes 22 watts, so it costs about 5 cents per day to run. I like that it helps keep the beer lines, shanks and taps colder. Previously, I would sometimes have problems with kegs icing up in the bottom, yet the kegs and lines weren't cold enough. The fan solved all those problems. It's an easy mod and inexpensive too.

Trouble with your Johnson, huh. They make blue pills for that....

I installed a tangential flow fan in my chamber. I has a flat, slim profile, and ideally fits in the corner so it is out of the way. My fan is wired into my Love controler so the fan is on only when the compressor is on. That was mostly because it was easy to do. On all the time is fine too

I've tried it both ways with the fan running intermittently and full time. IMO, running full time works better as it keeps the beer lines, shanks and taps colder and the temperature swings are very close to the differential setting. I have the digital Johnson controller and set the differential at 5 degrees. That's been working well, but I can't say with any certainty what the optimum setting should be. I do think that a bigger differential is probably easier on the compressor. I think it's a trade off between power consumption and temperature uniformity. The keg/beer temperature is very stable even with a 5 degree differential setting. I'm more concerned about the lines and taps warming excessively and the ambient temps have a lot to do with that.

You don't need a fan at all,just tape the probe to the side of your fermenter and tape some insulation over the probe. I use a johnson analog controller for fermenting and with this setup my temps stay where I set them. The thing I found with mine is I have to set the controller 4 deg. cooler than I want the fermentation temp to be. I have a kolsch fermenting at 60 deg. and my controller set at 56. Experiment with your controller to find the temp differential and you'll be good to go. Hope this helps.