Beer Forum

Hey all,

I poked around a bit and could not find a clear answer. I have an analog Johnson Controls Thermostat. I have the fridge at max temp and the probe is taped to the fermentation bucket with bubble wrap over it to insulate. I am getting large swings in temp. The manual states there is a +/- 3.5 temp diff. However, setting the thermo at 68, I see a range of 72.2 - 61. I am using two thermometers to validate. One digital that is used for cooking. It has an external probe and I taped the probe to the fermentation bucket just like the thermostat config. The 2nd thermometer is your standard boiling type. Both are reading close to each others temps. It appears they are accurate to +/- 2 degrees of each other. So my question is do I have a faulty thermostat? Or should I set it at 68 and let the thermostat do its thing and ignore those thermometers?

My first thought was that you were reading the ambient with your therms, but I see that isn't the case... what time frame are we talking that it's swinging those 11 degrees according to your other probes? Are some of your probes on the 'cold side' of the fridge?

Every cooling cycle. Temp is set to 68. Temp rises to 72ish, fridge turns on. Temp drops to 61ish, the fridge turns off. The thermometer with the probe is taped to the fermentation bucket just like the thermostat probe is. The other thermometer is sitting on top of the bucket.

With that said if I am getting ambient temps, am I safe to just set the thermostat to 68 and that gives me 64-72 range if there is a +/- 4 degree swing?

I found an old thread where people leave the probe in the open air and set the thermostat to 4-5 degrees below the desired fermentation temp. Then use a thermometer on the fermentor to monitor its temp. Is this the best way to set this up for best accuracy? Wouldn't the thermometer on the fermentor be reading ambient temps?

This problem has been one of the reasons I have held off on buying a temp controller. If the temp controller is only cycling at +/- 4 degrees, it seems as if it would not be particularly useful since the whole point is to measure the fermentation temp rather than the ambient temp.

skibikejunkie wrote:This problem has been one of the reasons I have held off on buying a temp controller. If the temp controller is only cycling at +/- 4 degrees, it seems as if it would not be particularly useful since the whole point is to measure the fermentation temp rather than the ambient temp.

Agreed. I am having a tough time finding how to accurately setup this type of system. Unfortunately, I dont have a cool area to ferment in and I have to go this route.

I wonder what would happen if you used a thermowell and your johnson controller? Would you still get those crazy swings? I have never had an issue using my ranco digital temp controllers.

brewinhard wrote:I wonder what would happen if you used a thermowell and your johnson controller? Would you still get those crazy swings? I have never had an issue using my ranco digital temp controllers.

Thanks for the reply. The thermowell will not work with the JC thermostat as the probe is too large. Without knowing the temp of the wort fermenting, I am not really sure how to react at this point. The thermometers I have are sitting on top of the bucket or I have the probe taped to it. I am questioning their accuracy. Time to maybe get a fermometer strip which is supposed to give an accuracy of the bucket contents within a few degrees. Then just ignore the settings and make sure I dial it in to keep that fermometer 65-68 degrees?