Guide to set internal chest freezer thermostat to >32F; Eliminate external control

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Typical keezer conversion instructions shown here on HBT and elsewhere indicate that an external temperature controller is required. However, this is not true. The frugal and patient DIYer can spend that money elsewhere.

The vast majority of chest freezers used for keezer conversions utilize a simple internal electromechanical thermostat. These thermostats are preset by the factory to operate with a typical range between -20F and 20F depending on the position of the user accessible dial. Keezer temperatures are in the 40F range, depending on taste. It is the inability to readily set the internal thermostat to keezer appropriate temperatures that drives the need for an external thermostat. Here is an example of a typical internal thermostat:



What is not widely known is how these electromechanical thermostats work, and that these internal freezer thermostats have a coarse temperature adjustment screw which can be used to widely adjust the temperature setting of the thermostat, even above freezing to keezer temps. The coarse adjust screw on the unit pictured above is in the 5 o'clock position relative to the fine adjust dial shaft. They are usually recessed within the unit and may be covered with tape. Some units have them on the side, but I have yet to find one without a coarse adjustment screw.

Electromechanical thermostats operate on the principle of the ideal gas law, PV=NkT, where P=Pressure, V=Volume, N=Number of gas particles, k=Boltzmann’s constant, and T=Temperature in Kelvin. The idea is that by sealing a quantity of gas in an enclosed vessel the variables V, N, and k are held constant resulting in the pressure of the gas being directly proportional to its temperature.

Freezer thermostats use a long hollow tube as the enclosed vessel that is stuffed inside the freezer wall, with a short portion extending into the compressor compartment where one end is connected to a pressure sensor. The pressure sensor works by movement of a diaphragm set against a spring. The important part is that to overcome manufacturing and material process variables and to allow use of the same design for different applications the spring tension is designed to be widely adjustable by manipulation of the coarse set screw.

From the ideal gas law we know that at higher temperatures equal higher pressure against the diaphragm. Thus, to adjust the thermostat to trigger at a higher temperature the opposing spring force must be increased. Increased spring tension can be felt as increased resistance when turning the coarse adjustment screw. Below is a simplified diagram detailing the principle.



Some trial and error can be required to get the setting correct, and this is where patience comes in as it can take several hours for the temperature inside the freezer to come to steady state. Quick coarse temperature measurements can be made with a dry thermometer, but since hysteresis can result in several degrees of error final measurements should be made with a thermometer with some thermal mass for final adjustment (i.e. a floating thermometer in a glass of water).

CAUTION: Since the thermostat switches line voltage to the compressor there is live electrical current inside the thermostat at all times when the freezer is powered. REMOVE POWER FROM THE FREEZER BEFORE ADJUSTING THE INTERNAL COARSE SET SCREW.

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pauljmccain

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I'm going to try this, thanks! So glad I saw it- I was literally just about to walk out to install my johnson controller on my in-progress keezer.
 

Reno_eNVy

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Subscribed! Awesome. I'm almost done with a chest freezer conversion, almost ready to attach the controller. I checked the compressor housing on the freezer and there is a little cover over where, I'm assuming, the internal controller and adjustment knobs are. Time for a bit of exploration.
 
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Glad to have helped. I haven't seen this information posted before.

Here's another thermostat with the coarse set screw on the side. This one was staked in place with some blue Loctite at the factory. It didn't take much to pop the Loctite.



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Jem

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Subscribed. This helps move the keezer project up from a budget standpoint.
 

carrotmalt

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So looking at the diagram, it looks like I'd be loosening (lefty loosy:eek:) the screw to get it above freezing right? In your experience, how many turns should I start off with for my first stab at 40 degrees? I'd love to not have to buy another controller for my keezer build!!!
 

ajwillys

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Excellent info! I already have a controller on my keezer, but I'm sure I could find other uses for it.
 

gregkeller

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Alright, so everyone talks about the accuracy of maintaining a temp with an external controller, seems the digital ones keep the temp accurate to within a degree or so, and the analong ones within 4 degrees. How accurate are the internal thermostats of a freezer? I'm assuming they aren't nearly as accurate, because it just needs to keep stuff freezing, so if there is a big variation in temps, then it's not a huge deal because stuff isn't going to be melting/defrosting. But when we want it to stay around 40, is the internal temp gonna swing too much? I'd imagine that even if the internal temp swings a few degrees the temp of the beer in the kegs will not because of the thermal mass of a few 5 gallon cornies. Any ideas?
 

JuanMoore

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Alright, so everyone talks about the accuracy of maintaining a temp with an external controller, seems the digital ones keep the temp accurate to within a degree or so, and the analong ones within 4 degrees. How accurate are the internal thermostats of a freezer? I'm assuming they aren't nearly as accurate, because it just needs to keep stuff freezing, so if there is a big variation in temps, then it's not a huge deal because stuff isn't going to be melting/defrosting. But when we want it to stay around 40, is the internal temp gonna swing too much? I'd imagine that even if the internal temp swings a few degrees the temp of the beer in the kegs will not because of the thermal mass of a few 5 gallon cornies. Any ideas?
What you're referring to is the hysteresis of the thermostat. With the external controllers you're able to set the hysteresis yourself to keep temp swings to a bare minimum. From what I gather most modern fridges have a hysteresis of 6-10F that cannot be easily altered. I'm not sure but I think freezers usually have an even larger hysteresis. IMO this is probably ok for a keezer where precise temp control is less of an issue, but not so great for fermentation chambers. As always, YMMV.
 

howabouttheiris

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Is this true of the standard Walmart/HomeDepot freezer?
Would think that a digital control with a thermocouple would be cheaper now than mechanical controls....
 

ajwillys

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I agree, hysteresis would be important for fermentation but in a keg its really not that important. If your temps swing from 37-43 degrees, 5 gallons of beer inside a steel keg is not going to change its temperature much, if at all. By the time the beer starts to noticeably warm or cool, the fridge temp is swinging the other way, counteracting it. The result is it will probably stay at a pretty tight average of the two extremes. Personally, I have mine set at 4 degrees with my Ranco, simply to reduce the cycling.
 
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Wow, this thread seems to have generated a lot of interest.

So looking at the diagram, it looks like I'd be loosening (lefty loosy:eek:) the screw to get it above freezing right? In your experience, how many turns should I start off with for my first stab at 40 degrees? I'd love to not have to buy another controller for my keezer build!!!
The direction you turn is dependent on the particular thermostat you have. It could be left or right. The diagram is just for the concept. Here is one way to find out; If the freezer is at temperature and the compressor is off, unplug the unit and turn the screw a few turns in each direction and listen for the thermostat relay to click on. When it does you know the direction you were turning when it went on makes it colder.

Would this work on refrigerators as well, to get them up to 65-68 degree range?
Absolutely. I used this method for a long time on my fermentation fridge (dorm style fridge) to ferment Kolsch during the summer.

Alright, so everyone talks about the accuracy of maintaining a temp with an external controller, seems the digital ones keep the temp accurate to within a degree or so, and the analong ones within 4 degrees. How accurate are the internal thermostats of a freezer? I'm assuming they aren't nearly as accurate, because it just needs to keep stuff freezing, so if there is a big variation in temps, then it's not a huge deal because stuff isn't going to be melting/defrosting. But when we want it to stay around 40, is the internal temp gonna swing too much? I'd imagine that even if the internal temp swings a few degrees the temp of the beer in the kegs will not because of the thermal mass of a few 5 gallon cornies. Any ideas?
One clear advantage of a digital thermostat is that you can set it where you want it without trial and error. The disadvantage is added complexity in wiring and cost. For my keezer once I've set the temperature I forget it. The ability to control "deadband" or hysteresis to 1 degree is unnecessary, IMO. The mechanical thermostats do have a several degree hysteresis. The reason is the manufacturer understands that several degrees of hysteresis prevents the compressor from rapid cycling thereby saving on the electrical bill and increasing the life of the compressor. Remember the volumetric heat capacity of air is about 1/3000 that of water so while the ambient air temperature in the freezer will vary by several degrees a corny (or even 12oz bottles) will vary relatively little. I see less than a degree of fluctuation in my keezer with a floating thermometer in a 12oz glass of water but around 5 degrees in the air temperature.

What you're referring to is the hysteresis of the thermostat. With the external controllers you're able to set the hysteresis yourself to keep temp swings to a bare minimum. From what I gather most modern fridges have a hysteresis of 6-10F that cannot be easily altered. I'm not sure but I think freezers usually have an even larger hysteresis. IMO this is probably ok for a keezer where precise temp control is less of an issue, but not so great for fermentation chambers. As always, YMMV.
I have yet to see one with a 10 degree deadband. My experience has all been right in the 5 degree ballpark. Remember, 5 gallons of wort has a very high heat capacity and will likely fluctuate only a fraction of a degree even with a 10F deadband. Think how long it takes to bring it up to boil with XX thousands of BTUs under it. And yes, of course, YMMV :)

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marzsit

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been there, done that... depending on the thermostat design and where the sensing bulb is located, the results can be hit or miss. i had an old whirlpool chest freezer that was re-adjusted to work between 35-46 degrees, it was fine as long as the ambient temperature of the room was kept between 60-70 degrees. outside of that range, it was pretty unpredictable and would get too warm or too cold.
 

newnick

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I'm guessing that when you reset one of these you want the fine control set in the middle of it's range? I've been needing to get a fermentation chamber to better control the temp of my beer, I think I'll try this.
 

MI_Craig

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Freezer coming tonight, going to have to try this. Thanks for the info!
 

blip01

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holy crap! A few twists of the coarse adjustment screw last night and after work today my chest freezer sits at a sweet 37 degrees!

too bad I had just ordered one of those aquarium temp controls off of ebay.

great advice! I vote this should be a sticky as it can save people $25-60!
 

hlumbard

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I have a cheap chi co dial temp controller on mine. I will be replacing with NOTHING this weekend! Thank you for the info! I will post back here with pics and updates.
 

wilserbrewer

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This is GREAT information...I would think by now you would no longer be the "assistant brewer"...nice posting. I have a commercial chest freezer that I salvaged from a supermarket that has fridge and freezer capability.
 

blip01

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Here's what the thermostat looks like on my Magic Chef 10cf chest freezer. I set the normal temp control dial to its midway point them used the coarse adjustment screw seen in the pic to bring the temp range up. Nothing scientific, I just screwed it in 4 or 5 full turns and let the freezer run overnight. Got pretty lucky. It was right at 37 degrees.

ForumRunner_20110611_110310.jpg
 
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Glad to see that this is working for others, too.

blip01: Thanks for posting a photo of the 'stat in the Magic Chef. That seems to be a popular brand for conversion.
 

GatorDad

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"cool"!!! :)

the level of expertise on this board never ceases to amaze me... :mug:
 

BangorBrewer

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Anyone have any idea what this looks like for a Frigidaire? I am mechanically stupid but a capable learner. Would love to do this rather than buy a temp control.
 

blip01

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BangorBrewer said:
Anyone have any idea what this looks like for a Frigidaire? I am mechanically stupid but a capable learner. Would love to do this rather than buy a temp control.
The thermostat unit on my magic chef just popped out of the front of the freezer. Once I pulled it out it was pretty easy to find the adjustment screw. Like Assistant Brewer mentioned, it was covered by a sticker.
 

blewis1984

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I actually used this method on a cheap kegerator I bought at Home Depot. I was having an issue with it not getting cold enough. Did a little research and found this same method on the MicroMatic forums. Just took the thermostat off, gave the screw a couple turns, and made a big difference.
 

blip01

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Update: everyone should give this a try if they're building a keezer. I finished my collar up over the weekend and since Saturday the temp has been holding steady at about 39-40 degrees with just the thermostat adjustment described by assistant brewer. No external temp controller!
 

JasonG

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So, after some searching, I found the screw that assistantbrewer was talking about. I tightened it some, no change. i loosened it some, no change. So i loosened it some more, then some more, then the screw came out completely!:eek:

I can see the spring mechanism swinging freely inside the "box" or relay or whatever. I can't seem to get the screw to catch the spring and so can't get the screw to tighten again. It seems like the freezer will still run even with the internal temp at 30 F.

Am I totally screwed? Is there any way to fix this? If so, can I assume that loosening the screw is not the right direction (since it is completely out and the freezer is still running at 30 F)? Any advice would be appreciated.

Of note to others - be careful how much you loosen this screw - apparently it can come out!

JG
 

emjay

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I'm confused... how long did you wait after turning the screw once, before you turned it the second time? And similarly, how long did you wait after that before turning it a third time?
 

austin_hubbell

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This is an awesome thread, just got a chest freezer today for free from a friend, was about to order an external and then I saw this. Thanks for the info!
 

JasonG

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I'm confused... how long did you wait after turning the screw once, before you turned it the second time? And similarly, how long did you wait after that before turning it a third time?
I was doing what assistantbrewer described in post # 17 of this thread - compressor off and freezer unplugged, then started turning the screw waiting for the compressor to click on. I never heard the click, but maybe it was just quiet or subtle and I missed it. I am going to have to work on it some again tonight. I can see where the screw needs to go, but I am not sure I can get it to screw back in again.

JG
 
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