Controlling Fermentation Freezer Temp Swings

[jordandemoss]

I haven't done a ton of research on the forum to see if anyone has proposed or experimented with these ideas before but here it goes. I'm also not a scientist so I think I explained the concept in layman's terms pretty well.

Chest freezers with external thermostats are great for fermentation because they can easily accommodate several carboys to ferment lots of beer, and it's not all that expensive. A refrigerators design is better suited to the job but to purchase enough refrigerators to handle the same volume of beer that a chest freezer can handle would cost more and take up more space.

One of the biggest drawbacks of using freezers for fermentation is that they're not designed for that temperature range which can give you appreciable temperature swings during fermentation. Freezers are not designed to hit a precise target temp like a refrigerator, a freezer just needs to be freezing or below to accomplish it's job - freezers just have more of a brute force job than a refrigerator.

Here's some issues I found with using chest freezers for fermentation.

1. The way home brewers use chest freezers doesn't leave the freezer with that internal thermal mass that it's expecting, its typically full of frozen food and typically ours are empty until we put a carboy or two in it.
2. The wort is at ground water temp and your chest freezer isn't (typically) which means that the freezer needs some time to cool it down. This can be prevented.
3. There's a great deal of air exchange when you put the carboy in. The room/outside temperature air typically won't be the same as your target temperature.

Here's some solutions I came up with to reduce the temperature swings.

1. Give the freezer more internal thermal mass.

If you added a cinderblock or two on top of that internal hump of the freezer on top of where the compressor is then you can greatly increase the freezers ability to maintain your desired temperature. This is a lot like having a bunch of food in the freezer that's already frozen, or in our case a cinderblock that's already at the target temperature. The internal air heats up when you opening it, the cinderblocks will assist in rapidly cooling that new hotter air and in turn cooling the carboys you just put in. You could also place some of those concrete patio slabs along the bottom of the freezer as well to help get that carboy to the right temperature.

2. Just wait for your wort to equalize with the target fermentation temperature.

The greatest temperature swings you're likely to experience are when your freezer is jockying between turning on and off until the wort equalizes with the target temperature. Your freezer is likely to do this several times as your wort gradually cools because the carboy is heating up the internal air and the freezer as a whole. If you just wait for a few hours, or overnight, before you pitch your yeast then you will be able to avoid temperature swings. This may introduce the possibility of an increased wort infection rate by waiting to pitch your yeast but I think it would continue to be small or non-existent, assuming your observing proper sanitation practices. Again, I think adding in these cinder blocks will assist in quickly equalizing the wort with the freezer.

3. Over cool the chest freezer before putting your carboy in.

A few hours prior to placing your carboy in the chest freezer set the temperature to 5-10 degrees cooler than the desired fermentation temperature to deal with the air exchange and warmer carboy that you'll be introducing to the freezer. Once the carboy is in there you can turn it back to the desired fermentation temperature. I'm sure many people do this already but I think it's worth mentioning again.

I'm about to go and buy my first chest freezer and temperature controller. I'll do some experiments and report back.

 

[sfgoat]

Or just tape the temp probe to the carboy so it only reads the temp of the wort. Takes quite some time to change the temp of 5 gallons of wort using only moderate ambient temperature differences. Any temp swings will be negligible and you will work your freezer less.

 

[FuzzeWuzze]

This is why you build a BrewPi like in my signature...because it uses PID to properly shut off the freezer before it chills your wort past its set point.

Although as sfgoat said as long as you insulate the probe against the thermal mass it gets you to pretty much the same spot.

 

[jordandemoss]

This is why you build a BrewPi like in my signature

RaspberryPi is really cool but I was going for a basic, logical approach.

(homedepot.com)
Cinder Block: $2.41 x 2 = $4.82
Patio Block: $4.27 x 2 = $8.54

For a grand total of $13.36, and installation/setup time is not much of a factor.

Or just tape the temp probe to the carboy so it only reads the temp of the wort.

I didn't really get in the the different methods of probe mounting. This will get you a more accurate reading of the wort as opposed to just dangling it in the freezer. I was aiming for solution that went a little beyond probe placement.

Obviously a thermowell goes a long way and solves a lot of issues. I wanted to present a simple solution that anyone currently using a freezer for fermentation (which seem to be quite a lot) could put to use with very little effort to improve their freezer performance and fermentation outcome.

 

[alphaomega]

5 gallons or so of wort is a pretty hefty thermal mass as it is. Even so, adding more and pre chilling still could be a good idea.
Probe placement, as pointed out earlier, is key. Sure, you may undershoot if you want to cool to pitch temp first, but once you are at your desired temp, the swings should not be that bad.
The temperature in the freezer is secondary, it does not really matter if it cycles from ambient to freezing (it won't, but just for the sake of argument), as long as wort/beer temp is stable.

 

[bigdaddybrew]

Fill empty milk jugs with water to fill much of the empty space in the freezer. This will add thermal mass and stabilize the internal temperature of the freezer.

 

[jhay_x7]

I have a better solution. Put more beer in the freezer to increase thermal mass