Building a Warmbox for Fermenting on the Cheap




Here's a quick pic-based wrapup of how we built a warmbox to keep kefir and sprouted brown rice warm during the winter.

Step 1:

We started with a steel rack so we didn't have to build a box.  First, we removed the upper shelf and inverted it so we didn't have to work around the internal bolts when making our measurements.  Next, we cut the upper and lower sheets of rigid foam insulation.

Step 2:

Inserted the back first, then the sides.

Step 3:

Figure out what wires you need and where you want them, and route them before you attach the sides and back panels!  If you don't, it's not the end of the world.

The base is just a sheet of plywood inside a heavy garbage bag.

Step 4:

We were going to do something fancy with the temperature sensor but this worked just fine instead.  I recommend having the sensor in direct sight of the heat source--otherwise it will be getting indirect heat while your warmed objects are getting direct heat.

Step 5:

Behind the lips of the top and bottom shelves we inserted a length of wood.  The one on the bottom shelf holds two L-hooks where the cover sits, as well as four webbing straps.

The top one just has four velcro squares stapled on for the straps to attach to.

Step 6:

The cover is a length of the rigid insulation covered with corrugated plastic.

Step 7:

We got some soft foam windblock material at the 100 yen store.

Step 8:

The foam insert allows the 2nd chamber to maintain a lower temperature via passive heat.  That small gap allows it to stay at 18 degrees or so with the main chamber set to 27 degrees, with an ambient temperature of 8-15 degrees.

Step 9:

That's it!  The temperature controller is an STC-1000, which is inexpensive and can be sourced easily online.



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    11 Discussions


    6 years ago on Step 9

    Why not use an electronic BBQ temp probe directly in your fermentables? They even have ones with remotes viewing modules that you could just Velcro to the outside of your box. Or you could use a thermostat that switches off the light when it gets too hot. I would have to make one somewhat larger than this, because I make 6-8 gallon batches, but all in all, nice instructable!

    2 replies

    Reply 6 years ago on Step 9

    The problem with putting the probe in the food itself is that the food will react more slowly to temperature changes than the air around it, so the air will get too warm or too cold while the food maintains its temperature. That could cause the food to then get too warm or cold while the heat source catches up.


    Reply 3 years ago

    Also the acid in the brine will easy the metal on the probe


    5 years ago on Introduction

    Is it possible to get more info of how to setup the temperature controller & the switching mechanism? I'm planning to do something different but utilizing the thermostat control. This is my first time doing something like this, so any info would be helpful. thanks.

    2 replies

    Reply 5 years ago on Introduction

    You can google the part number for setup instructions for that particular model. Homebrew sites should have some good ideas if you're going to be using a different thermostat.


    6 years ago on Introduction

    This would be very helpful in making sake, if you look up home brew supply stores you can find temp. controlllers.


    6 years ago on Introduction

    I like it! Lots of food for thought. I have been kicking around building a proofing box for sourdough, this has added to my ideas. Very nice job.

    1 reply

    Reply 6 years ago on Introduction

    apamplona, thanks for mentioning this. I mistyped--it's the STC-1000! Going to edit the instructable too. Good luck!