How cold can a mini-fridge ferm chamber get?

[Jonathan Daniel]

So I'm planning on building a frementation chamber using a mini-fridge. What I want to do is take the door off and then build an extended box off of the fridge so that i can fit about 3 carboys/buckets in it. I'll hook the fridge up to a temp controller, insulate the box and blamo! I've seen a few designs and how to's on this and i'm excited to get going with it. But here's my question:

I like to cold crash my beer. With the above set up and an Elitech STC-1000 Temp Controller powering the fridge, can I get it down to ~33/34 degrees F to cold crash? I guess I'm asking if I will be limited by the abilities of the mini-fridge, or if with a temp controller, it'll do whatever I set it to?

And to head off all the "get a chest freezer" comments...that's not out of the realm of possibility, but it's more of a cost factor...to get a chest freezer that will easily fit up to 3 batches kinda goes over the budget whereas buying a used mini-fridge for $50 and such keeps things a bit more comfortable. I got a buddy donating the wood, so it's really fridge, temp controller and insulation i need to worry about.

Thanks everyone.

 

[Gruel]

I think you could test this by disabling the built-in controller and measuring how low the fridge will go without the added heat load from the extension. If your fermentation chamber insulation is as good as the original fridge insulation, and you are making the total surface area a factor two bigger, then the fridge+extension can reach half the difference to ambient temperature that it could reach before (same cooling power, double the loss per degree, half the temperature difference).
So, if the fridge by itself could reach say 20F in an 80F environment, then after doubling the heat gain (because the surface area of the box to be cooled is now twice as large) the fridge+extension would reach 50F. If you can insulate your extension box really well, twice as good as the fridge, then you would increase the heat load only by a factor 1.5, and the fermentation chamber could reach 40F. Obviously these are made up numbers, but you can see that it depends a lot on how well that box is insulated.