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epateddy 08-13-2012 11:57 PM

Can't get fermentation chamber temp down
Hey all.

Got an Octoberfest fermenting in my homemade chamber at 56 degrees. Johnson controller is set at 52. Can't seem to get the temperature down.

Here's the setup: Got an old fridge and built a box around it with doors on the front - like most others have done. The inside of the box is insulated with two layers of 3/4" Dow Tuff-R foam insulation. (Assuming my R-value is ~10.) Basement temp is 68 degrees. Sensor is taped to the outside of the carboy and holding at 56. Fridge seems to be running 24/7 and it won't go any lower. The ice tray is already building up with tons of ice.

Any ideas? Is the heat byproduct of the fermentation keeping the carboy temp up?


day_trippr 08-14-2012 12:18 AM

If that beer is in the throes of primary, it can indeed try to run a good 5-10F above ambient, given the chance. So it's reasonable to believe it's keeping the compressor running.

The frost in that freezer section is killing your efficiency. I suspect the frost will be a big problem to solve. You might at least try sticking a 120mm computer fan in there, maybe it would help...


epateddy 08-14-2012 12:29 AM


Funny I thought that ice buildup would help, throwing more, below freezing air. I have a fan I was ready to put in my keezer. Maybe it's better used here? And let's assume the internal temp is running just 5 degrees warmer. I'm now above the yeast happy zone (48-58). Wonderful.

MalFet 08-14-2012 12:39 AM

With your probe taped to the side, you're getting a reasonably accurate reading on the beer, not just the ambient air. Lager yeasts don't tend to produce nearly as much heat as ale yeasts, in any case.

But, if you want to get that temperature down, as day_trippr says you've got to get that ice off the heat exchanger.

starman 08-14-2012 02:27 AM

How big is the chamber? The amount of cooling required increases with the surface area of the box. The fridge can only produce a finite amount of cooling, typically 100-150BTU/H. If you are picking up more heat from the surrounding area, you will never hold temp.

It's hard to tell from the picture but guesstimating would be 2' wide x 3' deep x 3'6" high of R-10 and a temperature delta of (68-52) 16. The original fridge looks 2'w x 2'd x 3'6"h and we'll guess it to be R-8 construction.

Heat loss/gain through a wall is surface area x temperature differential x U-value (or 1/R value)
Surface area of new chamber = 40 sq.ft. (7 sq.ft. is common with the original fridge, so no heat loss that side) x 16 degrees x 1 /10 = 64 BTU/H
Surface area of old fridge = 29 sq.ft. (minus the 7 sq.ft. of common) x 16 degrees x 1/8 = 58 BTU/H

My rough estimate has you needing 122 BTU/H and that completely excludes heat from air infiltration, what the yeast produces, and the fan motors inside the chamber. First double check everything is completely sealed, especially the door. Then you could consider increasing the R value of the walls or decreasing the surface area of the chamber.

epateddy 08-14-2012 03:22 PM

Wow, Starman. You blew my mind with that equation. Here are the specs (all outer dimensions).

Fridge: 33"h, 20w, 18d
Chamber: 36", 32w, 26d

Chipped away at as much ice as I could last night. Most of it is back this morning. Time to get the fan in there. Any suggestions were to place it? I'm assuming blowing air from fridge toward carboy. And just sitting down low? Colder air is at the bottom, right?


MalFet 08-14-2012 03:28 PM

If you are getting frost that quickly, you are probably leaking air pretty hard. Seal up your chamber better if you can, too. You want the fan blowing over the freezer plate.

Junkster 08-14-2012 04:17 PM

Yeah, any moist air coming in through any gaps in going to frost up the evaporator. Get the box sealed as tight as possible and vent your blowoff tube outside of the box if you can. Instead of chipping, try hot water from a garden sprayer to defrost it fairly quickly and wipe it down to get that moisture out of the cabinet.

epateddy 08-14-2012 05:24 PM

Fan installed. Already down to 53 degrees - which I haven't seen yet. Ice on the freezer coil (?) is disappearing as well.

AWESOME advice, folks. Thank you!

TimpanogosSlim 08-14-2012 05:28 PM

There are also dehumidifying devices you can get. Some are chemical based (open container, stick inside, eventually replace or refill) - some are powered and just need to be emptied from time to time.

And i second the advice to seal it up better, but that might not even be the problem. I only have evaporative cooling on my house, and in this heat wave it is running 24/7, so i have fairly high humidity in the house.

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