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Old 06-14-2010, 12:23 AM   #1
ThreeTaps's Avatar
Aug 2009
Pacific Beach, CA
Posts: 520
Liked 7 Times on 7 Posts

Hey all,

So, my last A/C unit on top of my ferm chamber started to freeze up every time, so I bought a new one today. It's a mechanical 5,000 BTU one, which seems to be working great, but the compressor kicks on and off frequently. It hasn't gone below 64^F ambient inside the ferm chamber all day. From what I know about the differences between ambient and wort temps, this should still be OK for the majority of brews, right? I understand that big beers or real active fermentations may need extra cooling, but short of re-wiring the unit so the compressor is always on (something I don't really want to do, just because I'm lazy and can't find a wiring diagram), I'm thinking that it should be OK.

FYI: I have a temp controller with a probe inside a 1L bottle of water, so if I need to do re-wiring for better results, it's not a problem. The temp controller is set to 62^F.
Justin H.
Brew Blog: Three Taps Brewing
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Old 06-14-2010, 06:01 PM   #2
Jan 2010
Minneapolis, Minnesota
Posts: 555
Liked 7 Times on 7 Posts

From what I understand, having the probe in water isn't the best option. The ambient temperature in the chamber could move way above the desired temp before the bottle of water raises to a point where the A/C will kick on...and be cooler ambient before it turns off. If you attach the probe to the outside of your fermentor with some insulation/paper towel over it you'll more accurately judge the fermentation temperatures as wort temps can be up to 10˚ higher than ambient temps. I'm working on my Son of Fermentation chiller right now.

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Old 06-14-2010, 06:05 PM   #3
strat_thru_marshall's Avatar
Mar 2010
Oklahoma City
Posts: 1,641
Liked 31 Times on 25 Posts

I had to open up the front grill of my window AC unit and re-locate it's internal temp sensor. It kept causing the unit to cycle the compressor on and off, basically defeating my Johnson digital controller that it is plugged into. I could not get below about 65 degrees without doing this.

I removed the temp probe from the front grill and ran it out the back of the AC unit into the open air. No re-wire needed, I just unclipped the probe and ran it out the back of the unit instead of the front. Now the AC always thinks that the temp is my ambient garage temp, so whenever my digital controller turns it on, the compressor runs full blast until the controller shuts it off. Basically tricked the AC unit into thinking it's much warmer than it is, so it never turns the compressor off.

I can hold fermenting wort in the 40's with it now if i want to!

Also, alot of mechanical AC units have a set screw inside on the thermostat that you can lower or raise the operating range. Try backing that screw all the way down to make the unit run cooler.

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Old 06-14-2010, 06:28 PM   #4
david_42's Avatar
Oct 2005
Oak Grove, Oregon, USA
Posts: 25,599
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Very cleaver strat. I lucked out when I purchased my A/C. I bought the smallest one there and its digital control can go down to 50F. Plus is has a 'mixing' cycle, where is kicks in the fan for a few minutes every hour, even if the temperature is okay. That prevents any hot/cold spots.
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