What about fermenting 3 gal batches in 5 gal carboys?
I would like to know your opinion about what could go wrong in fermenting 3 gal batches of beer in 5 gal carboys. I'm intending to do this because I don't want the krausen to get out of the air lock and all that kind of problems I have had sometimes when there's a lot of krausen forming, I know this is depending on the style, but with stouts I have allways had this problem.
I've fermented 3 gallon batches in 7.5 gallon buckets as well as a 6.5 gallon carboy. It was only primary though and I tried to keep it fairly short (2-3 weeks). So far I haven't noticed any problems despite the massive head space.
Excess headspace in the primary is a non-issue... in fact, there may be some added benefits, one of which includes a thicker/denser 02 blanket to protect the beer. Excess headspace in the secondary however is certainly an issue.
I could see it being a problem in the secondary if the batch is pretty much fermented out, but there's so much CO2 being pushed out the airlock it must be completely blanketing the wort. I could see if you move your containers around a lot, but if you don't invade the space I don't see a problem.