2.5 gallon batches in 6.5 gallon buckets?

Just wondering if it was a generally bad idea to ferment a 2.5 gallon batch in a 6.5 gallon bucket?

I'm surprised no one has commented on this yet. I'm hoping others will chime in. I think as a primary, you might be ok, but with that much head space, I'm not 100% certain. I honestly wouldn't use a fermenter that big for 2.5 gallons of brew.

I've never tried fermenting anything with that much headspace so not 100% sure but I think I'd go with a 3g carboy instead.

It is fine for a Primary. A lot of great beers are done in open fermenters. Just don't leave it in there for a long time as once active fermentation is over.

so how about buying a 4 gallon PET water jug at home depot for around $5 and using that? I saw them there the other day for that price. Is PET plastic safe?

B3 sells PET carboys: http://morebeer.com/search/103701/beerwinecoffee/coffeewinebeer/Plastic_Carboys

although they do say it's "special" PET. but sounds like it'd be fine.

in response to the question at hand, it's my understanding that 2.5 gal in 6.5 gal is fine for primary, but not desirable for secondary. so as long as you ferment and leave it alone, there should be a layer of CO2 protecting the beer from oxidation.

I asked this same question last week, and the consensus is, it doesn't matter. You have an airlock on the carboy... as the yeast ferments, it will eat all the oxygen and push out CO2. Since oxygen can't come back in the carboy, there is no issue with it.

I wouldn't rack to as secondary though, since there would be lots of headspace. Just keep it in the primary until ready to bottle.

But... what do I know, I haven't even brewed my first batch. Just going off of basic chemistry of an airlock pushing out oxygen