Is there a problem with fermenting 3 gallons in a 5 gallon fermentation bucket? I thought I heard or read at sometime that you wanted to have as little space in the bucket as possible, but I’m not sure about this. I have a grain bill that would be good for a 3 gal batch that I’d like to BIAB, but only have a 5 gal ferm bucket. What problems could this cause?
Any help is appreciated!
No problems. It’s good to have headspace unless you want to harvest yeast through a blowoff.
Yeah, what Euge said. If you’re conditioning a beer for a longer period of time in a secondary for some reason, like if it’s a very high alcohol beer, you would want as little headspace as possible to minimize the risk of oxidation. But for active fermentation you’re blowing off enough CO2 that it doesn’t matter.
The problem is you are going to only be drinking 3 gallons of beer. 
Nailed it.
I have been thinking about doing the same thing for my next wee heavy. That way I don’t have to deal with 25 pounds of grain in my mash tun. And I don’t drink high gravity beers enough for only 3 gallons of beer to be an issue. I just enjoy a good scottish wee heavy from time to time.
No problems with extra headspace. Even though there is a CO2 layer above the beer after fermentation, given enough time it will diffuse and mix with ambient air. If you let it sit too longer after fermentation shuts down, then you risk oxidation. Just be on top of the process and get it kegged or bottled as soon as you are able.
Thanks all…sounds like I am good to go then.
What if it’s beer he doesn’t like?
Or perhaps using the batch to step up yeast to be harvested for another batch (I do this with lagers - 2 liquid vials of lager yeast for a 2.5 gallon batch, then use the yeast cake in the next 5 gallon batch and then that yeast cake in a 10 gallon batch).
Of course big beers would not be a good idea for this step process based on yeast stress, but there are times when it makes sense (especially like when you are making a beer that is really experimental).
But if you end up really liking the beer, then, yes, that is a problem only having 3 gallons…
^^^^^^ In danger of taking this too seriously.