Is it possible to have to much headspace during primary fermentation? I am going to do some experiment all grain test batches like 2-2.5 gallons or so. Except the smallest carboy I have is 5gal. and I don’t want to go out and buy another. So would 2.5gal. of headspace cause possible problems with say oxygenation? It seems very unlikely to me, I just don’t want to brew a whole batch only for it to go bad from such a silly thing like this.
don’t worry. it will be fine. there is more than enough CO2 generated to fill and purge 2.5 gallons of headspace. enjoy the test batches. I have been thinking I should do more of those but it is just as easy to go ahead and do 5 gallons…
Yes, it is easy to just do the full 5 gal. I just don’t want 5 gallons of beer that doesn’t taste good. I’d much rather experiment with smaller batches so then if they go wrong it’s less ingredients lost. If it goes good I can just as easily brew it again as a bigger batch. I didn’t think the headspace was a problem I just want to for sure eliminate that as a factor to the finished beer when tasting test batches.
It’s OK. No oxidation worries.
I’m doing 2.8 gallons in an 8 gallon bucket right now. Decided I’m not doing any more 6 and 12 gallon test batches. 
Was thinking of something smaller myself but can’t be bothered to get yet another fermenter.
I do some 1-gallon batches on occasion, glass gallon jugs can be had for $6 full of juice :) A 2 gallon cooler, braided hose, you know the drill. Easy and fast. Especially since i have a 1 gallon corny ;D
+1
There are quite a few homebrewers on the forum that make one to two gallon batches routinely. I make my lager starters in 3 gallon carboys with no problem. There is a significant amount of CO2 generated during the fermentation that will consume the headspace fairly quick.
So crush and mash with a smile. 8)
I’ve fermented in the 2.5 gallon water jugs before. you just need to pop the spigot off and it works great.$2.50 for the water and you’ve got a pre-sanitized disposible fermenter after you drink the water.
I routinely do open fermentations in my buckets and my basement is roughly 124,363.636 gallons. No way is my fermentation producing enough CO2 to purge that space so I think your 2.5 gallons of head space is ok.
And then some. Every gallon of beer fermented will kick out about 20 gallons of CO2.