I have 3 carboys: 2 are 5 gallon and 1 is 6 gallon. I'm using the 6 gallon for my regular 5 gallon batch. I'm brewing today, regardless. I can make my new beer a 4.5 gallon batch but is there enough headspace for that in a 5 gallon carboy?
Better to be safe than sorry. If you can put 5 in a 6, you should be alright. 4.5 in a 5 may be okay, but it really depends on how crazy your yeast is. What yeast are you using, what temperature, and do you have past experience so that you will know how big the krausen will be? You can always just brew 4.25 or 4.5 gallons of slightly stronger beer and dilute with a few quarts water at packaging, too. Lots of ways to slay this beast.
It's an extract brew cream ale kit. US-05 yeast at around 70 degrees (i know it's high but I don't have temp control). I've used the 05 many times and it isn't insane but a good few inches of krausen for sure. i tweaked the recipe a little to account for the smaller volume. the beer is for my family who like lighter beer. a blowoff hose is simple enough, ehh?
Yeah, just blowoff. And if you set your fermenter into a tub with 2-3 inches of water in the bottom with a wet t-shirt over the top draped into the water to keep it wet, you'll cool your fermenter down to 65-66 F just by doing that simple thing. Cheers.
I always use a 6 gal or larger for a 5 gallon batch. Had too many clean ups by not doing so. Beers over 7 abv get split between 2 carboys, no blowoff required and no lost beer. Some use fermcap or some other product to keep the krausen down. I do not.
Blowoff hose and drop the temp and it will work. But if you can't get your temp down a few degrees you could still end up with a mess. There is a technique for catching the krausen in a container that then allows the liquids to flow back to the fermenter, but I can't remember where I saw it and what it's called. With it you can ferment almost the full 5 gallons in that 5 gallon carboy. With the miracle of the googler here's the link: https://byo.com/hops/item/351-build-a-burton-union-system-projects
Just reduce the volume by another half gallon (keep the grain and hop amounts the same as for the 4.5 gallon amount), and add a half gallon of boiled and cooled water back in after the most active fermentation has completed.
