Carboy size

Brewing 5 gallons of wort should, in theory, be able to fill a 5 gallon carboy. With a 6.5 how much, if any, do you need to alter a standard 5 gallon recipe in order to fill the carboy closer to the stopper?

dirkdover said:

Brewing 5 gallons of wort should, in theory, be able to fill a 5 gallon carboy. With a 6.5 how much, if any, do you need to alter a standard 5 gallon recipe in order to fill the carboy closer to the stopper?

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You want some head space in your primary fermenter. So just make your five gallon recipe for a 6.5 gallon carboy. Putting five gallons in a five gallon carboy is a good way to plug your airlock. If you use a blowoff tube I would think you'd be fine, you'll just have lots of blowoff to deal with.

To brew a 5 gallon batch in a 5 gallon primary you typically use a blow-off tube for the first few days to week. After the high activity has subsided, you can then fit it with an airlock.

Most people brew 5 gallons in a 6 or 6.5 gallon primary... I've done 5 gallons in 5 gallon, 6 gallon, and 6.5 gallon primaries.

If you're concerned that the blow-off tube won't be enough of a safety net, you can also use fermcap-s to keep the krausen under control... You can also use it in the boil so that you can get away with little head space there and not have a mess all over your stove.

If you are using a 6.5g carboy as a primary (which I do), there is no problem with head space. After pitching, the head space will be filled with air (which is not bad at the start of fermentation). When fermentation starts, the process generates a large amount of CO2 (which is heavier than air). This will force the air out of the airlock, leaving you with a nice protective blanket of CO2 on top of the beer.
So far (touch wood) I have never needed a blow off tube with a 5.25g batch in a 6.5g carboy. If I use a 6g carboy as a primary, then I have needed a blow off tube more often than not.

-a.