Just wondering why a kit might come with a 6.5 gal primary and then just a 5 gal carboy? Is it just best to have more room for primary due to it being so active and risk of blow off? Thanks
You are correct, the 6.5 would be for primary due to the concerns you listed. You want to have more headspace. The 5 gal would just be used for secondary or long term conditioning where there would be little to no activity and headspace is not critical.
I'd go further and say that a lack of headspace is important in secondary to limit exposure to oxygen. Once the yeast is done with fermentation, exposing the beer to oxygen will oxygenate it and can lead to reduced stability and off flavors. Hence the 6.5-gallon vessel for primary, leaving room for krausen, and the 5-gallon vessel for secondary, limiting exposure to oxygen.
