I have had a couple of real healthy fermentations in my 6.5 plastic primary that have actually plugged my airlock when fermenting a 5.25 gallon batch. I have miscalculated the increase when pitching my starter and been a bit above 5.25 gallons. I was wondering if there would be a downside to buying a 7.9 gal bucket for a 5.25 gallon batch? Since the entire headspace would be filled with CO2 I can’t see any negative drawbacks. Am I missing one?
There’s no downside to a bigger primary. You might try cold crashing your starter for a day or two, then pour off the top liquid layer. That way you’re only pitching yeast instead of 2 qts. of beer.
You can also keep your temps down, try Fermcap S, and use a blowoff tube.
I got a 7.9G bucket a while back for the same reasons, then found out that the 7.9G buckets are shorter and wider than the 6.5G, so while you have more headspace volume, you actually have less vertical headspace on a 5.25G batch in the 7.9G than in the 6.5G.
+1 to fermcap. I put 5.5G into my 6.5G buckets and have had no issues when using fermcap, even with big beers and notorious yeasts. I use about a drop/gallon.