I searched the forums and found a couple threads relating to this but not exactly the same. The other posts were about brewing 1gal in a 5gal carboy.
I want to try blending 2 recipes at secondary to allow distinct flavors to meld together. So I want to brew 2gal of the first recipe (14-21 days in primary) and 3gal of the second recipe (14 days in primary). They will be racked to the same secondary carboy for a total of 5gal.
the problem is I only have 5gal carboys. Will that amt of headspace really matter in primary (ie, max of 3gal headspace for up to 21 day). what if I purge the top of the carboy with CO2 after I add the liquid before airlocking?
You're fine. I ferment 3 gal test batches in a 5 gallon carboy. The co2 produced by fermentation will fill the headspace and push any air out of the airlock.
Rack as you normally do and everything should be fine.
Tanglewood Brewing Co.
primary: Off the Wagon IPA,
secondary:5 Turns Vienna Lager
Kegged: Barleywine, Lawn Chair Pilz