I think I know the answer, but....

[FedoraDave]

I figured I'd solicit opinions.

It's about a secondary fermenter. I make both 5 gallon batches and smaller batches, between 2.5 and 3 gallons. I like to keep my three carboys (2 small ones and one large one) active at all times, to keep the pipeline flowing. Sometimes, like when I make a small batch of a lager, I tie up one of the small ones for a little bit longer.

But it occurs to me that I do have a 5-gallon carboy that is meant to be a secondary. Any harm in using it for prolonged lagering of a smaller batch, which would free up that smaller carboy? Or is it not advisable to have that much head space, everything else being equal, i.e. sanitizing, air lock, etc.?

[mashani]

Headspace doesn't matter very much when CO2 is being produced, because the amount of CO2 will easily fill any headspace even 4 gallons of a 5 gallon fermenter would be easily filled.

When there is no CO2 being produced then the wort is going to be exposed to oxygen though with whatever effects that will bring.

So... what I would say is YMMV - if using an airlock and it's well sealed, and you move it to a secondary *BEFORE* it is completely done fermenting and still is letting off CO2 then you are probably in better shape. You could always also purge it with CO2 before you airlock it. This isn't going to give you "infinite" protection, but it might make it ok for a while in a stable environment.

If temperatures are going to swing, air pressure is going to change dramatically, then the gas laws come into play and the airlock will not totally protect you, as it is not a totally sealed cap so stuff will still move through it and all the liquid in the airlock will get sucked into your secondary eventually.

[HerbMeowing]

I wouldn't do it.
Thought the idea was to minimize 2ndary head space ...

[Beer-lord]

Yea, I'm in agreement with Herb. Unless you're fermenting under pressure, you're taking chances.

[berryman]

I use to secondary a lot but went from a 6.5 primary into a 5 gal. and with still some active fermentation going, 5 gal. batch. I would have mixed ideas on a 3 gal. going into a 5 for secondary unless it was still quite active. I would have no problem putting 3 gal in a 5 gal carboy for primary, have done it quite a few times.

[FedoraDave]

Yeah, that was my inclination, too. I've got a porter I want to condition for a lengthy time, and I hadn't wanted to bottle it, but perhaps that's my best option. Otherwise I'd have to either tie up a smaller carboy or a smaller keg, and I really don't want to do that.

Thanks, guys.