[phillamb168]

Anybody tried it? I've got a couple kegs' worth of lager in the fridge and I'm having to pull the release valve every few hours to keep them from carbing and stopping fermentation; seems like such a waste to let all that CO2 just float away. I have a check valve cask aspiration via co2, perhaps I could run that in reverse?

[hopfenundmalz]

When at Sierra Nevada we saw their recovery system.  It turns out the stuff you smell comming out of the fermenter is not what you want to put back in your beer.  They remove esters, and if your yeast was kicking out any sulfur you would want to get rid of that.  There is also some ethanol in the CO2, which they turned into fuel.  For me not worth the bother,  CO2 is cheap, and I don't have to brew by the Reinheisgebot.

[dcbc]

I agree with just using CO2.  Plus, if you adjust your water with acid (not made from grain) or salts, you're already outside of the purity law for all intents and purposes.  If it makes you happy, go for it.  If you're just worried about losing a little naturally produced CO2, I wouldn't bother.

[morticaixavier]

I used to recoup my co2 by keeping plants in the same room as the fermentors. They like co2