Ah yes, the disappointment of your own awesome all- grain beer only partially- filling the cornie keg! I'd like to say that was one of the reasons why the
MaxiBIAB method was developed (similar to the
20L Stovetop method), but its not as I was only bottling at the time. (Close enough though!
![Big Grin :D :D](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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If your kettle volume is the limiting factor (that's usually the reason this sort of question is asked), what you can do to increase the batch size is simply scale up the grain and hops, while leaving the water about as it was for the mash, but then including a sparge step and also diluting the wort at pitching. That's MaxiBIAB in a nutshell, its a hybrid which overcomes many of the limitations of conventional methodology and it means that in my cheap 19L kettle I often brew 24L batches of 1.055 OG into the fermenter.
You mention tannins, rest assured that there's no quality issues affecting MaxiBIAB either. This has been established by continual positive results at many competitions over the last three years, for instance this year I have two beers produced by this method which have qualified for the National amateur competition, that's because they placed at the Qld state comp, (plus four medals out of just six entries too! B) ).
So, perhaps try the method, as far as additional equipment goes all you need is a simple bucket for sparging, I recommend using a 15L or bigger as the ubiquitous 10L is just too small.
Hope this helps! :icon_cheers: