Last night, I made my first 20GAL batch of wort...well 19GAL....using my 20GAL kettle. I've done this technique before but not to this size, using the same summer ale recipe, without any noticeable change of taste, smell, fermentation, etc.
I'm sharing this in case any of you have a 50L kettle and just can't seem to make a 10GAL batch...
I mash out about 17GAL in my kettle and boil as normal, leaving a good 5-6GAL of mash temp water in my mash tun. Periodically throughout the boil, I open the ball valve and transfer from the mash tun into the kettle. The boil will slow and almost stop as the level rises back up to 17-18GAL.
By the end of the 1 hour boil, I transfer the remaining mash wort into the kettle, while turning the propane way up.
In the end, I was left with roughly 19.5-20GAL of slightly simmering wort in my kettle. A gentle whirlpool to the kettle, cover and let sit 10-15 minutes before cooling and transferring. The slight simmer in addition to the 10-15 minute rest ensures pasturization of the addition of the mash wort at the end of the boil.
I ended up with 4 fermentors, all at the 18L mark.
I just need 1 more GAL which could be preboiled and cooled water to top up the fermentor.
Mind you, you have to plan this out as it will change the anticipated gravity from the original recipe.