Originally Posted by RM-MN
Because they haven't learned good heat control and have issues with boilovers and boil off rate so they have to start with extra wort to avoid boiling off too much and with that much extra wort they need lots of space to avoid boiling over when hot break occurs. I do a 5 gallon batch in a 7.5 gallon pot and haven't boiled over yet nor have I ended up with less wort than I intend.
I strongly recommend the 15 gallon pot so you can brew any size beer (think DIPA, RIS, barley wine, etc.) with the equipment you have. As for heat control, the head space of a 15 gallon pot will allow you to achieve a good rolling boil without worry about the boil over...
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