I guess the standard answer I have seen and asked on here is that it will work, and is what i use for my 5-6 gallon batches, but is not ideal. Smaller is better, the bigger tun needs to be monitored and planned for after seeing what happens. for a normalish beer of around 16# grain and 1.5-1.75qt/#, i usually use less than 10 gallons of space in this cooler, but I know what it does and plan accordingly. I mashed my IPA in it back in at end of January, and went from 149.5°F down to around 147°F in 90 minutes, so I have no issues with that type of batch. It was a slightly higher drop with a 10# grain bill for my pilsner or saison, but most conversion happens early, so noticeable effect to my rookie taster senses. And everything stpug has to say as I write this