A few days ago, I brewed an American Stout (1.061) and for some reason (probably sloppy calculations on my part), I missed my target temp of 154ºF by 10 degrees. Since the temp difference was so big, I new I would need to boil a fair amount of water to raise the temp. So the mash sat at 144ºF for about 10-15 minutes until I was able to get enough boiling water to raise the temp to the target 154ºF. It then rested at 154ºF for the usual 60 minutes.
Obviously 10-15 minutes at 144ºF will make the beer somewhat more fermentable, but how much are we talking? Obviously this is a flexible style, so I probably won't cry if the beer ferments down 78% instead of the intended 73% (I used a huge dose of fresh S-04 slurry), but how likely is this to happen? Does anyone intentionally do this? And should I have just rested it for a shorter time at 154ºF to make up for the added mashing time?
Another weird thing about this fermentation - the beer is fermenting at about 62F with S-04, but it is the most aggressive and vigorous fermentation I have ever had. I am not used to having to use a blow-off, but this one kept shooting out of the airlock and forced me to blowoff. Now 44 hours since pitching, the beer is still out-of-control fermenting. The last time I had a beer ferment anything like this was when I was fermenting without controlling the temperature and was dropping 1.070+ beers onto full cakes of English yeast.