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I recently made an all-grain Russian Imperial Stout on a stovetop, and because of some volume miscalculations (or something), I boiled for 135 minutes to get down to my desired volume of 2.5 gallons. At the end, I discovered that my OG was about 1.115, rather than the expected 1.090, so I added water to get down to my intended OG.

However, after adding 1 package Wyeast ESB wet yeast, and waiting about 3 weeks, the FG is 1.036, which I worry will cause bottle bombs if I go for the bottle.

The fermentation could be stuck or slow (I made another yeast starter and pitched it in a few days ago with no noticeable increase in fermentation), but I wonder whether the long boil produced appreciable non-fermentable sugars, such as melanoidins.

My question is whether a boil this long could produce enough melanoidins (yes, you can taste them!) to increase the FG from an expected 1.010 to 1.036. If so, is there a way to calculate the expected FG based on boil time taking the production on non-fermentables into account?

Also, the brew now has a distinctly smokey flavor. Not terribly burned or anything, and there was no indication that anything burned on the bottom on the boiling pot, but you can definitely taste some smoke in there. I expect it will be sort of pleasant, but I'm just adding this to 1) know why it happened, and 2) perhaps it will help diagnose my high FG.

Hop flavor is good, added bittering close to 60 minutes before the end of boil.

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    Was your mash temperature good? Your fermentation temperature? – Philippe Mar 28 '17 at 14:31
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The problem is yeast, not unfermentables. Unless you made a starter, 1 pack for a 1.090 beer is way underpitching, assuming you made 5 gal. A single pack might work for 1 gal. at that gravity, but not 5. Also, a 1.010 FG for a 1.090 beer would make it very thin and bodiless. There is no accurate way to calculate FG.

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    Your measured FG corresponds to an apparent attenuation of 69%. If the yeast you used was Wyeast 1968 ESB then 69% attenuation is about what you can expect. I wouldn't worry about the sugar levels, but even if the primary ferment was perfect I would bulk age an Imperial Stout for at least 3 months. Since your yeast will be in bad shape adding yeast now (possibly the yeast cake from another brew) would be advantageous. – Kevin Sharp Mar 30 '17 at 7:56
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    FWIW, attenuation is much more dependent on wort than yeast. Using the same yeast, I can get from 60-80% attenuation depending on wort composition. – Denny Conn Mar 31 '17 at 15:30

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