Fusel Fermentation Question

[Koach]

I have a question about a problematic fermentation I'm on the tail end of. I was making 10 gallons of the classic "Janet's Brown Ale", using American Ale (1056) yeast. My OG was 1.070 and my target FG was 1.018. I followed my usual procedure, but as I take the beer off of primary for dry hopping, one of the carboys is very hot. I've never had a beer go fusel before, but I think this may be it. But the other carboy has no off flavors. I'm looking for reasons why - thanks in advance for any observations or pointers.

• I started with one packet of yeast, built it up on a stir plate in 2 stages with 1qt. starters each time. This may seem low compared to some online sites I've seen, but it's never been a problem before.
• My starters are made in bulk (with appropriate nutrients included) and canned in jars.
• I hit my wort with oxygen from a medical canister using an oxygen stone. I generally do 45 - 60 sec.
• I have a used fridge w/ exterior thermostat control. I set my fridge to 67degrees, but 2 days in I checked a backup thermometer in the fridge and saw 65 degrees - I didn't want fermentation to stall prematurely so I kicked the temp up to 70 degrees and left it there for 3 more weeks.

And my fermentation did fail to hit target. The carboys are at 1.020 and 1.021 respectively. So they are both maltier than I was looking for, but I think that will even out if I leave them at room temperature for a few weeks after bottling.

Thanks again for any thoughts and feedback!

 

[kombat]

Fermentation is exothermic - it generates its own heat. If you had the fridge set to 70° F, the beer itself could have been up to 10° warmer than that. 80° F is definitely hot enough to generate fusel alcohols.

Measure the temperature of the beer, not the ambient. Get yourself a thermowell, or at the very least, attach the temperature probe to the side of the fermenter and insulate it with bubble wrap or something.