I have a Kolsh that has been fermenting happily at 60* for the past 2 weeks in my refrigerator downstairs. Yesterday I lowered the Johnson control unit to 35*. This morning when I went to remove the blow off tube from the carboy to replace it with an airlock, I saw that the 6 gallon carboy was filled completely to the top, which was not the case 2 weeks ago when I poured ~5.5 gallons of chilled wort into it. My guess is that as the temperature dropped last night, water from the blow off bucket was sucked back into the carboy until it filled to capacity. I can’t think of any other explanation.
At this point I am guessing that the batch is probably lost and would at best be greatly diluted or at worst, infected (the blow off water that was sucked back into the carboy was pretty nasty looking). Would anyone have any thoughts on where to go from here?