OK, so I am bottling tomorrow and I was preparing my fermentation area for it. My area is an unused stall shower. I close the door and have a small electric heater in there to maintain 67 degrees for this brew. All fermentation is done. No more bubbling. So I took the heater out an cleared the area for the morning bottling. THe temp in the room dropped to 64 degrees. That must have caused the air in the carboy to cool and create a slight vacuum pulling air through the air lock and into the carboy. So the question is, how much exposure to fresh air is required to cause me trouble? Did I just damage 3 weeks of waiting?
Relax, don’t worry, bottle your home brew.
Your beer is fine. However, this is why people use sanitizer or vodka in their airlocks. Or better yet, replace the airlock with a solid stopper after fermentation and before cold crashing (I know you didn’t cold crash or drop temps on purpose. Just using this for future reference).
Or sanitize a piece of foil and tightly cover the carboy neck before cold crashing. It lets a bit of air in, but I’ve never had a problem doing this with lagers or when prepping an ale for gelatin.