I have an automatic brewery that I've been using for 7 batches now. It is all enclosed in a attempt to retain heat during the mash, and to not get my basement all steamy when I brew (all electric). I have a duct fan that vents out the window. This time I started with 6.5 gallons before the boil, and after 60 minutes I was shocked to find I had lost 3 gallons! My OG was 1.080 and I didn't have a yeast starter ready. I have a bad feeling about this batch.
The difference between this batch and the previous 6 is that I took the lid off of the boiler for the entire boil. Before I would leave it on, but off to the side a bit so about ~30% of the boiler was uncovered. I don't know if the vent fan increases the evaporation rate when I have no cover on the boiler but I'm going back to covering it to avoid disasters such as this (I presume it will be a disaster, but I'll have to wait until I drink it to make sure).
Moral of the story: If you have an enclosed brewery contraption with a duct fan attached, don't boil without a lid
Maybe I'll try again without the lid, but not boil so vigorously and leave the duct fan off.