A deliberate chimney.
I am wintering fewer hives in 10 frames and more colonies in nuc configuration. I have one combined hive that was in a couple of mediums and three deeps. But there wasn't much honey, most was in the medium. I had decided to shake it out when it gets frosty, then I changed my mind in favor of an experiment.
I put the heavy medium frames in 2 medium nucs, and topped it with 5 fairly full frames I gathered from the thee deeps. Then I pulled 5 full frames from three doomed colonies and put them on top.