There are apparently multiple air leakage paths between the basement and other rooms in the house. By opening up a huge path (the basement door) most of the flow now traveled through the conditioned first floor space with better mixing.
The real solution is to air seal and insulate both the basement walls and the top floor ceiling, and all plumbing / electrical / flue chases that penetrate either the basement ceiling or between the upper floor ceiling and attic. By minimizing the outdoor leakage at both the bottom and top of the house the total stack-effect infiltration drive gets dramatically reduced. By insulation the foundation walls the basement stays quite a bit warmer, as does the first floor's floor.
Balloon framing often has a basement-to-attic flue in every stud bay if left uninsulated and unsealed, and balloon framed partition walls could be drawing cool basement air into interior wall cavities, lowering the temperature of those walls, and the average radiant temperatures in those rooms.
I live in a 1923 bungalow (that was not balloon framed). After air sealing the basement and insulating to ~R15 with 3" of reclaimed roofing polyiso, even when it drops to -20C outside the basement stays north of +15C, and the whole house feels warmer.
