In many buildings, the mechanical systems are in the basement and whatever is carrying the heat runs up through the lower units. My old apartment in Cambridge was first floor and the 2nd and 3rd floor ducts were inside my living space and closets. On mild days, I could leave my heat entirely off and just the radiation and small leakage from their ducts was enough to keep my place comfortable. (The furnaces were all atmospheric gas burners with plenty of residual heat leaking into the basement as a result, making it an inadvertently heated space beneath as well.)

Heat leaves far more easily if there is air on the other side of the wall and(!) you have the benefit of ground isolating you on the bottom floor