Heat naturally flows from warmer to cooler. Things tend towards equilibrium.
Therefore heat vents are most efficiently placed on the floors of rooms so the heat might rise up throughout the room.
If the vents were on the ceiling, then all the heat would stay towards the top of the room.
And fans are then placed on the ceiling to circulate the air around more efficiently.
All this works according to the formula H=K*L/A(TH-TC) where K is the coefficient of thermal cooling for a substance.