hey there!:
The heat of the heat tends to rise, and the cold air tends to go down. The air in the vicinity of a heater gets hotter and rises. This favors the circulation of air in the environment. If the heating is near the ceiling, the air above the fiction heated and not would come down. Cold air, on the other hand, can be used at the bottom of the cycle and thus the appliance does not fulfill its purpose: to warm the environment. It is called thermal convection the process of heat transfer that happens thanks to a movement of a material.
Hope that helps!