Question

why ac is placed at the top of the room and room heater at floor of the room ?

Answer 1

because cool air is heavy than normal air so air comes from a.c. cover the whole room by starting from top and then bottom but hot air is lighter to normal air so hot air first cool downward then upward
if a.c. Set on bottom then top of room never get cool and if heater set on top then hot air dont comes down

Answer 2

Answer:

Warm air is lighter and goes up. The AC is placed at the top of the room to cool this warm air.

Cold air is heavier and lies on the floor surface region and room heater is placed there to warm this cold air.