Question

why are room heaters always kept at the floor??

Answer 1

 Hot air is lighter than air at room temperature. 
So if you keep the heater at the top of your room the hot air would remain at the roof level and lower air would remain cold. 
If you keep the heater at the bottom of your room, only then the air whole room would circulate and get thoroughly heated up.

hope help u spoorthi ji

Answer 2

as we know hot air Rises up and cold air settles down so room heater is placed near to the floor so the heat can move uniformly from down to up

Hope it helps