Why are radiators placed near the floor?
Correspondingly, how far off the floor should a radiator be?
Vertical radiators come in many sizes, but typically 1800mm high and 600mm long. Typically, standard radiators are mounted so that the bottom of the radiator is 150mm from the floor. The governing factor from an aesthetic point is the height of the skirting boards that will be present.
Beside this, why should a room heater be kept on the floor?
HEATER is placed near the floor because hot air rises up and the room will be heatness will spread even. The density of air depends upon the temperature. Hence, heater is placed near the floor because the air currents move upwards so the hot air fills up the whole room.
As described in the other answers, putting the radiators (or hot air vents in a forced-air system) under the windows offsets the greater heat loss of the windows, but there is another reason. As room air flows over the surface of a window, it will lose heat to the window (and the outside).