There is a good scientific reason behind this. We know that when air is heated, it increases in volume and as a result its density decreases and it becomes lighter and rises upwards. In other words, cold air comes down and hot air goes up. It is because of this principle that the fan or AC is placed on top and the heater is placed on the bottom. Since the fan or aircooler is mounted on top, it first cools the air at the top. That cold air, being heavy, descends to the bottom and sends the warm light air from below to the top. This time the hot air cools down again and comes down and thus the air in the whole house gradually cools down. Now if the fan had been put down instead of up , it would have just cooled the air below and that heavy cold air would have sat down. As a result, the hot air above would remain hot. Similarly the heater is kept down so it first heats the air below. That hot air, being light, rises to the top and sends the cold heavy air from above to the bottom. This time the cold air is heated again by the heat of the heater and rises to the top and thus the air of the whole house is heated periodically. Now if the heater was placed on top instead of on the bottom, it would only heat the air above and that hot air would stay on top. As a result, the cold air below would remain cold.