Question:Why do kettles and hot water boilers always have their heating elements at the bottom?
Heating and cooling elements are always placed in the best position to start a convection current; because this is the fasted mechanism of heat transfer in a liquid or gas.
Since heated fluids rise, the heating element must be put at the bottom.
If the heater was at the top then the heated water would just sit at the top and you would need to wait for conduction to carry the heat downwards, which would take a very long time.
It is actually possible to boil the surface of water and put keep your hand at the bottom, so long as the heater is at the top to prevent convection.