(a) very hot water from the boiler
travels through this copper coil. It loses heat energy to the water in the cylinder by
conduction through the walls of the tube.
(b) this small immersion heater only heats
the water at the top of the tank because hot water rises. You should use this one if you only
want a little hot water, for example for washing in the morning.
(b) this larger
immersion heater will heat all the water in the tank and should be used when you want a lot
of hot water. Of course the bigger energy output means that it costs more to run.