Hi Tony,
You can get away without turning off the mains as long as the solenoid valve is still connected as this will stop the water. However, as always, Isolate the electrical supply at the fusebox.
To replace the heater can you will need to loosen or remove the screws that hold the old heater can to the plastic backplate. (Put something over the showertray drain to stop you losing screws if they fall). The heater can water connections are push-fit with o-ring seals, you can pull all the water connections apart, you will probably need to use the o-rings again. You will also need to take out the Pressure Relief Device and plastic tubing to be re-used.
Then you need to remember how it was wired on top and transfer the wire connections, thermal-cut-out and earth wire to your new heater can. Then fit everything back in reverse. Make sure the electrical connections are tight as there is a lot of power here. I would advise looking at the electrical connections 6 months after you have changed them, if any of the insulation is discoloured from heat then replace them with new wires.
It's a bit fiddly but can be done with basic tools.
After you have fitted it the heater can will be empty, you need to turn the shower on cold to fill the can otherwise the elements could be damaged before the water fills the can up.
Good luck.
Paul M, April 2009
