Hi i have a electrican coming round tomorrow to qoute for changing the consumer unit at the moment there is a big old metal box with many unused wires in it thats been there since the mid 70s it has two feeds going into it one for the on peak and one for the off peak which only supplys the immersion heater at the moment will i need one unit for both or will they need seperate units i would like to know so that i don't get duped into something i don't need. thanks for your help