economy7

My Economy 7 tariff currently has off-peak hours from 1am to 8am. However the switch connected to my storage heater is currently set to turn on between 12am and 7am. How can I make adjustment to the timer, if there is one (no idea where please someone advise) to suit my Economy 7 tariff...

We've got a standard water tank with a bottom element fed from the economy7 wiring, and a top, "boost " element wired to the standard wiring. Unfortunately it's quite a small tank and if someone has a bath in the morning there is no hot water left, and the boost only heats the top of the tank...