The Economy 7 can control different types heater. Some heaters do not have off-peak ... but you can solve problem..
Copy following link:http://waterheatertimer.org/pdf/QE7-INSTRUCTION.pdf
If timer is wired to two different elements, then the night rate element or off-peak element might be burned out. Test element with multimeter.http://waterheatertimer.org/How-to-test-water-heater-element.html
Also this timer is wired so either Off-peak is getting power or Boost is getting power, but not both at same time.
If heater has off-peak element, then there will be a wire connected to terminal 5, going to heating unit.
If there is no wire on terminal 5, then there is no off-peak element, or wiring has not been connected. However you can change the timer function by moving wire on terminal 4 to terminal 5. Then only the off-peak timer function will operate, and not the boost.
You can also fool the timer, by setting time clock differently. Instead of setting timer for current time, rotate the dial and fudge current time by 10-12 hours so water heater thinks it's time for boost, but the actual time is during off-peak hours.
If terminals 4 and 5 both have wires, then either the element is burned out, or the timer has malfunctioned. Again, you can fool the timer by setting current time differently, and solve problem.