As long as you have not installed the heater so as to short out the coils on a metal surface, the only thing which will cause the heater to energize while the dryer is not running is a bad motor centrifugal switch. Your motor needs to be replaced if it is sending 240 volts to the heater while not running.
Heaters do not simply go out like yours has (twice). Beside the motor switch problem, you could also have venting issues that should be addressed before you go broke maintaining the dryer or cause a fire somewhere. The vent system should be cleaned out and you should not have foil or plastic duct on the Admiral machine. For more venting tips, click here