As I have learned, if you turn on the little water heater switch on the monitor panel, the little red LED next to it should temporarily illuminate and then go out when the unit fires on propane. If the LED comes back on later, the system is in lock-out and you have some kind of problem. You can operate the water heater on propane, 120V electric, or both at the same time. Just be sure there is water in the tank before turning on the 120V heating element.