Ok, here's what you may be missing: If you have switches inside for the Hot Water heater, one for AC and one for Propane. That AC switch does not have AC on it, only 12V which controls a relay on the rear of the Heater. So if you have 120 at the breaker, but no 120 at the heater that relay may be bad, or the 12V supply from the switch to the relay may be dead. Also those stupid little black switches on the outside of the heater are known for failure, so I would also check to see if you're getting any 120 to it.