We just opened our pool a few days ago and turned on the heater for the first time last night. This morning we noticed the the circuit breaker had tripped. After a couple of retries to start, we confirmed that the heater is tripping the breaker. Water temperature was 55, and air temperature dropped to high 40's last night. Heater is newer, 2.5 years old. Display flashes R13 for a second, before tripping. Can't recall if that code is normal on start-up, or is indicating a problem.