Hi, I have a power vent hot water heater. About once a week the circuit trips. It is a dedicated line with 15 amp circuit breaker. Any ideas why??

So this is a gas fired heater with a 120 v power vent, right? Easiest place to start is to shift the hot line in the panel to another 15 amp breaker. Cheap test of the breaker. Unscrew the black hot off the breaker and switch it with one,15 a, next to it in the panel. If the problem travels to the new breaker you have a bad vent motor. If it stays on this 1sy breaker, replace the breaker