Hello Does the heater turn on correctly? If the answer is yes; we discard obstruction in the venturi and its exit would have to be disconnected to verify its flow If the answer is no; You have to check your entrance, some have a filter that eventually gets clogged, then follow the vent and finally the serpentine. To rule out a problem in the hot water line, it is necessary to bypass inlet with outlet, that way we cancel the heater by checking the pipe and see if the flow improves. If the flow improves substantially and it is not covered or filter (if any) or venturi it is almost certain that the serpentine is covered. You have to take out and take to uncover. If the dissipation fins are cast, it is convenient to put new. Regards