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I am a Rinnai factory trained and certified repair technician. This is not normal, but not bad as far as your water heater is concerned. It is from your pump. I've had this happen to me after the rubber impeller in my pump had dried up do you lack of use. I'd clean your heater's inlet water filter and that's about it.
Do you have any other questions or concerns?
Sir, do you still have questions?