Kevin, next time you go out try turning on the water pump for your shower. It should supply enough pressure for proper operation of the water heater. It will tell you if its a pressure problem, which, like Laco says, it probably is. I have the same water heater as you and we have always used the water pump for showers without a problem.
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2017 Gemini 23TR Ford chassis w/ power stroke 3.2
DW, Daughter and 2 dogs, Sofie (black lab/boxer) and Phoebe (schnoodle)
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