Check that rating plate again. It states 78,000 BTU input. That's more than your typical 75 gallon tank. They figure on a rapid recovery but putting a flame thrower under it. I would be very concerned about gas pipe sizing and being able to deliver 78mbh with the house under full load. Otherwise, you could be under firing, which could result in lower recovery rate and poor combustion.

Any pics of the tank further back showing the venting and gas piping?
Bob

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Keep the fire in the fireplace.