I purchased an Eternal GU145S by Grand Hall. Very efficient in it's sweet spot, but I have had numerous problems I described in other posts. They now have a smaller one that can run on a 1/2" gas line. Might be ok if you swap out the plastic pressure switch for a metal one.
I got it because of the low head loss due to its combined tank/heat exchanger (which also eliminates "cold water sandwiches"), as I am using it in a combined DHW/hydronic heating system. Most tankless models have a tremendous amount of head loss due to the water path used to improve efficiency. A condensing storage heater such as Westinghouse's WGR050NG076 is also a good choice.
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"I‘d put my money on the sun and solar energy. What a source of power! I hope we don‘t have to
wait until oil and coal run out before we tackle that." Thomas Edison, 1847 — 1931
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