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Hi, the problem with these units is that most reasons for their failure is electronics, I am sure your plumber checked if the gas was present at the unit and if the power was on, beyond that we look for error codes on the circuit boards, some have a display with numbers others use flashing led's unless the error codes are in the manual you are stuck having to call tech support.
there is a manual online with a fault finding schematic and that would be the place to start
In the rinnai infinity 24 you need a digital controller to read the error codes, without that you don't have anything to go on and you will need a factory authorized repair person.