There are only a few manufacturers of the gas control valves, so many units are built from the same piece parts.

So it should not matter the brand? I am just concerned about hearing so many problems about AO Smith... is this just a "consumers only post bad experiences online" effect??

I haven't paid someone else to do that job in 20 years, but $1650 sounds high to me if it is just a plain simple tank. Seems like a 2-4 hour job including commute unless there is something difficult. Is this a direct vent unit, or just a simple tank?

HI attofarad - I think it is direct vent (goes up the flue?). Also as I understand per plumbing code I would need an expansion tank (the current system was installed over 10 years ago)..