What is your wintertime input cold water supply temperature? People look at the top line gpm ratings, but often fail to notice what they're using for inlet temperatures...the further north you go in the USA, the colder the supply water can get, and that can make a huge difference in the hot water output of a tankless. Think of it...9gpm is 8.34*9 then *60 for an hour, and that's over 4500# of water you're trying to heat up. 199000/4500=44 degrees max temperature rise...that would mean, to get 120-degree water, what's considered normal for a home at full hot, your incoming water would have to be 120-44, or 76-degrees...and, that's assuming the thing was perfect in the heat transfer (it's not). That 199.000 value is about the largest tankless burner. YOu can gang multiple ones together, but your inlet gas service will need to be huge.
I haven't kept up with the details, but didn't SF just implement some new regulations regarding NG appliances? Check those out carefully. Not sure when they start, or exactly what applies or how, but you may not be able to do this.