I've looked in to this fairly extensively regrading Rinnai heaters. I have a RL75 natural gas unit. It requires a dedicated 3/4" water line input and a 3/4" dedicated gas connection, plus a 20A 120VAC connection.
The residential controllers will limit you to 140ºF, but you can get a commercial controller
for these units and get 165-185ºF water depending on the unit. for my unit the RL75 my max water temp is 165. with the RL94 I believe you can get water out at 185º
My BIG disclaimer, is that it is really easy to forget you have the water heater cranked up past 140, which can result in very bad burns if your SWMBO decides to take a shower when you are brewing.