That Rinnai R85 looks like a better one than mine, it will heat 4.5 gpm at 75 degree temp rise, mine does half that at 77 degree rise. Good buy. I just hope it is designed to drain completely, if not you will need to build a well insulated place for it and a dependable small heat source.
I notice you said you are on spring water. Is it hard water? If yes, be prepared to flush the heat exchange coils or they will plug up with lime. At the very first hint of flow restriction , unhook it and pump white vinegar thru it until the flow returns, if you wait for the flow to get too low, sometimes it can not be cleaned and you have a pile of junk. Soft (not softened) water is all I've ever run in either of my tankless heaters, but I used to sell, install and service outdoor wood fired "boilers" with hot water coils for domestic hot water. Places with real bad water (hard) I had to flush the coils routinely. Those who waited until they only got a trickle of hot water sometimes ended up needing to buy a new coil at the tune of about $250 20-30 years ago. I don't know if the heat exchanger on the tankless heaters have replacement coils available.
If you do have hard water send me a P.M and I'll go thru the flushing equipment and process.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.