Tankless sizing, Noritz NRC98-DV versus NRC111-DV
I am having my house re-piped, and plan to install a tankless water heater. I had planned on getting a Noritz for a few reasons: thick copper in the exchanger, "dual combustion" burner, and the wide range of flame modulation (BTUs).
I am contemplating the sizing, NRC98 vs NRC111. It is common for us to use two showers simultaneously (5 GPM estimated) or even one shower and one bath (7.5 GPM estimated). Our groundwater is not that cold, probably no worse than 55 degrees F even in the winter. I would like to maintain a 115F output temperature under most conditions, although dropping to 110F would be acceptable under maximum load given the thermostatic shower valves.
At first I thought the NRC98 would be appropriate, given that I have heard it is best not to oversize a tankless HWH. However, looking at the temperature rise, flow rate, and especially the pressure drop, it appears the NRC111-DV would be the way to go, especially given the lower pressure drop. And this is their largest model!
Is my thinking correct? Do I really want the largest heater? Will it still be efficient under low-flow conditions? The minimum BTU rating is actually lower for the NRC111-DV (11,000) vs the NRC98-DV (16,000), although I don't know if this reflects the reality of their performance under low-flow conditions.
Lastly, I intend to use a Ready Temp hot water recirculator for the kitchen sink loop 50' away with a dedicated 1/2" return line. I am contemplating timing it to be on 10 hours a day, or pushbutton. The pipes will be insulated, of course. Would this cause unreasonable wear and/or excessive operation of the tankless?