Hi I installed a Navien NR-210A in my house this summer. I had some earlier problems that turned out to be due to shipping... fedex from New York to Seattle but the unit I have now doesn't have any damage that I can tell.
Anyways my system is a bit unconventional, I use hydronic heating for the house as well as my outside hot tub (with a HX). I can heat the ~500gallon hot tub from cold (~50) to hot in a couple hours I have the unit set to 140deg F with a mixing valve going into the house.
So here is the problem when my roomates/tenants take showers before the house is heating the water doesn't always come out hot. The fix is to crank up the heat at the thermostat to force the heater to come on.
So from reading and searching online I know in the past the Navien heaters had flow sensor problems by not using Stainless Steel but this unit is brand new so the sensors should be fine right?
I am considering adding a re-circ line so I can program the system to come on at certain times. Any ideas out there...
So it turns out the solenoids to switch off hot tub heating and house heating don't block back flow even though they seem to be direct acting (no off-center bump). So when I figured this out adding a check valve fixed my problem. What was happening was the cold water side went through the water pump and solenoids when they were off to mix with the hot water outlet from the heater. So my mixing valve was mixing cold with already mixed temp water to give fluctuating temperature water. Apparently the pressure drop through the WH is greater than the pressure drop going backwards through a non-active pump and the reverse direction of solenoid valves.
Before I figured this out I added a re-circ line but that didn't fix the problem.