Navien NPE vs. NCB: Different Hard Water Thresholds
Navien NPE series (tankless, water heating only): hardness threshold 11 GPG for warranty without treatment. Navien NCB series (combi-boiler, space + water heating): hardness threshold 14 GPG for warranty. The different thresholds reflect the different heat exchanger designs -- the NPE's primary heat exchanger is more susceptible than the NCB's secondary brazed plate design. In practice, both should be operated with water treatment in any US market above 8--10 GPG hardness for maximum longevity.
Navien NPE-A2 Series: Condensing Technology and Calcium
The Navien NPE-A2 series (NPE-180A2, NPE-210A2, NPE-240A2) uses a stainless steel primary heat exchanger and copper secondary heat exchanger in a condensing configuration that recovers heat from combustion gases. The secondary copper heat exchanger operates at lower temperatures (condensing zone, 90--110 degrees F) where calcium carbonate tends to precipitate as soft, loose deposits rather than hard scale -- making it somewhat more tolerant of occasional hard water exposure than the Rinnai primary copper heat exchanger. Despite this, Navien's warranty still requires water treatment above 11 GPG for the NPE-A2 series.