Heat exchanger

Condensing tankless units carry a primary and a secondary heat exchanger: the primary takes the burner's direct heat, and the secondary recovers latent heat from the flue gas, which is what makes the units condensing and produces the acidic condensate. Navien's A2 specifications list stainless steel for both; the material is the platform's corrosion story.

The exchanger's passages are narrow by design, which is their weakness: scale and debris that pass the inlet filter can deposit inside and block heat transfer.

Rinnai documents scale build-up as the cause behind heat-exchanger overheat codes and states that failure to flush will damage the exchanger and that lime damage is not covered by warranty. The annual flush through the isolation valves is the manufacturer's maintenance, and a scaled exchanger past flushing is usually replaced rather than repaired.

Metro Vancouver's soft water slows scale, which is why anode and corrosion management matter more here than in hard-water regions, but the flush interval still protects the warranty.