Heat exchanger
The part the whole appliance exists to protect. Replacing one costs a meaningful fraction of a new heater, and the most common reason it needs replacing is entirely preventable.
What it does
The heat exchanger is where the burner’s heat crosses into your water. On a condensing unit like Rinnai’s SENSEI RX there are two stages: a secondary exchanger that pre-heats incoming water by pulling heat out of the flue gas until it condenses, and a primary exchanger over the burner. That is where the efficiency comes from — and why a condensing unit produces condensate that has to be drained.
Why it fails
Scale, overwhelmingly. Hard water leaves mineral deposits on the water side that insulate the metal. Heat that should be entering the water stays in the exchanger wall instead — which shows up first as reduced performance, then as an overheat code or a high exhaust temperature code, and finally as metal that has been cooked past its design temperature.
The other causes are debris blocking the fin passages on the air side, and corrosion, either from water chemistry outside the manufacturer’s limits or from corrosive air being drawn into the combustion chamber.
The part that makes this preventable
Rinnai says it directly: damage caused by lime and scale build-up is not covered by the warranty, and above roughly 12 grains per gallon a softener or conditioner is required rather than recommended. A flush costs $25-40 in vinegar if you do it yourself. This part costs $400-900 before labor.
Why this is not a DIY part
The exchanger comes out with the burner assembly, on a gas appliance, and every gasket it touches — including the burner gasket — must be replaced rather than reused.
What a replacement usually costs
$400-900 for the part and $700-1,400 installed. At that point, on an out-of-warranty unit, compare it honestly against the price of a new heater.