Cold-water inlet filter

Tankless units draw cold water through the inlet, and the filter screen on that inlet catches the debris that would otherwise reach the flow sensor and the narrow heat-exchanger passages. It is the cheapest component in the unit and the one that protects the most expensive ones.

Rinnai's manual calls for cleaning the inlet filter periodically and lists it among the checks for several codes; its recirculation guidance adds the separate bypass filter in crossover-valve mode. A fouled filter reduces flow, so the unit fires late, weakly or not at all, and can report recirculation low-flow codes.

The filter is cleaned as part of the descaling and maintenance visit, where the technician can see whether debris is coming from the supply (a sign to check the supply side) or is a one-time event. Replacing a fouled filter without asking why debris arrived leaves the exchanger at risk.