Rinnai RE (non-condensing) and RX / RXP SENSEI (condensing) tankless: quick sheet

A fixture opens and cold water passes the inlet filter and the water flow sensor. Once the flow passes the activation threshold, the board starts the combustion fan, proves airflow, lights the burner and modulates gas input (RX199iN: 15,000 to 199,000 BTU/h; RE199iN: 10,400 to 199,000 BTU/h) to hold the set outlet temperature. A water flow servo valve can throttle flow when the burner is at full fire so the outlet temperature does not drop.

On the RX/RXP, a secondary heat exchanger pulls extra heat out of the flue gas until it condenses. That acidic condensate collects in an internal trap and leaves through the 1/2 in NPT condensate connection, normally to a neutralizer and a drain. If the trap or drain backs up, the unit shows code 25 and stops. The RE has no secondary heat exchanger; it vents hotter flue gas through a non-condensing vent system listed in the RE manual.

Temperature is read by thermistors on the inlet, the outlet and the heat exchanger, and the board compares them with the set point. Factory maximum is 120 F on the RX; Rinnai literature lists 140 F as available through a parameter change (retailer and Rinnai spec data agree). Over-temperature, ground faults, airflow problems, scale and leaks are reported as two-digit codes on the controller, and 'LC' means scale build-up was detected and the heat exchanger needs a flush.

The pump versions (REP, RXP) carry an internal circulator for recirculation programs; a standard RX can be upgraded with Rinnai's internal pump kit (RX2RXPCK, listed by a retailer). Without a dedicated return line, a thermal bypass valve at the far fixture is needed. Wi-Fi control is through a separately sold Rinnai module; initial setup on the RX uses Bluetooth and the Rinnai app.