Rinnai Error Code 18: Rinnai Code 18 - Exhaust Thermistor Fault

Severity
Warning
Type
digital display
Estimated repair cost: $100 — $250 (parts + labor)
Warning: Your Rinnai tankless water heater has detected a problem with its exhaust temperature sensor and has shut down. You will not have hot water until this is resolved.

How to Fix Rinnai Code 18

  1. Step 1: Turn the unit off and then back on again. This may reset the system and clear a temporary error.
  2. Step 2: Check the wiring connections on the outside of the unit for any obvious damage or corrosion. Do not open the unit.
  3. Step 3: If the error persists after resetting, contact a qualified HVAC technician to diagnose and repair the problem.

How to Reset Rinnai Code 18

  1. Turn off the unit using the power button on the front panel.
  2. Turn off the gas supply valve.
  3. Turn off the circuit breaker supplying power to the unit. Wait 30 seconds.
  4. Restore power at the breaker, then open the gas valve.
  5. Turn the unit back on. If the error returns, the underlying issue needs repair.

Resetting clears the code from the display but does not fix the underlying fault. If the code returns after reset, proceed to the diagnostic steps above.

What Does Rinnai Error Code 18 Mean?

Rinnai error code 18 indicates a fault with the exhaust thermistor, which is a temperature-sensitive resistor that monitors the temperature of the exhaust gases leaving your tankless water heater. The unit's control board relies on accurate readings from this thermistor to ensure safe and efficient operation. If the thermistor is faulty, sending incorrect or no data, the unit will shut down to prevent potential overheating or other problems. This shutdown is a safety measure. The control board needs to know the exhaust temperature to properly modulate the burner and fan speed. A faulty thermistor can lead to the unit running too hot, potentially damaging components or creating unsafe exhaust conditions. It's important to address this issue promptly to restore hot water and ensure the continued safe operation of your Rinnai tankless water heater.

Common Causes

  • Faulty exhaust thermistor
  • Loose or corroded wiring connections to the exhaust thermistor
  • Damage to the thermistor wiring harness
  • Problem with the main control board

Diagnostic Steps

If... Check Expected
Unit displays error code 18 Measure resistance across the exhaust thermistor terminals at the control board with the unit off. Resistance should be within the range specified in the Rinnai service manual for the measured ambient temperature. Typically around 10k ohms at room temperature.
Thermistor resistance is outside the specified range or reads open/shorted Inspect the wiring harness and connectors between the thermistor and the control board for damage or loose connections. Wiring and connectors should be intact and securely connected.
Wiring and connections are good, but the thermistor resistance is still incorrect Replace the exhaust thermistor. New thermistor should read within the correct resistance range.
Error persists after replacing the thermistor Check the voltage at the control board terminals for the thermistor circuit. Voltage should be within the range specified in the Rinnai service manual.

Applicable Rinnai Models

Replacement Parts

Part OEM Number Est. Cost Where to Buy
Exhaust Thermistor 812200068 — Find Part
Main Control Board — — —

Related Codes on Same Models

Other codes that affect the same Rinnai model families as 18.

Code Description Severity
02 Stagnant Water Detection Warning
10 Air Supply/Exhaust Blockage Warning
11 No Ignition Warning
12 Flame Failure Warning
14 Thermal Fuse Warning
16 Over-Temperature Warning
19 Electrical Grounding Warning
25 Condensate Overflow Warning
29 Condensate Neutralizer Warning
31 Inlet Thermistor Failure Warning