31 Burner thermocouple out of range

What the control is telling you

Burner thermocouple reading is outside its allowed range: above 30 mV (possible over-firing) or below 8 mV (low gas pressure or low-Btu gas) during combustion.

Applies to: RH180 (Hybrid Tank-Tankless Non-SENSEI Series); Non-SENSEI Series

Likely causes

• Manifold gas pressures set incorrectly for altitude
• DIP switches set incorrectly
• Over-firing from high manifold pressure
• Blocked heat exchanger fins
• Low gas pressure or low Btu gas content
• Failed thermocouple

Test and repair steps

1. Check low and high fire manifold pressures and set them for your altitude per the Service Manual or Technical Data Sheet inside the front cover.
2. Confirm all PC board DIP switches are set correctly for altitude (see Technical Data Sheet).
3. Measure thermocouple output during combustion; normal is 20–27 mV. Code 31 triggers above 30 mV or below 8 mV.
4. Above 30 mV verify high fire manifold pressure. If within spec, remove manifold and burners and check heat exchanger fins; clean or replace the heat exchanger if clogged.
5. Below 8 mV verify forced low fire gas pressure. If within spec, contact Rinnai Technical Support.
6. If the fault persists, replace the thermocouple.

Settings for these units

• RUCS/RUS unit clearances · Non-SENSEI
• RUCS/RUS water quality limits · Non-SENSEI