Rinnai continuous flow hot water system code 36: Lower tank thermistor fault

Continuous flow hot water system, Australia

The lower tank thermistor has failed or is reading out of range.

This needs a licensed gasfitter. There is nothing safe to check yourself. Leave the covers on and book a professional.

What causes it

A fault in the lower tank thermistor or its wiring. Rinnai lists this as needing a service call.

Book a licensed gasfitter

Turn the unit off and leave the covers on. Gas components must only be serviced by a licensed technician.
Note the code, how often it appears, and the model and serial numbers from the rating label.
Book a licensed gasfitter and give them those details.

6 models in 2 ranges

Range	Models
Heavy Duty (commercial)	REU-NB3237WC-AK (Heavy Duty 32C), REU-NB3237FFC-AK, REU-NB3237WC-50-AK, REU-NB3237FFC-50-AK
Infinity Enviro	REU-NB3237W-AK (Infinity Enviro 32), REU-NB3237W-50-AK

Checked against the manufacturer's manual on 9 October 2026.

Code	Meaning	What you can do
21	Data transfer or dip switch error	Call a pro
22	Gas valve adjustment fault	Call a pro
25	Sensor fault or condensate blockage (depends on model)	Depends on your model
32	Outgoing water temperature sensor fault	Call a pro
33	Heat exchanger thermistor fault	Call a pro
34	Temperature sensor fault (depends on model)	Call a pro
38	Exhaust temperature thermistor fault	Call a pro
41	Frost protection (ambient) sensor fault	Call a pro
51	Inlet water thermistor fault	Call a pro
52	Gas modulating valve fault	Call a pro
54	High exhaust gas temperature	Safe checks first, then a pro
55	Service reminder	Safe checks first, then a pro