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Over time after about 5 to 6 years many Noritz heaters tend to display error code 90 and stop working. The unit has to be shut off and back on for it to work again for a short time. Error code 90 is indication of abnormal combustion and can have a few causes, including blockage in the exhaust vent. However the problems lies mostly with the combustion fan. that is the fan that you hear when the unit is running. It is unable to deliver enough air to normal combustion.
Replace the fan and while doing it clean the air pathway with compressed air. In addition remove the ignition and heat senors (all in one assembly, in the middle of the unit) and sand them very slightly with very fine sandpaper (400 or higher) to remove build up on them.
While the fan is being ordered you can at your own risk operate the unit with the cover slightly open (about 2 in.) at the bottom and the unit will run without code 90. It is highly recommended to hire a trained technician for the above work as gas appliances pose a significant danger to human life and property if not installed or maintained properly. This solution is for informational purposes only. Please be safe. Tankless Water Heater Buying Guide Best Prices on Gas and Electric...
The error is a HIGH LIMIT THERMAL ERROR. That means Calcium buildup has appeared in the unit. It is also called scale. To clear the error you have to power it off. That is done by the circuit breakers. You should descale the unit first though.
The Noritz field manual says that error code 66 is Bypass Water Control Valve failure. Call support at 866-766-7489 with your model #, serial # (both on the unit face plate), date of purchase, to see if the valve can be replaced under warranty.
1) flashing burner light - a flashing burner light indicates the unit is getting an error code common error codes: A. continuous flashing - ignition failure or abnormal combustion; look for visible water in the machine indicating a leak, check gas pressure, debris or buildup on flame rod/igniter plug, or clean unit with compressed air B. 3 flashes - error 16, abnormal outlet water temperature; typically this code indicates you'll need to descale you unit C. 4 or 5 flashes - exotic error codes; you should contact Noritz technical support to deal with these codes
2. If there is no flashing code, try increasing the hot water flow. In the summer time you typically use less hot water; this can result in intermittent operation of your tankless heater. Increase water flow by either: A. cleaning your showerhead and removing scale deposits B. widening your flow restrictor C. Remove your flow restrictor in the showerhead
3.If none of these resolves your problem, you probably have debris in your flow sensor. Contact Noritz technical support for a walkthrough on removing debris from a flow sensor.
I called Noritz error code 16 is a overheat problem. Noritz say's to flush your coil with vinegar or CLR.
Use a small pump to recirculate the CLR or vinegar through the system for 45 minutes and flush with fresh water. This worked for me.