Rinnai Water Heater Error Code 85: How to Fix It

Q: What does error code 85 mean on a Rinnai tankless water heater?

Error code 85 on a Rinnai tankless water heater means the outlet water temperature has exceeded the unit's safe upper limit, typically 167°F (75°C). The heater shuts down automatically to prevent scalding and internal damage. Common causes include a temperature setting that is too high, restricted water flow, scale buildup in the heat exchanger, or a faulty thermistor.

Q: How do I reset my Rinnai water heater after error code 85?

To reset a Rinnai water heater after error code 85, press and hold the On/Off or Reset button for 3–5 seconds, then release. The unit should power cycle and attempt to restart. If the error clears and does not return, the issue may have been a temporary temperature spike. If the error comes back, you will need to investigate the underlying cause such as temperature settings, flow rate, or the outlet thermistor.

Q: Can scale buildup cause Rinnai error code 85?

Yes. Scale buildup from hard water minerals inside the heat exchanger is one of the most common causes of error code 85. When scale coats the interior walls of the heat exchanger, it acts as an insulator and prevents heat from transferring efficiently to the water. This causes the water to overheat. Rinnai recommends descaling the unit at least once a year in hard water areas to prevent this problem.

Q: Is Rinnai error code 85 dangerous?

Error code 85 itself is a safety feature — the unit is shutting down to prevent harm, so the heater is working as intended. However, the underlying condition that triggered it (excessively hot outlet water) can be dangerous if the unit were to continue operating without intervention. Address the cause promptly. Until the issue is resolved, do not override or bypass the safety shutdown, and do not attempt to force the unit to run continuously.

Q: How often should I descale my Rinnai tankless water heater to prevent overheating errors?

Rinnai recommends descaling your tankless water heater at least once a year if you have hard water (above 7 grains per gallon). If your water is moderately hard or you have a whole-house water softener, every 2 years may be sufficient. Installing a water softener or a scale inhibitor filter on the cold water inlet can significantly reduce scale accumulation and help prevent error codes like 85 from occurring.

Abstract

Rinnai error code 85 indicates that the outlet water temperature from your tankless water heater has exceeded a safe threshold — typically above 167°F (75°C). This is a protective shutdown triggered by the unit's built-in safety system to prevent scalding or internal damage. The root causes usually involve an incorrect temperature setting, a faulty thermistor, scale buildup inside the heat exchanger, or a malfunctioning flow sensor restricting water flow.