Rinnai Water Heater Error Code 21: How to Fix It

Q: Can I use my Rinnai water heater with error code 21?

No, error code 21 prevents the heater from operating safely. Attempting to use it could result in no hot water or a potential gas leak. Address the issue before use.

Q: How much does it cost to fix error code 21?

If you DIY, a replacement solenoid valve costs around $50–$100. Hiring a professional may cost $150–$300 including labor and parts. Control board replacement can be $200–$400.

Q: Can a dirty air filter cause error code 21?

No, error code 21 specifically relates to the solenoid valve circuit. However, a dirty air filter can cause other errors like code 11 (no ignition). Always clean filters regularly.

Q: Will error code 21 clear itself?

Sometimes a temporary electrical glitch may clear after a power reset, but if the solenoid is faulty, the error will return. Perform the diagnostic steps to determine the root cause.

Q: Where is the solenoid valve located on my Rinnai tankless water heater?

The solenoid valve is mounted on the gas manifold assembly, typically behind the front panel and near the gas inlet. Refer to your owner's manual for exact location.

Abstract

Error code 21 on your Rinnai tankless water heater indicates a problem with the solenoid valve circuit. This typically means the gas solenoid valve is not opening or closing properly, which can prevent the heater from igniting. The issue is often caused by a faulty solenoid valve, a loose wiring connection, or a problem with the control board.