Rinnai code 05: bypass flow control fault

Inside many Rinnai heaters, a small motorized bypass valve blends some cold water with the heated water. Code 05 means the control board is not getting the response it expects from that valve.

The meaning and service steps below come from Rinnai's own diagnostic code guide for its tankless water heaters.

What to try first

1. Turn off the power to the heater, then turn it back on.
2. Open a hot tap and let the heater fire.
3. If code 05 appears again, the valve likely needs replacing.

What a technician does

On SENSEI models, Rinnai tells technicians to measure the valve's resistance at the control board before replacing it. On older models, the fix after a failed power cycle is a new bypass valve.

A note on hybrid models

Rinnai lists different checks for code 05 on its RH180 hybrid tank-tankless units: cleaning the air filter and inspecting the venting for obstructions and correct clearances. If you have one of those, start there.

Before you call

Have the model and serial number from the rating label ready, along with the exact code and when it appears: at startup, during use, or only at high demand. Mention any recent work on the gas, water or electrical supply and when the heater was last descaled. This helps the technician bring the right parts and often saves a second visit.

See every code on our Rinnai error codes page.