Rinnai code 54: high exhaust temperature

Exhaust from a tankless heater should stay within a safe temperature range. When it gets too hot, the heater protects itself and its vent pipe by shutting down.

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

How the shutdown works

According to Rinnai, when exhaust temperature passes 163°F the heater reduces its gas input. If the exhaust is still above 163°F after 120 seconds, it shuts off and shows code 54.

What Rinnai lists as causes

• Vent termination clearances not met, or restrictions in the intake or exhaust
• Venting longer or different from Rinnai's specifications
• A heater running on a different gas type than its rating plate shows
• Incoming water hotter than 160°F
• Scale inside the heat exchanger, which shows up as hot spots on its surface

The scale connection

When scale lines the heat exchanger, heat cannot move into the water, so more of it leaves with the exhaust. Rinnai's fix is to flush the heat exchanger with vinegar following the owner's manual. If you have hard water and the unit has not been flushed in a year or more, this is the likely cause.

How to reset it

1. Close all hot water taps.
2. Turn the water heater off at the temperature controller, wait about a minute, then turn it back on.
3. Open a hot tap and watch whether the code returns.

If the same code keeps coming back, the cause is still there. Repeated resets are not a repair.

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.