Tankless Water Heater Shuts Off During a Shower

Look at the display first

A unit that fires and then quits has almost always logged a code, even if the display has since cleared. On a Rinnai SENSEI RX you can read the last nine codes back: with the heater off, press and hold the On/Off button for 2 seconds and then press the up arrow at the same time. The stored codes flash in sequence. Repeat to exit.

That history is the single most useful thing you can bring to a service call — and often it means you do not need one.

What each pattern usually means

It cuts out after a few minutes, every time. Something is reaching a limit as the unit warms up. Overheat and high exhaust temperature faults behave exactly like this, and scale in the heat exchanger is the most common reason behind both. On a Rinnai those are codes 14 and 54, and both are a stop-using-it-and-call-someone answer rather than a reset-and-continue one.

It cuts out when someone opens another tap. Flow through the heater drops below the minimum it needs to keep firing. Check the inlet water filter, and be realistic about how many fixtures the unit was sized for.

It cuts out when the furnace or another gas appliance starts. That is a gas supply symptom, not a heater fault — inlet pressure sagging when total demand rises. It needs a manometer and a look at how the gas system was sized.

It quits within seconds of lighting, repeatedly. The unit is losing the flame or cannot prove it is there. On a Rinnai that is code 11 or 12, and the usual findings are a dirty or loose flame rod, or gas pressure. Not a DIY repair — the flame rod lives inside the sealed combustion chamber.

It only happens in cold weather. Suspect the vent terminal and the condensate line before the heater. Snow across a vent terminal and a frozen condensate line both produce a shutdown, and both clear themselves once things thaw — which is why the fault seems to come and go.

What not to do

Do not keep resetting it. Every reset asks a gas appliance to fire again with whatever caused the shutdown still present, and on overheat faults the safety device is specifically designed to keep tripping until someone fixes the cause. Two shutdowns in a row is the point to stop and read the code history.