Leak detector (water leak sensor)
A sensor in the bottom of the cabinet that notices water where there should not be any. When it triggers, the cheap outcome is a failed sensor and the expensive one is a leaking heat exchanger — and you cannot tell which from outside.
What it does
Sits in the bottom pan of the heater and detects standing water. It exists because of what is directly above it: a sealed gas appliance full of mains electronics, where a slow leak from the heat exchanger or a fitting will find the control board before it finds the floor.
When it triggers, the unit stops.
How it shows up in the codes
Rinnai gives it code 79, and only on the non-SENSEI generation — the number is not assigned on the SENSEI or SENSEI RX ranges.
Rinnai’s diagnostic order is worth knowing because it tells you how the manufacturer ranks the possibilities: the wiring harness first, then the heat exchanger and all internal piping, then the bottom pan for pooled water, and only after all of that the idea that the sensor itself has failed. The sensor is the last suspect, not the first.
Why that ordering matters to you
Because the bill depends entirely on which one it is. A failed sensor is a $30-120 part. A leaking heat exchanger on a unit old enough to be a non-SENSEI Rinnai is often the point where replacement beats repair.
So a code 79 is a page where the honest advice is not “here is the part” but “find out which of these two it is before you spend anything”.
What to do now
- Switch the unit off at the breaker. Do not run it to see whether the code clears — the whole point of the sensor is that running it is the bad option.
- Close the cold water isolation valve to the unit if you can reach it safely.
- Look for water on the floor beneath the unit and for staining on the wall behind it, and tell the technician what you found. Where the water is narrows the diagnosis considerably.
Why this is not a DIY part
Water and mains electricity inside a sealed gas appliance. Even if the sensor turns out to be the fault, confirming that means ruling out a leak first, and that is an inspection inside the cabinet.
What a replacement usually costs
$30-120 for the sensor and screw. $150-400 installed if that is genuinely all it is — and materially more if the inspection finds the leak the sensor was built to catch.