Quick answer
Rinnai error code 79 means water leakage has been detected inside the heater; it applies to Non-SENSEI series models. Do not keep running it: water inside a gas appliance cabinet reaches the control board and burner. Close the cold-water isolation valve below the heater and look for water under the unit, then call a technician right away to find the leak.
Water where it should not be: Rinnai error code 79 means the leak detector inside the cabinet has sensed water. Rinnai's diagnostic guide titles it “Water Leakage Detected” and applies it to Non-SENSEI series models, Rinnai's name for its heaters outside the SENSEI line. If your heater is a SENSEI or SENSEI RX/CX, go by the label inside the panel, which is the authority on what each code means for that model.
Should you shut off a Rinnai that shows code 79?
Yes, and do not reset it to see what happens. Inside the cabinet the water pipes, the control board, the wiring harnesses and the burner share one small box. Water that reaches the board or its connectors causes shorts, and water on the burner upsets combustion. The code exists to stop the heater before a small drip becomes a failed board.
What to do in the first five minutes
- Close the cold-water isolation valve below the heater. It is the valve on the cold inlet pipe; a quarter turn puts the handle crossways to the pipe.
- Switch the heater off at the controller and unplug it.
- Look under the unit and down the wall below it for drips, a wet streak, or white or green crust on the fittings.
- Put a bucket or towel under any drip and photograph where the water is coming from.
- Do not remove the front cover. If water is running and the isolation valve will not stop it, close the main water shut-off for the house.
The valve positions and what to do if the handle is seized are in how to turn off a leaking water heater.
Where the water comes from
- A leaking heat exchanger or internal pipe joint. Rinnai's procedure checks the heat exchanger and all piping inside the heater, and this is the serious end of the list.
- Water pooled in the bottom pan. Rinnai tells technicians to look for it when no active leak is found; it can be left over from an earlier drip or from moisture that ran down inside the cabinet.
- A wet or faulty leak sensor. If the cabinet is dry, the sensor is dried and the heater restarted; a sensor that still reports water is replaced.
- A damaged sensor harness: loose, broken or damaged wiring at the leak detector produces a false alarm.
How a technician finds the leak
Rinnai's sequence runs from the simplest check to the most serious. The wiring harness at the leak detector is inspected first. Then the heat exchanger and every pipe and joint in the cabinet are examined for leaks. If nothing is leaking, the bottom pan is wiped or blown dry, the sensor is dried, and the heater is restarted.
If the pan was dry all along, the sensor is unplugged as a test and the heater fired; a heater that runs with the sensor unplugged gets a new sensor. That test is a diagnostic step for the technician, not a way to keep using the heater.
Repair or replace: the heat exchanger decision
A sensor, a harness or a joint is a repair, and most tankless repairs land between $150 and $800, with a $90–$150 diagnostic visit credited to the job. A leaking heat exchanger is different. It is the major part in the heater, so it becomes a repair-or-replace decision and a warranty question at once.
- Find the model and serial number on the rating plate on the side of the heater, and the install date on your invoice.
- Ask whether the heat exchanger is still under the manufacturer's warranty and whether the unit was registered; the terms are on your warranty card.
- Get the heat exchanger repair as a written quote with parts and labour shown separately.
- Compare it with a new tankless at $3,500–$6,500 installed. A working rule: replace when the repair is more than half the cost of a new unit and the heater is over 12 years old.
A leak that has already reached the floor is also an insurance question, and whether home insurance covers a leaking water heater goes through it. ZK Mechanical handles Rinnai leaks under tankless water heater repair: call the 24/7 line at (647) 801-1252 or send a message. The other Rinnai codes, including code 14 for overheating, are in the Rinnai error code guide.