Quick answer
Rinnai error code 16 means high outgoing water temperature: a safety shutdown because the outlet water exceeded 95 °C (203 °F). Do not keep running it; water that hot is a scald hazard and the shutdown is deliberate. Turn the heater off, let it cool, and check that nothing is blocking the outside vent terminal. If the code repeats, call before restarting: it needs a flush and a gas-type check.
Water leaving the heater above 95 °C (203 °F) is what triggers Rinnai error code 16, which Rinnai's diagnostic guide calls “High Outgoing Water Temperature” and describes as a safety shutdown. That is close to boiling and far above anything a household setting asks for. The code applies across the SENSEI, SENSEI RX/CX and Non-SENSEI series, but codes vary by model: the label inside the panel, not this page, is specific to your heater.
Is code 16 dangerous?
The water was hot enough to scald on contact, and the heater caught it and shut down. Both halves matter. Do not keep resetting, because the shutdown is the control refusing to deliver water at that temperature.
The first water out of a tap after a code 16 can still be very hot. Open a tap slowly, keep children away from it, and do not use the shower until the heater has cooled and run normally at a sink.
What to do first: off, cool, and look at the vent terminal
- Turn the heater off at the controller and let it cool.
- Go outside and look at the vent terminal. Rinnai's first check for code 16 is a restriction in airflow around it: snow, ice, leaves, a nest, or a storage bin or shrub pushed up against it.
- Clear what you can reach by hand, without pushing anything into the pipe.
- Turn the heater back on and run a sink tap, testing the water with care.
- If 16 comes back, switch it off and call before restarting again.
Causes of high outlet temperature on a Rinnai
- A scale-restricted heat exchanger. Scale narrows the water passages, less water moves past the same flame, and what does get through leaves much hotter.
- Restricted airflow at the vent terminal, or foreign material in the combustion chamber and exhaust piping.
- A fan fault, or a check valve behind the fan motor that is stuck and restricting air.
- The wrong gas-type setting. The rating plate on the side of the heater names the fuel it was built for; on SENSEI models the setting is Parameter 10.
- A failed gas valve or fan motor, which Rinnai lists as replacement items once the checks above have passed.
Halton and Hamilton have hard water, which is why scale is the first thing to rule out on a code 16 in Burlington, Oakville, Milton or Stoney Creek. A heater that has gone several years without a flush is the usual candidate.
What the flush and combustion check involve
A repeat code 16 needs two things: a descale and a combustion and gas-type check. For the flush, the technician closes the isolation valves under the heater, connects hoses and a pump to the service ports, circulates a descaling solution through the heat exchanger, then rinses it and cleans the inlet water filter. Gas and power to the heater stay off while the solution circulates.
For the combustion side, the rating plate is compared with the gas supply and the gas-type setting, the fan and its check valve are inspected, and the combustion chamber and vent are checked for debris. Gas valve or fan replacement comes only after those pass. This is licensed gas work in Ontario, carried out by a TSSA-certified technician.
What a code 16 visit costs
- Descale flush through the isolation valves: $150–$300.
- Diagnostic visit: $90–$150, which goes toward the repair if you proceed.
- Fan, gas valve and other tankless repairs: $150–$800 for most jobs.
A heater installed without isolation valves needs them added before it can be flushed, and that work is quoted separately in writing. When you call, have three things ready: the model number from the rating plate, the date of the last flush if you know it, and whether the code appeared during a long shower or at a single tap.
Our post on hard water and tankless heaters in Hamilton and Halton explains how to slow scale down once the heater is clean. To have a Rinnai flushed and checked, see tankless water heater repair, contact ZK Mechanical or call (647) 801-1252. Two related faults are worth knowing: code 14 is an overheat trip at the heat exchanger and code LC is the scale warning that usually comes first. The complete list is in the Rinnai error code guide.