What does Rinnai error code 54 mean?

Rinnai titles code 54 High Exhaust Gas Temperature, and its diagnostic manual spells out the sequence. If the exhaust temperature exceeds 163 degrees Fahrenheit, the water heater reduces its input Btu firing rate. If after 120 seconds the exhaust gas temperature remains above 163 degrees, the heater shuts down and flashes code 54. Rinnai lists it for both the SENSEI series and the non-SENSEI models.

The logic is straightforward. Exhaust that runs hot means heat is not getting into the water, so it goes up the vent instead. Something is either keeping the heat from transferring or keeping the exhaust from leaving.

The manual lists ten steps. Grouped here by what they point at, and not in Rinnai's own numbering:

The vent and the air

• Check around the vent termination to be sure clearances were met.
• Check for restrictions in the vent system, on both the intake and exhaust ports.
• Verify venting is within Rinnai's clearance specifications and lengths.
• Check for fan air leakage, meaning leakage between the air intake and the combustion chamber.

The fuel and the water

• Ensure the heater was installed on the gas type that matches the rating plate.
• Ensure the inlet water temperature is not too high.

The heat exchanger

• Check the heat exchanger surface for hot spots. Rinnai says hot spots indicate a blockage inside the heat exchanger due to scale build up, and it directs a flush with four gallons of vinegar per the flushing instructions in the owner's manual. If the heat exchanger shows damage from poor maintenance, it is replaced.
• If scale caused the code, treat the water supply to prevent future issues.
• Remove the heat exchanger, separate the burner from the primary heat exchanger and inspect the fins for blockage. Debris is cleaned out with 120 psi compressed air, and if it cannot be cleaned, the heat exchanger and all gaskets are replaced. Rinnai notes the burner gasket must not be reused after removal.

The last resort

• If code 54 still appears after all of the above, replace the circuit board.

The shorter list in the owner section of the same manual reads: make sure the heat exchanger fins are clean and not blocked, confirm the inlet water temperature is not too high, clear the code by resetting the main power supply to the water heater, and ensure the condensate is not blocked.

Start with the gas smell rule. A gas smell changes everything: skip the checklist, avoid switches and phones inside the building, step outside, and call Southwest Gas at 877-860-6020 or 911. Rinnai's manual restricts service instructions to trained and qualified professionals and says to shut off power, gas and water before servicing.

With no gas smell, there are three reasonable owner steps:

• Reset the power once. Rinnai's own owner list says the code is cleared by resetting the main power supply. If the heater restarts and the code does not return, note the date. A code that returns is not a one time event.
• Look at the vent terminal from the ground. Rinnai's first two steps are about clearances and restrictions at the termination. Anything stacked against the terminal or covering the intake or exhaust is worth clearing with the heater off, and worth telling the technician about.
• Look at the condensate line. If your heater has a condensate drain, Rinnai's list includes a blocked condensate line. A drain that has come loose, kinked or backed up is visible without opening anything. Our page on Rinnai error code 25 goes through what a good condensate drain looks like.

Do not take the front cover off, do not try to flush the heater unless you are sure of the procedure, and do not attempt a gas type or altitude change. Those are technician tasks.

### Why is scale the likely story in Phoenix?

The heat exchanger hot spot check is the one to think about. Rinnai's owner manual says scale build up is caused by hard water and can be made faster by a high temperature setting, and that damage from lime build up is not covered by the warranty. It also says the heater includes a service reminder that, when selected, shows the SS code to say it is time to flush. Failure to flush, in Rinnai's words, will cause damage to the heat exchanger.

The City of Phoenix publishes water quality reports every year, and hardness is part of them. The Pacific Northwest National Laboratory's study of hard water and water heater efficiency, done for the Department of Energy, says scale accumulation will impact the efficiency of tankless and gas storage water heaters. Rinnai's diagnostic steps make the same connection from the other direction: hot spots on the heat exchanger mean scale is blocking the flow of heat into the water, and a heater that cannot pass its heat into the water sends more of it up the vent.

That is why a heater that has never been flushed is the first suspect for 54 here. Our page on how often to descale a tankless water heater in Phoenix gives a schedule, and our page on the Rinnai LC code explains the earlier warning.

### What does Rinnai's flush involve?

The owner's manual gives the steps for a heater with isolation valves. In short, power is turned off and disconnected, the hot and cold shutoff valves are closed, a pump is connected to the service valves, and four gallons of undiluted, food grade white vinegar are circulated through the heat exchanger for at least one hour at about four gallons per minute. The heater is then rinsed with water for five minutes, and the inlet filter is removed and cleaned. Rinnai says it strongly recommends isolation valves so the exchanger can be flushed. If your heater has none, that is a good reason to have a plumber do the work.

### What does 54 look like before it shows up?

Rinnai's description says the heater cuts its input first. From the shower, a heater in that stage may deliver water that is a little cooler than the setting, or that cycles between warmer and cooler, because it is firing at a reduced rate. The manual does not describe what the user sees during those 120 seconds, so treat that as the likely effect of the reduced input and not a promise. What Rinnai does say is that the response is automatic and that the shutdown follows if the exhaust stays hot. If you notice weaker hot water on a heater that has gone a few years without a flush, it is worth having it looked at before the code appears.

Call if code 54 comes back after one reset, if the heater has never been flushed, if there is no isolation valve set to flush through, or if you smell gas. Also call if the vent is damaged or the terminal is out of reach. Many of Rinnai's items, such as removing the heat exchanger to inspect the fins, need a technician.

HQ Plumbing and Air is licensed and insured and works emergencies 24/7. Free estimates and upfront pricing come standard. For every code on one page, see the Rinnai tankless code list. Our page on Rinnai error code 14 covers the other overheat safety.