What causes Rinnai Tankless Water Heater 55 (SS)?

The service interval the installer set in Parameter 03 (6 months, 1 year or 2 years) has elapsed since the heater was installed or the code was last reset. The heat exchanger is due for its descaling flush, which Rinnai says matters most with hard water and higher set temperatures, since both speed up scale build-up. The code was reset without a flush, so it returns at the next interval.

How do I fix Rinnai Tankless Water Heater 55 (SS)?

Confirm the display is SS (it reads like 55) and not another number. Codes such as 10, 11, 12 or LC point to different problems. To clear the reminder, press the On/Off button on the temperature controller five times within five seconds. Rinnai notes it will display again when the heater reaches the interval set in the parameter settings. Schedule the flush. Rinnai's owner instruction is to contact your local service provider to flush and service the water heater when SS appears, and says failure to flush will damage the heat exchanger, with lime damage not covered by the warranty. If you flush it yourself and your heater has isolation (service) valves, Rinnai's procedure is: turn off power at the controller and disconnect power; close the hot and cold shutoff valves; connect a pump hose to the cold-side service valve and a drain hose to the hot-side service valve; circulate four gallons of undiluted food-grade white vinegar through the heater for at least 1 hour at about 4 gallons per minute; then rinse with fresh water for five minutes, clean the in-line cold water inlet filter, reopen the valves, disconnect the hoses and restore power. Clean the cold water inlet filter as part of the flush. A clogged filter also restricts flow. If you have hard water, plan for treatment. Rinnai says hard water must be treated to prevent scale build-up, and it may require a more frequent flushing schedule.

When should I call a pro for Rinnai Tankless Water Heater 55 (SS)?

Your heater has no isolation valves, so there's no easy way to connect a flush kit. Rinnai strongly recommends having them installed. You want the reminder interval changed or turned off. Parameter 03 is set in the heater's parameter settings during installation, which is an installer task. The heater shows LC, LC0 to LC9 or 00 instead of SS. Those mean scale has already built up, and on newer non-SENSEI products the heater locks out until it is flushed and reset. Hot water is fluctuating, the heater is noisy, or other codes appear along with SS. You smell gas: leave the house and call your gas supplier from outside.

Code: 55 (SS)🚿 Rinnai Tankless Water Heater

Rinnai Tankless Code 55 (SS): Service Soon, Time to Flush the Heat Exchanger (Meaning, Reset, Fixes)

Q: What does Rinnai Tankless Water Heater 55 (SS) mean?

"55" on a Rinnai SENSEI tankless water heater is really "SS", which looks like 55 on the controller's display. It stands for Service Soon: a time-based reminder the installer turned on, telling you it's time to flush the heat exchanger and service the heater. It isn't a burner or ignition fault. You clear it by pressing the On/Off button on the temperature controller five times within five seconds, but Rinnai says the heat exchanger should actually be flushed, because scale from hard water damages it and that damage isn't covered by the warranty. Rinnai's SENSEI installation and operation manual calls this code the Service Indicator (Service Soon, SS), and its own figures print it as "Service Soon (55)" because the letters SS look like 55 on the display. Rinnai's diagnostic-code troubleshooting manual lists SS for the SENSEI series as "a time-based service indicator set during installation" that "indicates that it is time for service," and says the heat exchanger should be flushed to prevent damage and hard water must be treated to prevent scale build-up. The interval is Parameter 03 (Service Soon) in the parameter settings: A = not active (the default), B = 0.5 year, C = 1 year, D = 2 years. Rinnai says the choice is the installer's preference based on water conditions, and the code will come back each time the heater reaches the selected interval after a reset. Don't confuse it with the LC, LC0 to LC9 codes on non-SENSEI models: those mean scale has already built up in the heat exchanger, and on newer non-SENSEI products they lock the heater out until it's reset and flushed.

Reviewed 2026-10-23 · Based on 3 manufacturer & service sources

What the code means

Rinnai's SENSEI installation and operation manual calls this code the Service Indicator (Service Soon, SS), and its own figures print it as "Service Soon (55)" because the letters SS look like 55 on the display. Rinnai's diagnostic-code troubleshooting manual lists SS for the SENSEI series as "a time-based service indicator set during installation" that "indicates that it is time for service," and says the heat exchanger should be flushed to prevent damage and hard water must be treated to prevent scale build-up. The interval is Parameter 03 (Service Soon) in the parameter settings: A = not active (the default), B = 0.5 year, C = 1 year, D = 2 years. Rinnai says the choice is the installer's preference based on water conditions, and the code will come back each time the heater reaches the selected interval after a reset. Don't confuse it with the LC, LC0 to LC9 codes on non-SENSEI models: those mean scale has already built up in the heat exchanger, and on newer non-SENSEI products they lock the heater out until it's reset and flushed.

Likely causes

The service interval the installer set in Parameter 03 (6 months, 1 year or 2 years) has elapsed since the heater was installed or the code was last reset
The heat exchanger is due for its descaling flush, which Rinnai says matters most with hard water and higher set temperatures, since both speed up scale build-up
The code was reset without a flush, so it returns at the next interval

Safe DIY checks

⚠ Before you start

Unplug the appliance (or switch off its breaker) and shut off the water supply valves before removing panels or disconnecting hoses. Keep towels handy for residual water.

Confirm the display is SS (it reads like 55) and not another number. Codes such as 10, 11, 12 or LC point to different problems.
To clear the reminder, press the On/Off button on the temperature controller five times within five seconds. Rinnai notes it will display again when the heater reaches the interval set in the parameter settings.
Schedule the flush. Rinnai's owner instruction is to contact your local service provider to flush and service the water heater when SS appears, and says failure to flush will damage the heat exchanger, with lime damage not covered by the warranty.
If you flush it yourself and your heater has isolation (service) valves, Rinnai's procedure is: turn off power at the controller and disconnect power; close the hot and cold shutoff valves; connect a pump hose to the cold-side service valve and a drain hose to the hot-side service valve; circulate four gallons of undiluted food-grade white vinegar through the heater for at least 1 hour at about 4 gallons per minute; then rinse with fresh water for five minutes, clean the in-line cold water inlet filter, reopen the valves, disconnect the hoses and restore power.
Clean the cold water inlet filter as part of the flush. A clogged filter also restricts flow.
If you have hard water, plan for treatment. Rinnai says hard water must be treated to prevent scale build-up, and it may require a more frequent flushing schedule.

🛠 When to call a pro

Your heater has no isolation valves, so there's no easy way to connect a flush kit. Rinnai strongly recommends having them installed.
You want the reminder interval changed or turned off. Parameter 03 is set in the heater's parameter settings during installation, which is an installer task.
The heater shows LC, LC0 to LC9 or 00 instead of SS. Those mean scale has already built up, and on newer non-SENSEI products the heater locks out until it is flushed and reset.
Hot water is fluctuating, the heater is noisy, or other codes appear along with SS.
You smell gas: leave the house and call your gas supplier from outside.

Sources

Last reviewed 2026-10-23. Always confirm against the manual for your exact model number.

Rinnai Tankless Code 55 (SS): Service Soon, Time to Flush the Heat Exchanger (Meaning, Reset, Fixes)

What the code means

Likely causes

Safe DIY checks

🛠 When to call a pro

Related codes

Sources