Rinnai shuts off during use

hsanchez97

Hello all, long time lurker, 1st time poster. I've been dealing with this issue for a few weeks now. I have sifted through years of posts here and have learned so much but still failed to solve my issue, I hoping someone can share their insight.

My Rinnai RUC98i is turning off during a shower, it turns off after a few mins for about 30 seconds then turns back on, and just repeats this cycle. No error codes, just shows the 120 temp setting.

Unit was installed about 8 years ago by the previous owners, have not had any issues besides some weird train horn sound when the unit starts up in the winter, but never affected its ability to heat.

1 Shower is running about 2.3 the other about 1.8 Gallons a minute based on the unit readout, it is happening on both.

Here are some things I tried.

Tired opening multiple fixtures, got GPM up to 3.5 and issue remains

I run flow-aid through it about every 9 months, just did one last week hoping it would solve issue no luck.

Replaced water servo and flow sensor and the screen filter at the cold water inlet based on a lot of what I read here and at other sites. Lots of people solved similar issues replacing the servo, but no luck.

I inspected the exhaust with a snake cam I see no obstructions.

Checked condensation line for obstructions

Pulled and cleaned exhaust fan and the inlet tray

I am standing in front of the unit while running and I can see the "In Use" light just turn off for no reason, flame goes out, fan continues to exhaust, turns back on after 30 seconds then repeat.

I am at a loss of what to do next, has anyone had a similar experience? or can suggest any additional steps I should take?

hsanchez97

**Update, so the unit is set to 120 degrees. Using the temp reading in the menu I see the temp heat up, then it shuts off at 122 degrees, drops back down to about 80, then fires up.... heats back up to 122, then shuts off. It repeats this over and over. The previous owner had it set to 125, I lowered to 120 when we moved in 2 years ago and have had no issues until now.

Lowering the setting to 115 degrees prevents it from reaching 122 degrees and shutting off. It only gets up to 117-18 so we are able to take showers without the unit shutting down.

While this is great I feel like it is only a band-aid for the underlying problem and may come back to haunt me if I don't resolve.

EdTheHeaterMan

There may be a bad sensor that is sending an incorrect temperature reading. There is more than one. Perhaps the High Limit is reading 122° as a much higher temperature and shutting down the burner. That reading will not show on the display because it ia a limit control, not an operating control that modulates the flame based on the operating temperature sensor. I will look at the wiring logic to see if there is a high limit that could cause this BRB.

EdTheHeaterMan

From the limited information available from Rinnai on this particular item, it appears that there may be as many as 3 sensors that could cause your issue. There are 2 outgoing water temperature sensors and a heat exchanger sensor, of the thermistor type, that could possibly cause a High Limit failure that might shut the burner down. I am not sure why there would not be an error code that is associates with that. I might suspect the sensor that is at the heat exchanger may be sensing a higher than normal condition that might interrupt the burner. But I'm not familiar enough with the product to offer a fix. Perhaps you can talk it over with Tech Support to get their take on the problem.

hsanchez97

EdTheHeaterMan said:

From the limited information available from Rinnai on this particular item, it appears that there may be as many as 3 sensors that could cause your issue. There are 2 outgoing water temperature sensors and a heat exchanger sensor, of the thermistor type, that could possibly cause a High Limit failure that might shut the burner down. I am not sure why there would not be an error code that is associates with that. I might suspect the sensor that is at the heat exchanger may be sensing a higher than normal condition that might interrupt the burner. But I'm not familiar enough with the product to offer a fix. Perhaps you can talk it over with Tech Support to get their take on the problem.

Thank you, will give them a shout and report back.

hsanchez97

Tech support recommended cleaning the thermistor, will report back. 

hsanchez97

Thermistors looked new. Cleaned them anyways and problem persist.

Contacted Rinnai and they offered no help. Said I need to get a technician out to trouble shoot with them over the phone.

Intplm.

@hsanchez97

When you are done with the tech and Rinnai, drop a post here to let us know how things worked out.

hilltown

While your running the water, take a look @ #11 on the controller that is the hex temp. What is it reading?

hsanchez97

hilltown said:

While your running the water, take a look @ #11 on the controller that is the hex temp. What is it reading?

It reads to 203-204 before it turns off

hilltown

That’s a issue, it is overheating and shutting down, once it cools off it will go back in use as you described.. sounds like you have been good at flushing, may need a bypass valve, could be sticking open not allowing enough water thru the heat exchanger..

Are you giving the flush a full hour?

hsanchez97

hilltown said:

That’s a issue, it is overheating and shutting down, once it cools off it will go back in use as you described.. sounds like you have been good at flushing, may need a bypass valve, could be sticking open not allowing enough water thru the heat exchanger..

Are you giving the flush a full hour?

Yes, sometimes 90 mins. I replaced the water servo valve, is this a different valve?

hsanchez97

I had a tech scheduled today, He said he was familiar with Navien's, so didn't really know his way around this Rinnai. I helped him troubleshoot and recreate the problem with Rinnai rep through video. Rinnai said heat exchanger getting too hot.

Recommendation was to flush again and replace Bypass valve as Hilltown recommended. If that doesn't work, will look at replacing heat exchanger.

Part ordered, will report back.

hsanchez97

Success, replaced the bypass valve 5 days ago and issue is gone. Water is piping hot and Heat Exchanger temps stay in the 160's.

Takeaways for those experiencing similar problems is to monitor the HEX temp to determine if the unit is turning off because of over heating. Rinnai should have the unit produce some sort of code for this.

Replacing they bypass valve was very simple, took 15 mins. 4 out of 10 difficulty on this unit, was able to access and remove part without removing anything else. Replaced all O-rings also would recommend if changing any part.

Replacing the Servo was a 6.5 out of 10 difficulty, required really getting into the unit.

Another thing to mention is my unit used to produce a truly annoying air horn sound when cold starting. Here is a clip.
https://www.youtube.com/shorts/vgzstxI2pbQ
Replacing the servo solved the issue, so an unexpected win there after dealing with it for 2 years.

I saw multiple post regarding the same issue, but none really nailed down a solutions. I hope my experience will help someone in the future and I am able to contribute something to this great community.

Kickstand55

Based on what I've read here, you might consider a particulate water filter and a descaling filter just before the unit. I've done it to many units with hard low pH water and it works great.
Be sure to have the unit serviced every year. That means taking things apart and cleaning the inside well. Also have a gasket kit ready.