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Rinnai RU98i - Intermittent Cold Water Suddenly After Years Working

JasonZ Member Posts: 1
edited March 2019 in Domestic Hot Water

Anyone have any ideas on troubleshooting this problem? We have a Rinnai RU98i that has been working for 4 1/2 years now, and has been working just fine without any issues the entire time. Maintenance of cleaning the inlet filter and performing the service flushing has been done annually (last done 6 months ago).

Recently in the last two months, when taking showers (or washing dishes) the hot water suddenly goes cold for about 5-10 minutes. I just now followed to manual instructions to check the controller for error codes (model # MC-91-2US-S), but the 9 recent codes all show as blank with _ _.

Does anyone have any ideas to troubleshoot this problem, short of calling a technician? Someone came by today and quoted $ to perform maintenance, but they couldn't even ID the problem, so now I'm resorting to online help to try to narrow it down and ID the issue, and then pay a technician if necessary to actually fix the problem.

Any suggestions would be greatly appreciated! Thanks in advance


  • Intplm.
    Intplm. Member Posts: 2,011
    Troubleshooting the problem may need to take place with a service tech. speaking with Rinnai technical Support via a phone call.
    These codes do not always show up on a Rinnai trouble shooting manual. Tech support, with the knowledge conveyed to your on sight tech. Should be able correct the problem.

    rick in Alaska
  • Slimpickins
    Slimpickins Member Posts: 340
    What temperature is your Rinnai set to? If it's at 135 or 140, you might try turning on dip switch 7 on the upper bank of tan switches.
  • nibs
    nibs Member Posts: 516
    The problem with talking to Rinnai, is that they do not want to talk to anyone but a licensed installer, because of liability problems. I know much more about my Rinnai than my local plumber/gas fitter (who is a great asset), but they dont want to answer even the simplest question.