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Rinnai tankless - intermittent hot water and no error code

I have a Rinnai tankless RC80HPi water heater. It was installed in my house back in 2009. It has been working flawlessly up until about 2 weeks ago. Then all of a sudden while trying to run hot water in either of the showers or in the kitchen sink, the hot water would start cycling between cold and hot. This cycling happens randomly and for various amounts of time. Sometime the cold water will last for 30 second, sometime it will last for a minute.
With the hot water at the kitchen sink and/or the shower turned on - if I am standing in front of my Rinnai water heater, I can see the water heater turn off and the In Use light go out. Meanwhile I can still hear water running through the water heater. The heat is just off. Then about 30 seconds to a minute later, the Rinnai water heater will fire up and start producing hot water again.
There are no error codes. The display consistently and always displays "120". I've checked to see what the water flow is through the water heater, it is 26 when I have the shower on and 11 when I have the sink on. I've checked to see what the outlet water temperature is, it is between 108 and 110 always.
I regularly flush this water heater, thus it is generally well maintained.
Twice in the past 3 days I have opened up the Condensate trap drain plug, and maybe a cup or two of water has flushed out. Is that normal?
Does anybody have any advice on what I can check next or what may be going on? I am at my wits end with this and want to get back to having an endless supply of hot water during showers or while doing dishes!


  • DZoroDZoro Posts: 768Member
    When you flush the water side are you using a cleaner?
    Has the burner side been maintained annually?
    There should always be some condensate inside the trap. When the system is running is there a steady flow of condensate running out?
  • lchmblchmb Posts: 2,932Member
    go into the control and check your gpm. I'd start guessing your flow servo is failing..
  • lukekr12lukekr12 Posts: 3Member
    Thanks DZoro!
    I didn't realize there was a "water side" and a "burner side". When I flush my tankless water heater annually I "flush the heat exchanger" with a solution of white vinegar and water as described in the Operation and Installation Manual.
  • lukekr12lukekr12 Posts: 3Member
    Thanks Ichmb!
    When you say "check your gpm" that done by pressing the up arrow for 2 seconds and then pressing the "On/Off" button? Or is there a different way to check gpm?
    I've done the way I described...and with the shower on the digital display shows 26; with the kitchen sink on it shows 11. I'm guessing that is gpm...
  • Intplm.Intplm. Posts: 760Member
    Might be that the fan and or the exhaust is dirty. Also, I'm not sure, but the gpm has to be just around 3 gallons per minute. (That's water flow rate feeding the unit.) If it fluxuates lower, the unit will tend to "hunt" for a reason to stay on which I think is what you are describing. If it stays on longer, you might find that it will show an error code and then shut down.
    So. Try checking the fan and the fan housing for dust build up.Check if water use has changed possibly reducing the water flow to the heater.
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