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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!