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Rinnai RU98 issues

Hello everyone!

I recently bought a house that has a Rinnai RU98 tankless water heater. During my first shower the water was hot for about 3 minutes then went ice cold. I bolted to the water heater and saw an error code 25. I turned the heater off and on and it fired back up, a couple minutes later the water went cold again. This has happened off and on for the last 4 days.
I had a friend come over last night who is pretty knowledgeable with water heaters and we cleaned the water intake filter (the filter was pretty gross) and we tested the water. Hot water ran for 8-10 minutes continuously without issue.

This morning the water went cold again within 4-5 minutes and then would go cold every couple minutes after I shut the unit off and back on.

We noticed some debris similar to what was in the filter, on the hardware inside the unit where the filter was.

Does anyone think flushing the heat exchange will fix this?

Thanks for all of your time.


  • JackJack Posts: 1,029Member
    That unit is internally trapped on the condensate side. Therefore, the field supplied drain line cannot have any additional traps or sags in it. As well, the termination of the drain cannot end up in standing water as it has the same affect. If there is a sag/trap/submerged end, the condensate builds up in the internal receiver and triggers the 25 code shutting it down. Disconnect the drain below the heater. It is probably piped with 3/4" pvc. Put a 3/4x1 1/2-2" bell reducer on the building drain side and let the unit drain go into that. Again, you need a gap in there.

    Oh, also, if you are using a condensate pump on this a submerged drain will do the same thing.
  • JohnnieBeeJohnnieBee Posts: 4Member
    Thank you so much for the quick response Jack! I will try look in to that tonight if I do I will post the results!

    Thanks again!
  • JohnnieBeeJohnnieBee Posts: 4Member
    Hey Jack,

    Quick question from my friend; what is the easiest way to inspect or remove the condensate tank/float?

    Thanks for all your help!
  • JackJack Posts: 1,029Member
    Well, if you look at it, there is a connection in and an outlet. There are also wires connected. It sounds like your friend wants to correct a drain issue by altering the unit. Yes?

    If so, get a new friend. You own the unit and can do as you please, but I would suggest you just fix the drain properly. It should be pretty straightforward. Certainly easier than getting into the unit piping and electronics. Also that tank is there for a reason. It separates the combustion side from atmosphere. There is a warranty associated with that unit. You begin removing parts and......well, you can connect the dots.

    Btw, there is no float.
  • JohnnieBeeJohnnieBee Posts: 4Member
    Thanks again Jack.
    He wasn't wanting to alter the unit, just trying to get a better understanding.

    It was a combination of the condensate drain being submerged and the hose running to the condensate neutralizer not being laid out right.

    Thanks again for all your help! It was nice to finally take a hot shower!
  • JackJack Posts: 1,029Member
    Sorry to jump at you there. Glad it is operating properly. That's a good unit.
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