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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.