Rinnai Gas Tankless Water Heater / Temp Fluctuation

[Hopefulrepair]

I have had a Rinnai Gas Tankless Water Heater in my house for 5-6 years now. In general, it has been fine. Hot water forever....

Since the temperature has dropped below 40 the heater has not been functioning correctly.
This is most noticeable in the shower. Here's how it goes:
Turn on hot water only
wait for water to travel from the basement up to the 2nd level
hot water for 20 seconds
cold water
repeat and rinse

USUALLY after 3 or 4 cycles of this I'll get hot water and it will stay hot. I say usually because this morning it did not stay hot even after 15 minutes. I always go through things like turning the hot water off and then turning it back on. I don't know if this does anything....it probably doesn't. It definitely didn't this morning. It is 7 degrees this morning.
The amount of water flowing does not change at all.
I emailed Rinnai and they told me to call a plumber.
Does anyone have any experience like this?
Once again, this only happens when it's cold.

Thanks,
Chris

[HayZee518]

not too familiar with a tankless heater but is there a flow switch in the inlet cold water pipe? if the paddle inside the pipe is missing you'll get erroneous signals to the heater.

[adnadeau]

Try standing next to the heater with the hot water running at some faucet in your house. Can you hear the burner in the heater firing? Does it keep firing or does it shut down after a few minutes?
There is a pdf service manual available on the rinnai web site for a lot of different models.
There is also a similar problem discussed on the forum dot heatinghelp dot com. Here is a direct quote from that site:

"What error codes are you getting? Pull the immersion sensors on the upper rt and at the hot water exit of the unit. It is the 1/8" ss sensore in the well connected with two white wires. Two small phillips head screw. There is an O-ring in the well. Sometimes it stays on the sensor, sometimes in the well, sometimes you don't see it fall to the floor . Is the sensor coated? Sounds like you have climbing temps and it is tripping on limit. Does the unit restart when temps drop? Read the output temp on the unit (push down arrow for at least two seconds and then while holding the down arrow push the on/off button )and monitor the output temps. Are there any indications of condensation coming back to the unit. If there is any debris in the hx fins. You can check by removing the flame rod and sparker. Get a light look up into the bottom of the hx fins. Remove the vent and examine top of the hx for same. If condensation has come back onto the hx and cooked off it will leave a residue which inhibits the the flow of flue gases, which can increase water temps. If a Rinnai exceeds its set-point by 6F it will shut down and restart when temps come down. In this condition it will not generate an error code. It is possible that the gas valve may be malfunctioning, but I'd look elsewhere first."