Rinnai Persistent Cold Water Sandwich: Firmware upgrade or other troubleshooting?

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The Rinnai is fixed!! 😄

It was the bypass flow control valve. The tech reviewed all the testing with Rinnai and they agreed it was likely the flow valve, shipped him one, he swapped it in, and it's working great now:
-Output temp comes up to seet temp way faster
-HEX output temp stays much closer to heater output temp (typically ~150sF for 125F output temp
-No longer drops temp dramatically when a vanity faucet is turned on while showering
-No more flame cutoff when the vanity then is turned off
-Overall dramatically more stable output temps.

I kept the old valve and disassembled it last night. It was totally stuck. I first took out the screws that hold on the electric motor, exposing the gear that rotates the valve in the housing. Could not spin it with my fingers at all. Pulled the valve part out of the brass housing so that I could have good leverage, holding the outer plastic part while trying to spin the actual valve part. With the leverage I now had I was able to force it to turn, back and forth, and after a few full rotations I was able to spin it open and close pretty freely. The valve would probably work fine now, at least for a little while. No idea why it failed in the first place, but it was absolutely stuck in the open position, with little if any actual motion based on whatever input signal the motor was getting

The tech did ask Rinnai about the lack of error messages, and apparently this isn't unusual in this situation; the HEX was definitely getting hotter than usual but not enough to trip the error code, and because the output of the heater was (eventually) close enough to set point (though unstable when flow changed), it didn't make any other codes. The flame going out when flow drops a bit was due to incorrect understanding by the brain of how much flow there actually was; it thought more water was going through the HEX than actually was.

It's unclear how much this will cost me; the parts are under warranty but probably not the labor. There's one more part backordered that he is going to do on a precautionary basis due to a buildup of crust around the pressure relief valve (inside the heater); there is a tiny leak. After that, he will submit paperwork and find out what is actually covered.

I did a bunch of tests today; so far so good. Thanks for all the input, ideas, and support.

I will update again once the other part is replaced and I get the bill.
 
It should’ve thrown a code If the outlet temp went totally cold while you were in the shower just because another faucet was open and shut.

That’s what I thought you had complained about…
 
For anyone interested here is the disassembled valve:

1741922508422.png

This picture was taken after I had exercised the valve to loosen it up, but this is approximately the position in which it was frozen. There's a lot of grease packing in there, and working it back and forth really loosened up the spinning.


After I put the valve back into the valve housing body, l could easily spin with my fingers the gear shown at the right of the plastic valve assembly. I imagine the stuck valve might have caused the electric drive motor to fail; I don't have a simple way to test it as-is, so I should just throw this thing away, but I'm a packrat and will probably hang onto the thing for a while (like, until I sell the house sometime before I die...).

If you call Rinnai, this is what the plumbing tech reported to them:
-Sluggish getting up to temp when ~1.8gpm flow through the heater is called (shower hot water flow rate). Input water temp 50F.
-When close to set output temp, HEX temp ranges between 170-190F typically
-System reports bypass valve % never below 9%, sometimes gets up 35% but seems to act funky (he had better details, sorry)
-When flow raised to about 2.5 GPM ("add vanity sink flow") output temp drops to 100 or even lower before gradually rebounding
-Shutting off the vanity flow and resuming just 1.8 GPM typically causes flameout for 5-30 seconds with super cold water coming through

He had other details ready to go based on the script he is used to responding to; I don't have those details handy.
 
I’m not going to worry with giving heat exchanger temps as that’s not necessary.

My main concern is…. You said…..while in the shower the water temp is fine but when someone turns on another demand like a lavatory faucet and then turns it off, the temp goes cold and the burner cuts off then it fires itself back up. No codes thrown period.

My position is this shouldn’t be happening without throwing a trouble code. The heater shouldn’t completely shut down then fire back up like that.

The outgoing temp should be stable….this is a safety issue. It can cause a chill affect that can make a person jump out of the cold water and fall……

Same if the opposite was happening and the water got too hot. The outgoing temp is constantly being monitored and if it gets a degree or few out of spec the code should throw. You’re saying the burner is shutting off…..that means out of spec temp water is flowing to the faucet.
 
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