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

[ColdWaterSandwich]

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 set 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.

 

[Twowaxhack]

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…

 

[Twowaxhack]

I’ll for sure be calling Rinnai in the morning and present this same scenario and see what they tell me.

You’ve peaked my interest.

 

[ColdWaterSandwich]

For anyone interested here is the disassembled valve:



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.

 

[Twowaxhack]

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.

 

[Twowaxhack]

I didn’t get around to this but I will. I had an emergency replacement of a 100’ water main this morning and just got finished.

 

[ColdWaterSandwich]

I agree it would be good for the unit to throw a code if the output temp drops too far below the set temp, but I don't think it does that. It does throw codes if the flame doesn't light (like, because I have the gas turned off!) or if the recirc pump can't run properly due to air in the lines.

Anyway, 1 full day in: it's like night and day. And, I have two years worth of temperature trend records from a temp sensor taped to the outlet pipe (one temp reading per minute), and I can tell you that in that two years worth of data, the output temp has never been as stable during sustained use as it is now.

I believe the valve may have been defective from the factory, as in a bit "arthritic" and didn't do a great job balancing the temp in real time. Then it got progressively stickier until for the last year I've been aware there was something going on, and eventually terribly wrong. When it was first installed I noticed some moderate deviations in temp, and just figured that was what the technology was capable of. Now, it's SO SOLID. I ran the shower while doing all sorts of things around the house intermittently using different amounts of hot water in addition to the shower, on and off, and my pipe-mounted temp sensor didn't budge more than a few tenths of a degree. At any point in the last two years I would have seen much more variation.

Note my pipe-mounted sensor maxes out around 100F... that's as hot as the sensor gets in partial contact with a brass angle fitting on the hot water outlet pipe, in a 65F basement. Sensor is wrapped with tape to hold it against the fitting. So 99F here equates to about 125 actual water temp. But the trends are clear.

Here is that run I did today, turning on and off water at faucets (in addition to the constant shower) every couple minutes:



And Here's my morning shower two days ago while my wife was doing stuff in the kitchen using hot water:



Just wow. So much better now!!!