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Rinnai cycles on/off without error code

My symptoms are similar to the original question in https://forum.heatinghelp.com/discussion/138604/rinnai-cycles-hot-cold, I have a Rinnai V65e that heats a 40g tank, which is used for DHW and subfloor radiant (please don't flame me). It has worked great for 2 years and now turns on, ramps up to what sounds like max power, brings the outflow water temp rapidly above the 140F setpoint in ~20-30 secs, then turns off, cools down approx 20-30 secs, and repeats. This started rather abruptly 5 days ago.
Some details: circ pump flow is 1.6-2gpm as measured by the rinnai (variable speed, I tried both settings), inflow goes from 90F up to 115F when the circ pump is shut off. I can see the outflow temp rise on the rinnai controller and it shuts off when that exceeds 140F (which is the setpoint).
Diagnostic so far: I took the heat exchanger out and removed the vent (a PITA with the exterior mount version). Burners look clean. Top of heat exchanger has some white scale stuff which I blew out. Air circ seems fine to me.
The min BTU on the unit is 10.5KBTU, 1.6gpm with 40F rise takes 32kBTU, I have the feeling that it somehow operates at too high a power. I have not tried the vinegar flush. We have hard water, I'm using a TAC anti-scale filter, no LC code to be seen.
I would appreciate any tips on what to troubleshoot. Barring any input, I'll do the vinegar flush just because and then pull out some test tools to see which burner solenoids fire to see how many BTU it's firing at.
NB: I'm sure I'm gonna get flamed for using a Rinnai for this non-approved application... I started with a Polaris, which is exactly designed for my application. But I went through 3 units in 6 years (warranty, fortunately). First corroded at bottom, which seems to be a common problem. Second developed a leak in side-seam. Third corroded again like first. Each replacement was a total pain work-wise and we were out of hot water for days. AWH wouldn't replace the third unit on warranty. I replaced the polaris by a cheap electric 40g tank and use the Rinnai to heat that tank, using the electric element as back-up. I can go through 3 Rinnais for the price of one polaris and swapping the Rinnai....
Some details: circ pump flow is 1.6-2gpm as measured by the rinnai (variable speed, I tried both settings), inflow goes from 90F up to 115F when the circ pump is shut off. I can see the outflow temp rise on the rinnai controller and it shuts off when that exceeds 140F (which is the setpoint).
Diagnostic so far: I took the heat exchanger out and removed the vent (a PITA with the exterior mount version). Burners look clean. Top of heat exchanger has some white scale stuff which I blew out. Air circ seems fine to me.
The min BTU on the unit is 10.5KBTU, 1.6gpm with 40F rise takes 32kBTU, I have the feeling that it somehow operates at too high a power. I have not tried the vinegar flush. We have hard water, I'm using a TAC anti-scale filter, no LC code to be seen.
I would appreciate any tips on what to troubleshoot. Barring any input, I'll do the vinegar flush just because and then pull out some test tools to see which burner solenoids fire to see how many BTU it's firing at.
NB: I'm sure I'm gonna get flamed for using a Rinnai for this non-approved application... I started with a Polaris, which is exactly designed for my application. But I went through 3 units in 6 years (warranty, fortunately). First corroded at bottom, which seems to be a common problem. Second developed a leak in side-seam. Third corroded again like first. Each replacement was a total pain work-wise and we were out of hot water for days. AWH wouldn't replace the third unit on warranty. I replaced the polaris by a cheap electric 40g tank and use the Rinnai to heat that tank, using the electric element as back-up. I can go through 3 Rinnais for the price of one polaris and swapping the Rinnai....
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Comments
This is how a heating cycle looks like. In yellow the outflow temp (my sensor, not the rinnai's), clearly visible is how it goes up & down. In green the inflow temp. The red shows the duration of the circ pump being on. X axis is time, i.e., a little more than 3 minutes total.
Here is a heating cycle a month ago. This one lasted a bit longer, but the outflow is constant after the initial ramp up.
That lead me to try and read the thermistor directly to see why it would malfunction like that (the input water temp is stable). To my surprise, it turns out that the V65e does not have an input water thermistor! I thought that the firing rate is calculated from the inlet temp and flow rate, hmmmm. So the value reported must be estimated? Seems like it's estimating incorrectly and thus over-firing and then over-heating. I'm not sure I understand the control algorithm it uses....
So... as far as I can tell it appears that the v65e does not really have the ability to flag LC error codes. Because it doesn't have an inlet temp sensor and it also doesn't have active control over the bypass I don't see how the controller could possibly determine that the heat exchanger is plugged up... Sigh.