Rinnai cycles on/off without error code

tve

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....

tve

Oh, I didn't notice I could attach images...

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.

tve

Something odd I noticed while troubleshooting further is that the inlet water temperature reported on the controller drops when the heater turns on. It starts out around 90F and when the heater turns on drops to ~50F within 3-4 seconds. Then when the heater overheats and turns off again the temp reported goes back up to ~90F. (Water flow rate and water temp are stable all the time.)
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....

tve

Another update... I did an acid flush of the heat exchanger and now the Rinnai seems to be working normally, or at least a lot better. I need some more cycles to be sure and I have the feeling that I should do another flush.
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.

Jack

You could buy an MV-100 controller for the V65 and it will give you a boat load more info. You bought the Value series and it doesn't have the feature set the other models, as the name implies. All of the other Rinnai models for the past few years have all the info of the MV-100 built into the standard control. As well, on anything but the V series you can go wireless to see what is going on with the water heaters on any unit back to '06 production. I guess you are now on a tighter vinegar flush schedule.