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....