Getting to the Bottom of a SENSEI mystery with Sense

About a month ago had a Rinnai SENSEI RU160iP tankless water heater installed. Installers did a good job, works great.
I’m down in the basement and occasionally I hear the unit start to spin-up but it doesn’t start and no one is using water. This has happened more than once. No error codes, everything is working.
I did a bunch of research on this issue checking the manuals, searching forums and it could be several things from check valves to water hammer to pex pipes but there was no conclusive answer. I do not have a recirculation pump and checked that recirculation setting were “Off”.
I also contacted Rinnai Support, they responded the only time the water heater should ever start is if there is a call for hot water. Their support page also states: By design, the fan will keep running for a short time after the flow of water stops. This helps ensure constant water temperatures during rapid starting and stopping, as well as exhausting any residual gas flue products from the unit. It will turn off on its own after a few moments.
Rich Trethewey wasn’t returning my calls so guess I’ll have to figure this out on my own.
The Rinnai manual states the tankless unit will start when it senses .04 GPM water flow, some forums suggest maybe a leak in the plumbing causing these phantom starts- thanks to Sense I validated my well pump isn’t cycling on and off overnight or when we aren’t home so we can rule out leaks.
When the tankless was doing this were there any other devices running at the same time, before or after? Ice maker, well pump,etc? Sense can help with that and confirmed nothing was running during the time of these false starts.
Since the Rinnai is connected to an HS110 I looked at the signature of a normal start, including turning the hot water on then off quickly. These phantom starts are different, they peak between 9-12 watts whereas a normal start is 55 watts. The phantom starts last for 30 seconds.
Looking at the meter a pattern was emerging. In my second test I predicted the exact time each one of these phantom starts would happen. After the purge fan shuts off first phantom is 10 minutes after that 15 minutes later the second phantom starts, 30 minutes after that the third phantom starts and finally 1 hour after the third phantom is the fourth. I’m not an expert on plumbing issues but if this were a leak or some physics related hot water issue it wouldn’t run on a perfect schedule especially since I used to different taps each test, one was upstairs in the bathroom, the other in the kitchen.
If Rinnai says it should only start if there is a call for hot water, then what is happening? With information gathered using Sense I contacted Rinnai Support again.

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Rinnai support responded to my updated ticket I sent with the Sense usage graph:
With our SENSEI units this is normal operation. The unit does this as a safety measure to keep condensation from building up in the units heat exchanger. It should only last about 30-45 seconds


Thanks for the update. I have an Rinnai RUR199iN, but I operate it with recirc, so my waveforms are fairly regular. Still interesting to understand different operating modes.

And this behavior is undocumented but it is consistent. It will start over any time the water is used which is why it seemed to be happening all the time and randomly. If the cycle makes it to the 4th mini purge which is about after 2 hours the power consumption drops down to 2 watts after that.