Weird 27W device running every hour for 30m...identified?

Saw this strange identified device crop up the other day. I thought it kind of odd that it would repeat so regularly, running for 30min every hour, though not quite…some hours are skipped and there’s some skew where the start time changes by a few minutes over days.

This was all happening while I was on vacation, and nothing I could think of would be waking up so regularly (but imperfectly). My computers were off and draw much more power besides. The clock-based repeats didn’t feel like thermostat-derived, and the imperfect time keeping didn’t feel like a clock-based scheduler.

Then I turned my mini split heat pumps on remotely, and the 27W repeater stopped. After a day I turned them off again, and after a while the 28W repeater started up again. That seemed to indicate the heat pumps were the source, which I thought odd since the heat pumps draw on the order of 1kW normally.

But that gave me a hint; the mini splits must be using some bit of standby power, so why on a schedule? I did a search for “Mitsubishi mini splits standby power” and found a link from the DOE discussing exactly that, indicating some units draw 27W in standby mode…others up to 180W!

So Mitsubishi heat pumps use a bit of power to keep the condenser from freezing when not in use. 28W seems like not much power, but that must be it. The clock skew is probably from a timed process running on durations rather than clock time.

Now if only Sense would find the other heat pump modes I could get some real data to see how it stacks up to oil heat when it’s freezing outside.

It definitely looks like a DC transformer detection and from what you are saying seems likely it’s the Mitsubishi standby … probably useful to refer back to the Mains Power Meter and correlate the cycles with whatever else is going on since what you are looking at is not “ground truth”.

There’s another undetected wattage of right around 30W in the “other” device that goes away when the 27W one does, so they might end up being added together eventually. “Other” is otherwise between 0-2W so they’re clearly correlated to my eye. Ground truth here via smart plugs is not possible with 240V devices wired directly into the breaker, so sensing it from mains is the best we can do without something like a smart breaker panel.

These low wattages are pretty much noise when in the house, what with hall lights or other devices constantly turning on and off. I just thought the not-quite-30min schedule was strange.

What I mean by ground truth in this case is that the Mains Power Meter is showing you ground truth on your entire supply so that’s the only thing you can really trust when sleuthing this.

20-30W is the realm of a typical lithium charger; NAS & network gear and so on … noisy territory.

What model Mitsubishi (indoor/outdoor) do you have?

That would be the MXZ-3C30NAHZ2 outside with 3 indoor units, all in standby mode as they coast down to the setpoint. Combined with “other” the device is probably closer to 60W in standby mode but I can’t find anything in its docs with a specification. Good thing about being on vacation is there are no extra sources to muddy the signal. The variable speed everything in the hvac system is going to be a pain to sense even without all that noise.

In the 50W realm you might be seeing this …

Yeah saw that but it doesn’t look like a normal compressor on the full power meter, though it’s probably that or something like it. I’d guess a resistive load without any access to phase information…once I’m back I can read the current and voltage on its breaker and see if I can match it up to an element in the schematic. Other docs make mention of a defrost capability, but I’d expect that to be much higher power.

When they say “the compressor is energized” they don’t mean that the shaft is rotating, they mean that a small current is being run through the windings to generate heat and vaporize refrigerant that pools in the compressor sump. Many HVAC systems have this feature, especially ones that are designed to operate in winter conditions (heat pump or low-ambient cooling). Usually the feature is implemented with a separate resistive heater element attached to the compressor housing, but Mitsubishi is being clever here and using the actual motor windings as the element.

Also note that many mini-splits with heat pump or low ambient cooling capability will also have a heater element in the bottom of the case to keep the drain holes there from freezing up.

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Given the analytical detail that Mitsubishi and others these days give to techs to service equipment, you’ve got to wonder how long before “standard” high resolution electrical signatures become part of the tool kit. Are they already?

And wouldn’t that be nice for Sense to have access to … just plug the data in and implement failure alerts and service calls. Mitsubishi has this for pressure monitoring and so on and of course you need that for larger scale commercial systems but at the mini-split level where internal monitoring is limited, the aggregated data via Sense could surely, beyond the device disaggregation “issue”, be very useful, in turn, to Mitsubishi! A bit of a chicken-and-egg problem, but you get the idea.

FYI: When I finally get my mini-split installed the first thing I’m doing is dedicating a Sense to it for “ground truth” beyond what smartplugs can do.

Had the same one and it took a while to track it down. We just shut off the breaker for the heat pumps during the winter when they’re not in use to save power.

Meh, 25W constant isn’t too bad (50W for 50% of the time) and the Mitsubishi heat pumps controlled by Kumo will automatically disable themselves below a configurable outside temp to allow the backup heat to do it’s job. That’s set around 30F right now though hypothetically the HyperHeat system can produce heat all the way down to -15F outside, with an efficiency penalty. It’s nowhere nearly that cold in MA, and the cold snaps aren’t that frequent so the heat pumps may be able to handle the majority of the heating season, with good ol’ steam radiators for the really cold times