Mitsubishi Ductless Mini split winter losses found

Thanks to sense I noticed a mysterious 70w device come on for 30 mins every 30 mins. Sense only labeled one “device” at 30 watts but I noticed that whenever that Device turned on the load was actually 70 watts. Turned out that the two ductless units that run off one outdoor unit have a heating cycle in the winter for the compressor oil. The units do both heat and AC but we do not use them for heat in the winter. Pulled the disconnect and killed the power use on them till summer. Good for about 20 kwh/month as well as at least 10 watts standby power.


Nice catch!

That actually seems like a potentially “easy” target for a Sense-based alert. If you combine the knowledge that a household “Home Details” has, say, a mini-split + furnace then an undefined 30W cycle might be weighted toward being “Your mini-split is on unnecessarily”.

That said, as @MaheshAtSense explained last year, nothing about AC detection is particularly “easy”.

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@nlanzano I have two Mitsubishi mini splits as well, but think they only do cooling. Can I ask what model number you have and how you were able to figure out that 70w usage was then? Thank you.

Mitsubishi publishes detailed specs on all their equipment so if you have the model# and configuration it’s reasonably straightforward to determine what electrical signatures you should be seeing. The cycling pre-heat use is easier to identify above background noise than, say, the constant-on of the controller boards. Variable speed fans and compressors are more difficult for Sense and more difficult for humans to see … it really depends upon whether you (or Sense) can isolate the pattern from other usage. There is, in essence, a “detection cycle” as well. For example, and to state the obvious, Sense is only going to have the potential to detect an HVACs cooling mode in Summer.

The device detection that @nlanzano got is a clear case of the potential for Sense, especially for those among us who want to be lazy after installing Sense and just wait for a notification. The interesting and not necessarily intuitive thing here is that component detection or, indeed, component phenomena detection is not dependent upon a full “device” detection. Sense can take the phenomena in this case and with enough instances determine that it’s likely a “Mini-split left on” and notify you without ever having been able to disaggregate the mini-split with enough confidence to designate it as a device.

Hi, probably unrelated info, but I was told by a Mitsubishi person (he gives classes to installers here in NE) to keep my condensers plugged-in during winter. Electric motors need some power to prevent damaging during very cold temps. I didn`t ask for more details but he said that power consumption is very little

I’ve never heard this, but it doesn’t give those of us who have to “winterize” summer cottages much encouragement. There is no way I can leave the power on all winter. If actually true (and not an “old wives tale”), shame on Mitsubishi!

BTW, in decades of turning off the power all winter, I’ve never had a problem.

That is correct. I think though that only applies to using the unit in winter. Since I don’t use the units for heat at all in the winter I figured I’d just shut them off till it gets warm out. Hoping And assuming that doesn’t affect them much.

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Sorry for the delay. Been dealing with this crisis going on.

I’ll post pics of the spec plates on the indoor and outdoor unit.

@nlanzano This is the “Pre-heat Control” detail for your unit:

@andy I would guess that the control boards and other solid-state stuff can be de-energized without concern and that the “Keep it on” advice from a Mitsubishi installer is probably about what’s being quoted above: “To improve start-up condition, the compressor is energized even while it is not operating.”
I suppose you can take “improve” in a couple of ways, but I would guess, based on your experience, that an initial seasonal startup might be slightly longer (?) or slightly less guaranteed (?) but otherwise not impacted by a winter power-down.

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I think this has come up before on this forum. The heater is indeed to keep the oil from becoming too viscous and the refrigerant from pooling as a liquid in the compressor sump. If the compressor attempts to pump liquid refrigerant, damage can occur. So, if the unit is being used actively, the heater must remain energized. If not, the breaker to the unit can be shut off to save energy. However, when it’s time to start the system up for the season, most manufacturers recommend that the system is energized 24 hours prior to running it for the first time. This allows the heater to warm the compressor sump and evaporate the liquid refrigerant from that area before the compressor turns on.

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So why don’t you use them for heat?

We did for a little bit before my daughter was born. But we were able to leave part of the house cold then. Now that we are using that part of the house turning these on for heat would freeze out the other part of the house so we just stick with the forced hot air on propane. The house was older and updated throughout the years and the forced hot air ducts aren’t designed for AC so the ductless units are set up in the parts of the house that couldn’t be ducted for AC. We have a couple ductless units for AC and one section ducted.

Oh okay I understand. I have a wall unit in each bedroom and one for the living room and kitchen which is a shared space. So I got rid of my oil boiler in baseboard heat, actually in the process of removing baseboard from another room today. after a couple of winters with the mini splits and not needing the baseboard I decided to get rid of it.

I’ve done a couple of mini-splits in a vacation home, and turned off all the baseboard heaters. No Sense monitoring there, but the power bills in the spring and fall have been MUCH lower than previously. We use them for A/C during the hottest parts of the summer, but on Lake Champlain that’s only a few weeks each year. We don’t use the place in the winter, so everything is powered down.

Wish I’d done a multiple head system in the first place, but we did them bit by bit, so two heat pumps so far and another one coming.

In my experience no matter what you do you usually have a few “wish I dones”. I’ve been happy with them. The first winner I kept my oil boiler for domestic hot water but after two winters not using it for heat I took it out and put a HPWH in. Storage is at a premium at my house so getting The boiler in the oil tank which were both in my storage room out was a plus.