In the last three years I have had four Mitsubishi low-temperature mini-split heat pumps installed in Maine. Two of these are smaller, 6 kBtu/h units, one is an 18 kBtu/h unit, and the oldest of these is a 15 kBtu/h unit, located in my living room.

I have been monitoring the electric use of the two cottage heat pumps for more than a year. In March 2023 I began monitoring the energy use for the two older heat pumps in the house.

This post looks at the standby power of the four units. One of the units displays, what I would characterize as strange behavior. I have reached out both to Mitsubishi and to Dave’s Appliance who installed my units and no one has explained the behavior. Perhaps someone reading this post will be able to shed light on this.

These days all four of my heat pumps have power but are turned OFF so that they provide neither cooling nor heating. They are essentially in stand-by mode. As mentioned, three of these heat pumps use about 4W of standby power. A graph of P(t) for the last day or so for my 18 kBtu/h unit in the cottage is shown below.

In contrast to the 4W standby power of the above heat pump, the 15 kBtu/h heat pump in my house living room displays the behavior shown below.

The above heat pump has standby power of about 5W during the day, then starting at midnight, has periodic spikes of close to 70W. These bursts last for only about 10-12 minutes, then re-appear about 2 house later. These bursts stop sometime the next morning, then the pattern repeats the following night.

The excessive energy use is not much on a day-to-day basis. My three heat pumps use about 0.11 kWh per day per unit in standby mode. The one with bursts of power uses about 0.23 kWh per day in standby mode. The excessive use is about 44 kWh per year which, at $0.30/kWh, costs about $13 per year. Again, the excessive energy is not that much — but what is its origin? What is it about the control of this heat pump that is different from the other three?

These graphs just illustrate that a lot is going on in these heat pumps that you would not notice if you don’t measure their power use. They all seem to be operating normally, otherwise.

I would welcome any input if anyone can explain the behavior.

For reference, the model numbers for the 15 kBtu/h units that show anomalous behavior are MUZ-FH15NA for the outdoor unit and MSZ–FH15NA for the indoor unit.

All of the heat pumps are 220VAC units. I monitor their electric use with iammeter single-phase units. The data from these units are regularly logged with Home Assistant.