On December 7th, I installed an egauge energy monitor on twelve circuits; the two lines supplying power to the house (2), the two heat pumps (4), the water heater (2), the ERV (1), the well pump (1), the family room circuits (1), and the dryer (1).
At about the same time, I set the first floor mini-split at 70 degrees and the second floor unit at 66 degrees.
One of the first things that I noticed when reviewing the egauge data, was that the heat pumps were both “short-cycling;” that is, they were turning on and off at intervals as short as three to five minutes. When “on,” the unit would draw about 1kW. When “off,” the unit would draw about 20 watts; just enough to power the internal fan on the wall unit (which runs continuously). Here are a couple of examples:
This didn’t always happen. Occasionally, one heat pump or the other would “flat line” at 300-400 watts. This could go on for hours, and in a couple of instances, days. But then the unit would revert back to the short-cycling.
The short cycling didn’t seem to have a strong correlation to outdoor temperature. For instance, sometimes the first floor unit would short-cycle while the outdoor temperature was in the 30s and flatline when it was in the 40s. At other times, just the opposite would occur.
Having said that, the first floor unit stopped short-cycling when the temperatures dropped below 20 degrees or so. But the 2nd floor unit continued to short-cycle even at those low temperatures.
I contacted both the local HVAC company that installed the units and Mitsubishi’s National Sales Support Manager John Bart, and presented both with a summary of the data. I had learned about John when we were beginning construction of the house. He had participated in a Passive House webinar where he discussed the Mitsubishi mini-splits. Shortly after that webinar, I contacted him with questions about the type of mini-split system that might be best for my house, and he was very responsive. In fact, it was his responsiveness that caused me to go with Mitsubishi over the less expensive Fujitsu units.
It appeared to me that the local HVAC guys (who installed the system) didn’t know quite what to make of the data. However, John, who was clearly more familiar with the Passive House concept, asked for additional information; a floor plan, photos of the wall units, and the Passive House heat load computation, which was (is) about 8,000 btu/hr. Given that information, he surmised that the units were oversized, and sent the information over to his Northeast Tech office.
To back up, both mini-splits are 12,000 btu units, and can power down to somewhere around 3,500 btus. John suggested that, at the more moderate temperatures, even 3,500 btus was probably too much for the house. However, he acknowledged that it was perplexing that the short-cycling stopped for sometimes lengthy periods of time, even at those more moderate temperatures. He added that even the smallest (9,000 btu) mini-splits would probably have had the same issue because their low end was very close to the low end on the 12,000 btu units. He suggested that a Mitsubishi regional tech visit my house to assess the situation and to install remote thermostats for both units.
Mini-splits (at least Mitsubishi mini-splits) come with a remote control. However, contrary to what some may believe (in fact, even the local HVAC contractor believed it), the remote does not have a built-in thermostat. In fact, the thermostat is located in the “cassette” that hangs on the wall and dispenses the heat. As I understand it, the sensor measures the incoming air, and in doing so, provides the data that helps the unit decide when to power up or down. But apparently it isn’t like the old days, when a single thermostat turned the furnace on or off. Rather, as I understand it, with these units there are five sensors; three on the outdoor condenser and two on the indoor wall unit, that collectively tell the unit what to do. In other words, things are much more complex than they used to be.
At any rate, last Friday the Mitsubishi tech showed up and installed the two remote thermostats. These thermostats (product MHK1) are manufactured by Honeywell specifically for the Mitsubishi mini-splits, and it appears that they resolved the issue. In my opinion, Mitsubishi provided outstanding customer service in dealing with this issue; something that I find sorely lacking with many companies these days.
Since last Friday, the 2nd floor unit has been idling; meaning that the compressor has not turned on. In other words, even during the evening lows, which have gone down to the low 20s, the first-floor unit has handled the entire job of heating the house, and the second floor has not gone below 68 degrees. So the picture, from the egauge perspective now looks like this for both heat pumps:
A couple of notes…The second floor heat pump is the horizontal red line at the bottom of the graph. It’s showing about 30 watts. As I said earlier, this heat pump has been in idle mode since the remote thermostat was installed. The more prominent red line is the first floor heat pump. The outdoor temperature early this morning was in the high 20’s, so my guess is that the three vertical jagged lines are defrost modes. However, they’re the only three such lines that occurred during that night. So I could be wrong. They may instead be cycling blips. But at one hour intervals, they’re a far cry from the three to five minute intervals that were occurring.