Short-Cycling Mini-Splits

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:

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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.

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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:

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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.

3 thoughts on “Short-Cycling Mini-Splits

  1. While I don’t have a Passive House, I do have an extremely energy efficient metal SIP house experiencing the same problem. I suspected it might, having read your article. So we set up energy monitoring today, and we are already seeing the short-cycling, just as you describe. Longer periods of 20 watts, followed by 1000 watts bumps roughly every 5 minutes.

    Here’s what we see:

    Looks like I might need to get in touch with Jack, also. Any other advice?

    • Hi Jeff – For some reason, I couldn’t see your data. But it sounds like you are experiencing exactly what I was experiencing last winter. The best I have to offer is buy a wireless remote thermostat and mount it as far from the wall unit as is practical. As I said in my post, that reduced the problem significantly. Fortunately, even though the problem isn’t completely eliminated, comfort isn’t an issue. The temperature in our house stays very steady, and frankly, if I didn’t have an energy monitor I wouldn’t even know that the unit isn’t quite working at its most optimal level.

      About a week ago, I spoke with someone who who designs HVAC systems and is very familiar with minisplits and high-efficiency homes. He was familiar with the cycling issue, and told me that he also had the same issue with a Mitsubishi in his own home. He said that the problem is basically as I described; Mitsubishis are programmed to turn on at the higher rate and then eventually power down, and in super efficient homes where even the smallest minisplits can still be oversized, they just don’t run long enough to power down to a lower level. He also said that it was his understanding (he emphasized that he wasn’t certain) that Fujitsu minisplits are programed with a “soft start” and might therefore be less prone to the short-cycling issue. And like the Mitsubishi rep, he said that the best way to address the problem is to use a ducted minisplit. But of course that’s not a practical option for those of us who already have a ductless unit installed. I guess HVAC systems still have a way to go to catch up with exceptionally efficient homes. And at the moment, the demand isn’t yet at the point where they’re willing to design and produce the smaller systems that would more efficiently meet our needs. I’m sure we’ll get there eventually. But for now, I guess we have to live with a system that, while pretty impressive, is still less than ideal for our particular application. I wish I could be more helpful, but that’s the best I have.

      Good luck, and let me know if you learn anything new!

  2. Thanks for the additional information. That explains exactly what we’re seeing. I assume the thermostat fixes it because it can 1) judges the heat with a single input, and 2) likely has a setback of 2 degrees so it waits longer to turn on the unit so the BTUs needed to reach shut-off becomes greater.

    Were you able to note if there is a more than negligible difference in power usage before and after the thermostat was installed? We might go through the winter and see how it performs once the temperature drops and the load increases, then make the call. I don’t mind installing the thermostat, but, we’re rural in Missouri, so the installer charges $75 travel for a visit on top of every installation charge. We could easily be looking at $500 per thermostat, and we have two units: a 9K and 15K unit running off two separate compressors.

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