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Community and Q&A

Visualizing mini-split performance data

Kevin Kircher | Posted in Mechanicals on

I’ve been digging through Mitsubishi M-Series performance data for a project I’m working on. I made some graphs of the capacity and efficiency of these ductless mini-split heat pumps. I figured I’d share the graphs here, since many GBA community members are interested in mini-splits.

Take-aways

1. Capacity and efficiency depend on the indoor and outdoor air temperatures. (See graphs #1 and #2.)

2. Efficiency also depends on how hard the heat pump is working. Efficiency degrades at very low load, when the heat pump starts cycling off and on rather than modulating continuously, and at very high load. (See graph #3.)

3. Maximum capacity is significantly higher than rated capacity. For heating, maximum capacity is typically 150% to 185% of rated capacity; for cooling, 110% to 150%. (See graph #4.)

4. Minimum capacity varies by model. For heating, minimum capacity is typically 15% to 30% of rated capacity; for cooling, 20% to 40%. (See graph #5.)

Terminology

By capacity, I mean output thermal power (in kBTU/h). By coefficient of performance (COP), I mean the ratio of output thermal power (converted from kBTU/h to kW) to input electrical power (in kW). COP is a dimensionless quantity.

By nameplate capacity and COP, I mean the capacity and COP that the manufacturer reports at the specific test conditions of 60 F indoor/47 F outdoor (for heating) and 80 F indoor/95 F outdoor (for cooling). By rated capacity, I mean the capacity measured at given indoor/outdoor temperatures. Rated capacity is less than maximum capacity. By rated COP, I mean the COP measured at rated capacity at given temperatures. Rated COP is usually the highest COP the heat pump achieves at given temperatures. In other words, rated capacity is an efficiency ‘sweet spot’.

Data sources

The primary source is Mitsubishi’s reported performance data, pulled from their 2019 submittals, their longer technical specifications, and/or the NEEP “Cold-climate air-source heat pump specification product listing” spreadsheet. This data is for the 6, 9, 12 and 15 kBTU/h Hyper-Heating models. I focused on this manufacturer and product line mainly because the M-Series is popular, efficient, and known to heat well in cold climates. (Also to keep the project size manageable.) I sanity-checked against Jon Winkler’s 2011 report, “Laboratory test report for Fujitsu 12RLS and Mitsubishi FE12NA mini-split heat pumps”, and various real-world monitoring studies (Roth et al. 2013, Faesy et al. 2014, Ueno et al. 2015, Williamson et al. 2015, Korn et al. 2016, Sutherland et al. 2016). I can share data or link to these studies on request.

Caveat

There isn’t much empirical data on COP while cycling at very low load. The best information I could find was from Winkler 2011, who found in two heating experiments that the 12 kBTU/h model’s COP degraded by 10 to 37% when it started cycling. The lines below 20% capacity in graph #3 are not precise. I’d love to see more data on this.

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Replies

  1. Kevin Kircher | | #1

    Oh, another take-away: bigger models are less efficient. (See attached.)

  2. James Howison | | #2

    Thanks! Is this just for indoor units or this just one-to-one mini-splits as opposed to multi-splits? I (and my installer) had a seemingly impossible time finding the minimum capacities for heads attached to M-series multi-splits.

  3. Kevin Kircher | | #3

    My pleasure!

    This is just for indoor units paired one-to-one with outdoor units. I haven't looked carefully at multi-splits, for a couple of reasons. Partly to keep the analysis manageable, partly because the minimum capacities on the multi-splits are reportedly pretty high, which can cause cycling and hurt efficiency. But what I've read is mostly anecdotal.

  4. Jon R | | #4

    Data I've seen indicates that COP loss from short cycling is < 10%. With little effect on latent capacity.

    I believe that the cycling test in Jon Winkler’s 2011 report is very misleading. They actually cycled the power to the entire unit which causes it to start up in high speed mode. This hurts COP. But this isn't what happens when you allow the unit to cycle itself. See "The transient startup behavior observed in the laboratory can explain the high degradation coefficients. "

    Chiltrix has told me that they cycle their compressor even when the inverter drive could operate it continuously - because at some point it's more efficient to do so. So buying a HP with lower minimum capacity can reduce efficiency. And it may have no noticeable effect on anything else (eg temp swing, motor life or sensible heat ratio). Short cycling can be beneficial!

    1. Expert Member
      Dana Dorsett | | #5

      >"Chiltrix has told me that they cycle the compressor even when the inverter drive could operate it continuously - because at some point it's more efficient to do so. So buying a HP with lower minimum capacity may actually be harmful."

      This is demonstrably the case with some of LG's Art Cool series minisplits that modulate down to ~1000 BTU/hr out @ +47F out, with COPs in the 1.5 range at min-output, less than half the COP at max, looking at the NEEP spreadsheet numbers. At +47F the 3/4 ton LSU/LAN090HSV5 has a COP of

      1.50 @ 1023 BTU/hr (min)

      5.60 @ 10,900 BTU/hr (rated)

      2.65 @ 17,061 BTU/hr (max)

      The somewhat comparable Mitsubishi FH09 does quite a better at +47 though it's only modulating down to 1600BTU/hr. That's a higher minimum modulation but at a COP of

      4.26 @ 1600 BTU/hr (min)

      4.50 @ 10,900 BTU/hr (rated)

      3.59 @ 18,000 BTU/hr (max)

      I'm sure they could spin it down below 1600 BTU/hr, but I suspect they're trying to avoid the huge efficiency hit if they did.

      Given how well the LG performed at 10,900 BTU/hr (5.60 is pretty excellent!) I wonder where the actual COP curve steepens, and whether it would do better if they never let it drop below 2000 BTU/hr.

  5. Jon R | | #6

    More support for "cycling has a small effect":

    https://core.ac.uk/reader/4822580

  6. Robert Opaluch | | #7

    Maybe Kevin and Dana, Jon, or somebody could go back and forth and write up an article on this topic, with some recommendations for those of us with less expertise?? Could be helpful for us to select products more optimally and operate more efficiently.

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