Modulating vs short cycling ?
It seems that when mini-splits modulate down to low levels, they often become much less efficient. Maybe it would be better to let them short cycle?
For example, a Mr. Cool compact ducted unit that produces 7.7K BTU at 5 degrees can modulate down to 1.9K BTU at 17 degrees, but its COP at that level is only 1.37 while at 6.8K BTU at the same temperature the COP is 2.93. Might it be more efficient if it did not modulate down as far?
Or is the reason that the COP is lower when it is producing less heat because the pan heater has to stay on and draws the same power whether it is running hard or barely idling? And would the pan heater need to stay on even if the unit was cycling on and off?
It does seem that the data on the NEEP web site are inconsistent regarding the power for the pan heater. For example, the Mitsubishi MUZ-FH09NA shows that at 17 degrees the minimum heat is 920 BTU and the power draw is only 80 watts (COP 3.37) , while the MUZ-FH09NAH produces the same minimum heat but uses 340 watts (COP 0.79). I wonder if they included the heater draw on one unit and not the other?
Thanks for any thoughts.
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Can you provide hyperlinks for the documents where you found this info?
Here are the links for the Mitsubishi units:
https://ashp.neep.org/#!/product/34379/7/25000///0
https://ashp.neep.org/#!/product/34381/7/25000///0
And this is the Mr. Cool:
https://ashp.neep.org/#!/product/30932/7/25000///0
The NEEP database is not 100%, lot of times base pan heater is included and sometimes it is not. Also if the description on how the heater operates on the Mitsubishi units is correct (that is all the time on when cold), that is a terrible efficiency hit and very bad design.
Most units I have run the heater only during defrost and for some time after, so it won't effect the min output COP that much.
P.S. The Mr Cool units are rebadged Midea or Gree units. Usually they are from the budget lineup, for a couple of dollars more you can get a top tier Midea/Gree which has much better performance. There are some exceptions, the Mr Cool Universal (rebaged Gree Flexx) is a pretty good product.
I do see the inconsistencies in the NEEP database. Is there any other source that show the minimum heat and associated power at low temperatures?
Is this what you would look at?
https://www.ecomfort.com/manuals/mitsubishi-908495e6de91dc6667ef9a62abcacae5.pdf
https://iwae.com/media/manuals/mitsubishi/MUZ-FH09NA-1-specifications.pdf
These specification sheets do include useful data. But they often do not show minimum heat levels at all and almost never the power draw or COP at the minimum heat or minimum heat at lower temperatures. I seldom have seen that info anywhere but in the NEEP database.
I appreciate that you took the time to explain this. Thank you.
You need the engineering manual form Mylinkdrive, this normally has extended temperature data including min operating point:
https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10005\M-Series_Engineering_Manual.pdf