Replies

1. 
   Josh_Cavanagh | Aug 19, 2018 09:12am | #1

   I had the same question several years ago with my own home while I was adding a supplemental mini split in my vaulted great room. I have a very well insulated and air sealed home with a very low cooling load. I wanted to be able to heat and cool as well as dehumidify the space, as well as shift the load from my conventional ducted system to the mini split. I’m in coastal SC, so we have some serious humidity here. I opted for the next size up for the increased coil size and sensible heat ratio. What I found is that when a mini split with a very low minimum btu output is cooling a low load space, the coil doesn’t get very cold at all. While a conventional HVAC system evaporator coil can get very cold (40 degrees or so), my low output mini split’s coil was running at around 60 degrees during low load periods and even shutting off occasionally. What I’ve learned from this is that during the cooling season you can expect the DewPoint temperature of the air in the home to track pretty closely with the evaporator coil temperature (at least in a well sealed home). When it’s 95 degrees and above outside, my mini split dehumidifies just fine because the coil temperature is much lower for an extended time. My advice is to go with the smaller system, which will make it run a little harder and keep coil temperatures lower for increased dehumidification. You may still find you need a small dehumidifier to take care of the latent load at times. Good luck!
   Josh

2. GBA Editor
   Martin Holladay | Aug 20, 2018 05:45am | #2

   Josh and Josh,
   For more information on this issue, see "All About Dehumidifiers."

3. 
   joshdurston | Aug 20, 2018 09:08am | #3

   Thanks that was a good article on dehumidifiers. I guess I shouldn't worry as much about the SHR since I have a dehumidifier already. My main concern was that I would end up running the AC and dehum at the some time if the AC doesn't have adequete latent capacity.

   The FH06 only has a 0.2 pints/hr moisture removal rate. Looking back at some of the previous gen units, the moisture removal rates were much higher (closer to 2pints/hr, even on the small units). The cynical part of me wondered if Mitsubishi might be inflating there EER/SEER numbers by ignoring the latent loads, I a suspect it take less energy to deliver 9000btu/hr cooing with a higher airflow/wamer coil, than a cold coil and lower airflow (more latent capacity). The smaller FH06/09 units seem to have disproportionately higher cfm/ton than the larger FH12 and larger units, and the FH06/09 are the ones with the almost too good to be true 30+ SEER ratings.

   That being said I think I will go with the FH09. Perhaps the reduced latent capacity will help me avoid mold issues, which seem to be a major pain to deal with in some cases.

4. Expert Member
   Dana Dorsett | Aug 21, 2018 03:11pm | #4

   All vendors game the SEER test to hit the highest numbers the best they can. But that's not to say that it won't run at or near the stated efficiency when the outdoor dew points are reasonably low, and still manage the latent load at least most of the time in standard cooling mode. Not every location in North America has humidity levels as high as the peak humidity days in southern Ontario, and even there most days aren't really terrible for latent load. Upsizing to a 1-ton unit than won't modulate as low as the FH09 would be a mistake.

   If the humidity gets ahead of SHR during the highest-humidity weeks you still have the option of running it in "DRY" mode, which changes the SHR pretty dramatically compared to it's normal cooling mode. DRY mode lowers the overall cooling efficiency, but usually not as much as running a separate dehumidifier would. Consider the dehumidifier as the "Hail Mary" backup if for some reason DRY mode doesn't keep up.

5. 
   joshdurston | Aug 27, 2018 08:16am | #5

   I found some more info that makes me feel better. I think I'm going to go with the MFZ-KJ09NA instead of the FH. It has a bit better HSPF and slightly lower SEER, probably due to being a floor standing model instead of a high wall mount. Drawing in a cold return from near floor level should help efficiency and comfort in heating mode. The biggest reason though is that it appears to be easier to disassemble to clean if necessary. The coil is flat (vertical) with the fan on stop, instead of curved around the fan like the FH models. The mildew cleaning procedures look pretty invasive and scary.

   
6. Expert Member
   Dana Dorsett | Aug 27, 2018 03:01pm | #6

   The published HSPF of the FH09 is 13.5, to the KJ09's 13.0, so it's a slightly WORSE (though still excellent) HSPF number. The FH edges out the KJ a bit on both SEER and HSPF efficiency, probably related to the comparatively higher minimum modulated output of the KJ:

   http://meus1.mylinkdrive.com/files/MSZ-FH09NA_MUZ-FH09NA_Submittal.pdf

   http://meus1.mylinkdrive.com/files/MFZ-KJ09NA-U1-MUFZ-KJ09NAHZ-U1_ProductDataSheet.pdf

   The minimum modulated output of the KJ is 2900 BTU/hr @ 47F, compared to 1600 BTU/hr for the FH. That's nearly 2x the minimum output, which means it will have a bit more shoulder season cycling.

   Being a floor unit it may indeed have an increased "as-used" HSPF due to the intake of colder air near the floor despite the higher minimum modulation levels, but it isn't beating the FH09 in a bench test.

   With either of those units you'll most likely have to run it in DRY mode during the more humid weeks.