Mitsubishi Mini Splits
We recently purchased a home where the prior owner had installed $24,000 of Mitsubishi hyper heat mini splits for heating. So far the experience has been terrible. The units can kick out heat, it just seems they don’t often want to. A couple observations:
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The main issue is heat circulation. If I set the 24000 btu head unit’s temperature to 78 degrees, there are large pockets of the room measuring as low as 66 degrees. The area immediately around the mini split itself measures 78 however, so the mini split goes into a low power “maintain” heating mode where the fan runs at minimum and 78 degrees in the immediate vicinity of the mini split is maintained.
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Manually setting the fan to “high” setting does nothing unless the mini split is provoked out of the low power “maintain” heating mode. We’re using a portable box fan with our 24000 btu head unit, which seems really bootleg for a $24k HVAC system. It seems like this is just bad firmware?? Why can’t I just run the fan on high when I want to??
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All the mini split installation guides I’ve seen seem to recommend placement of the mini splits near the ceiling – our are installed exactly per manufacturer recommendations. But this seems to exacerbate the issue of localized heat trapping near the mini split unit because heat rises and essentially the mini split is sitting in an elevated layer of hot air in the room that is not mixing with the rest of the room. Again, higher fan power (if firmware didn’t suck), or an external fan help with this, but this is not what I’d expect from an expensive HVAC system. It seems like the room positioning is optimized for AC performance and not heating performance – cool air falls, mixes with room.
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The idea that the thermostat can sit in the unit itself seems to be a major defect in how this system is thought out. There is a wireless thermostat option, but at ~$355 a pop, that’s a lot of money for a system that is touted to be comfortable room-by-room temperature setting. But the alternative is I’m manually messing with a bootleg system of indoor weather thermometers, box fans and adjustment of the aspirational internal thermostat of the mini split itself to say keep the room at a comfortable 70 degrees. My $150 window mount AC unit has a remote control with a thermostat sensor built into the remote. Why can’t I get similar for a multi thousand dollar system??
Any advice, tips or things I can do differently to make this system more livable? At this point I’m tempted to just buy a new steam boiler and completely forget the prior owner’s mini split “investment.”
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Replies
Install ceiling fans to equalize temps.
… or install remote thermostats to sense temp at more suitable/ representative location.
You should be able to run the indoor fans on High/ med/ low/ auto and the system should not override your choice and the fan should run at the speed you have chosen. If the fan is running non- stop there is a setting that can be configured to not run when there is no call for heat/ cool from thermostat.
Mitsu sells wired remote temperature sensor for some of its indoor units ie:
https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10009/M_Submittal_M21EAA307.pdf
You can mount this somewhere away from the unit where it will more accurately sense room temperature.
The other option is the wire remote thermostat. These can be configured to use the temperature at the thermostat as control.
Unfortunately the many wall mounts on a multi split have a lot of limitations and these are not clearly communicated.
I have a hyper heat and am able to set the fan at whatever setting I choose. I do find it annoying that you can press a button on the remote and it will beep, but unless you are pointing the remote directly at the unit it doesn't register the button press.
99% of the remotes are strictly transmitters… thus you have to watch the leds on the head blinking or whatever to confirm command was received by the system
Yeah, I know, I just don't like how it beeps as if it registered the command even if you aren't pointing it at the unit.
Remote temperature sensing thermostat, Mitsubishi or a third party that uses the CN105 control port, is the real answer. Additional air circulation can also certainly help, but the root of the problem is what you noticed, the indoor unit doesn't know the temperature elsewhere in the room, so it doesn't know to keep running. I have a 5 zone mitsubishi system (Smart Multi 42k outdoor units), and before we added remote temp sensing the units cycled very frequently and struggled to maintain even temperatures. With the remote sensing they work considerably better, and run gradual multi-hour cycles instead of the fast ramps up and down that they did before. The additional fan run time and speed mixes the air better as well. All in all a far better experience.
I do think remote sensing thermostats should be standard, or at least strongly recommended, in all but the smallest spaces. I think a lot of people experience poor performance from the system that could be resolved with a simple accessory.