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  1. #1
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    Mitsubishi HyperHeat System - Very HIGH Humidity Levels

    Hi everyone, I have a very challenging situation that I hope someone can help me with. About six months ago I have a very expensive 5-wall-unit Mitsubishi HyperHeat mini split system installed in my small, Boston, Massachusetts home. The house is listed as 1,150 sq. feet, officially. It's stucco and very old, built around 1880. It has insulation and I've never considered it to be horribly performing or horribly inefficient, but obviously it's not a new, high performing house. Here's the stats:

    1st Floor: Kitchen, Dining Room, Living Room: 500 sq.ft. with pretty low ceilings of about 7.5 feet. In the living room, one 18kbtu wall unit with external thermostat. The unit was installed about 12 inches too far into the living room and as a result the airflow is almost 100% into that room. There is virtually no airflow into the dining room and kitchen, so temps in those rooms are three degrees higher. While not the subject of this thread so much, I'm NOT happy about that install error.

    2nd Floor: Three small 100 sq.ft. bedrooms, and some hallway/common space and bathroom adding another 175 sq.ft. or so. Higher ceilings of about 8.5 feet. Each bedroom has a 9kbtu wall unit with external thermostat. Two bedrooms are south facing.

    Basement: 500sq.ft. mostly finished, 100% used as valuable space. 12kbtu wall unit mostly for heat, but in the summer, the humidity gets high down there and we run a dehumidifier 24/7 that really heats up the space. I currently keep the relative humidity at 45% in the basement and have the mini split set to 75. It's perfectly comfortable.

    Here's the problem:

    Downstairs the RH is always around 55 to 65%. It's tolerable, but not great. As I indicated, I wonder how much better it would be if the unit was able to blow into the whole space instead of just the 200 sq. ft. living room. It's a relatively open layout, but not completely open and as mentioned, the temperature difference is very noticeable. It's too cold in the living room and too warm in the kitchen. Would moving the unit to the left 12 inches, allowing half the airflow to go into the DR/kicthen allow longer run times and get that humidity down to 50% or below? Any idea how much effort that would entail? I guess they'd have to braise the lines to add the extra 12 inches of line set. Is this worth pursuing?

    Upstairs has a **ridiculous** humidity problem. With only two units running (leaving the middle bedroom off) and all thermostats set to 73, the relative humidity varies between 60 (on a good, hot day) to 75% (at night, when I wake up.) Occasionally I have seen it drop into the high 50s--rarely. It's not comfortable...at all. It's actually pretty gross. The second you get about halfway up the stairs you can begin to feel it. My bedroom flat out smells of high humidity, but all the rooms feel damp. Sleeping isn't comfortable no matter what we set the temperature to. With our old "system" (three window units, one in each room: 2 x 6kbtu and 1 x 7k) it was fine, and we've lived in the house for 12 years so I've been through all sorts of summer weather. I'm not saying the window units were 100% perfect--there were times the relative humidity might seem a bit high, but absolutely NOTHING like the misery we're in now. Relative humidity problems were not anything I ever thought about until this Mr. Slim system began running.

    After reading many threads on here and everywhere else, and educating myself about these systems, and the science of relative humidity, it seems that I was sold a ridiculously oversized system. The outdoor unit is the 48kbtu HyperHeat. It can "ramp down" (as the sales guy would say) to 6kbtu for cooling. Apparently the indoor units can go down to as low as 2k in cooling, but that could only be achieved (given the 6k minimum system-wide) if at least three units are running. Regardless, 65 to 75% RH is simply uncomfortable and we never had such levels with the old window units.

    Questions:
    1) Is this solvable without ripping out the system?
    2) If so, is this solvable without installing a whole house dehumidifier system with ducts? The reason this system was so compelling to me, even at a VERY high price, was because it didn't really take up any space, and allowed me to remove my heating system from the basement and radiators from all the rooms. Adding ducts for a whole house dehumidifier defeats the purpose of the system. I have researched them, and I could install one in my basement with a duct up to the first floor fairly easily. Or I could install one in the attic pretty easily with a duct into the hallway on the 2nd floor. I could also duct into each bedroom with more work. But it seems the basement sceanrio leaves the upstairs less dehumidified, if at all, and the attic scenario leaves the basement needing it, and the basement *really* needs dehumidification in the summer. While I *could* duct from the basement to the attic though my no-longer-needed chimney space, I want to USE that space for something good like closets. That was the point of spending all this money--to reclaim desperately needed space in my small house.
    3) It seems the installer may be willing to replace the three bedroom wall units (9k each) with 6k units. Will this make a difference?

    Any help would be great. The installer is being reasonable, so far. They understand the situation is not acceptable. But they obviously want to spend as little time and money on this as they can. Every second they think about it, their profit margin on the sale goes down, as you guys know. Thanks in advance for your expert advice!

  2. #2
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    there is usually a tradeoff between hearing capacity in the winter versus moisture removal In the summer. 1st and foremost, have an energy audit done on your home. lack if bath fans, kitchen exhaust,number of occupants make a difference. I don't think that reducing the 9's to 6's are going to be a smoking gun difference. you can try running the units in dry mode. this will allow limited cooling and additional dehumidification. this may help. I do see a dehumidifier in your future however. perhaps 2 ducts (1 for the first floor and 1 for the 2nd floor) may be a good approach and the best of both worlds. teddybear will be along soon with a highly educated response.
    ...

  3. #3
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    Quote Originally Posted by kangaroogod View Post
    I don't think that reducing the 9's to 6's are going to be a smoking gun difference.
    Well, a bit of an update here. Today was a pretty hot day and the humidity was a bit lower inside the house. I believe at this point I can say that the hotter the weather, the longer the run-times for the wall units, the lower the humidity in the rooms. If the 6k units would make any difference, then I feel like I need to swap them out. The 9k btu for heating does NOTHING for me in the winter because the outdoor unit itself will be the weak link, not the wall units. the 100sq.ft. bedrooms will never need more than 6k btu of heat, unless the outside temp is below -5 or -10 f. Once that happens, the outdoor unit won't be able to keep up anyway. So if 6k will help--at all--in this humidity issue, I think I should swap them out. But...will 6k help, even a bit? Or is the low end of their range the same as the 9k? I have no idea and have been stymied in trying to find out.

    Quote Originally Posted by kangaroogod View Post
    you can try running the units in dry mode. this will allow limited cooling and additional dehumidification. this may help.
    No good. I've done it. It works, a bit, but the Misubishi's have neutered this mode by not allowing any temp. set point. They just run forever in this mode. When I was testing it, the temp got down to like 67 degrees and it was freezing. It's not a solution to "ride" the thermostat like that in every room. My wife would kill me if that were the solution! It's not workable. I was sold "set it and forget it." Constantly switching between cooling and dry mode is not going to work. But I appreciate the suggestion.

    Quote Originally Posted by kangaroogod View Post
    I do see a dehumidifier in your future however. perhaps 2 ducts (1 for the first floor and 1 for the 2nd floor) may be a good approach and the best of both worlds.
    I can live with this, if that's what has to happen. How large would the ducts be? They don't need to be insulated, right? How much electrical use would this add to the mix? It does feel like it defeats the purpose of spending all this money on this incredibly efficient Mitsubishi system only to have to supplement it with another compressor. But if that will work, and make the family happy and comfortable, maybe I can work with the installer to heavily discount the labor, given that they did not say this would be necessary. Any thoughts on the Ultra-Aire 98H? That one seemed appropriate in my research.

    Thanks for the suggestions!

  4. #4
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    it would likely be a 8-10" duct for each. the interesting thing about the 6k is that they are only rated at 6,000 btus for cooling while the 9k is rated 3,800-9000. the 6k indoor units are new to the market (last year I believe) and I heard a rumor that they do not modulate well although I do not know the truth of this. first and foremost,, set the blower speeds to low.. I suspect you already have.
    ...

  5. #5
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    Quote Originally Posted by kangaroogod View Post
    it would likely be a 8-10" duct for each. the interesting thing about the 6k is that they are only rated at 6,000 btus for cooling while the 9k is rated 3,800-9000. the 6k indoor units are new to the market (last year I believe) and I heard a rumor that they do not modulate well although I do not know the truth of this. first and foremost,, set the blower speeds to low.. I suspect you already have.
    The reason the 6k shows as having only that static number for cooling, is because it is only sold as part of multi systems. The specs you're getting for the 9k are based on a single-zone, single wall unit system with a dedicated compressor outside. Because the 6k is not sold as such a system, Mits will not/has not published data on them in this regard. I have not heard anything about them being new, nor not modulating well. That's interesting. I thought they had been around forever.

    Regarding fan speeds, definitely: all set to low. The 18k unit seems to have a faster slow setting then the others. Does that make sense? BUt they're all set to the slowest setting.

    8-10" duct? That's pretty big. I hate to run that through my first floor. There's just no good space for it. Maybe a unit in the basement for the basement and 1st floor and then a second unit in the attic for the bedrooms? More expensive, but no need for the duct running from the basement to the attic. Would running two smaller ducted units (Sante Fe Compact?) vs one larger (Ultra-Aire 98H?) significantly reduce the efficiency?

  6. #6
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    Quote Originally Posted by dansminisplits View Post
    Hi everyone, I have a very challenging situation that I hope someone can help me with. About six months ago I have a very expensive 5-wall-unit Mitsubishi HyperHeat mini split system installed in my small, Boston, Massachusetts home. The house is listed as 1,150 sq. feet, officially. It's stucco and very old, built around 1880. It has insulation and I've never considered it to be horribly performing or horribly inefficient, but obviously it's not a new, high performing house. Here's the stats:

    1st Floor: Kitchen, Dining Room, Living Room: 500 sq.ft. with pretty low ceilings of about 7.5 feet. In the living room, one 18kbtu wall unit with external thermostat. The unit was installed about 12 inches too far into the living room and as a result the airflow is almost 100% into that room. There is virtually no airflow into the dining room and kitchen, so temps in those rooms are three degrees higher. While not the subject of this thread so much, I'm NOT happy about that install error.

    2nd Floor: Three small 100 sq.ft. bedrooms, and some hallway/common space and bathroom adding another 175 sq.ft. or so. Higher ceilings of about 8.5 feet. Each bedroom has a 9kbtu wall unit with external thermostat. Two bedrooms are south facing.

    Basement: 500sq.ft. mostly finished, 100% used as valuable space. 12kbtu wall unit mostly for heat, but in the summer, the humidity gets high down there and we run a dehumidifier 24/7 that really heats up the space. I currently keep the relative humidity at 45% in the basement and have the mini split set to 75. It's perfectly comfortable.

    Here's the problem:

    Downstairs the RH is always around 55 to 65%. It's tolerable, but not great. As I indicated, I wonder how much better it would be if the unit was able to blow into the whole space instead of just the 200 sq. ft. living room. It's a relatively open layout, but not completely open and as mentioned, the temperature difference is very noticeable. It's too cold in the living room and too warm in the kitchen. Would moving the unit to the left 12 inches, allowing half the airflow to go into the DR/kicthen allow longer run times and get that humidity down to 50% or below? Any idea how much effort that would entail? I guess they'd have to braise the lines to add the extra 12 inches of line set. Is this worth pursuing?

    Upstairs has a **ridiculous** humidity problem. With only two units running (leaving the middle bedroom off) and all thermostats set to 73, the relative humidity varies between 60 (on a good, hot day) to 75% (at night, when I wake up.) Occasionally I have seen it drop into the high 50s--rarely. It's not comfortable...at all. It's actually pretty gross. The second you get about halfway up the stairs you can begin to feel it. My bedroom flat out smells of high humidity, but all the rooms feel damp. Sleeping isn't comfortable no matter what we set the temperature to. With our old "system" (three window units, one in each room: 2 x 6kbtu and 1 x 7k) it was fine, and we've lived in the house for 12 years so I've been through all sorts of summer weather. I'm not saying the window units were 100% perfect--there were times the relative humidity might seem a bit high, but absolutely NOTHING like the misery we're in now. Relative humidity problems were not anything I ever thought about until this Mr. Slim system began running.

    After reading many threads on here and everywhere else, and educating myself about these systems, and the science of relative humidity, it seems that I was sold a ridiculously oversized system. The outdoor unit is the 48kbtu HyperHeat. It can "ramp down" (as the sales guy would say) to 6kbtu for cooling. Apparently the indoor units can go down to as low as 2k in cooling, but that could only be achieved (given the 6k minimum system-wide) if at least three units are running. Regardless, 65 to 75% RH is simply uncomfortable and we never had such levels with the old window units.

    Questions:
    1) Is this solvable without ripping out the system?
    2) If so, is this solvable without installing a whole house dehumidifier system with ducts? The reason this system was so compelling to me, even at a VERY high price, was because it didn't really take up any space, and allowed me to remove my heating system from the basement and radiators from all the rooms. Adding ducts for a whole house dehumidifier defeats the purpose of the system. I have researched them, and I could install one in my basement with a duct up to the first floor fairly easily. Or I could install one in the attic pretty easily with a duct into the hallway on the 2nd floor. I could also duct into each bedroom with more work. But it seems the basement sceanrio leaves the upstairs less dehumidified, if at all, and the attic scenario leaves the basement needing it, and the basement *really* needs dehumidification in the summer. While I *could* duct from the basement to the attic though my no-longer-needed chimney space, I want to USE that space for something good like closets. That was the point of spending all this money--to reclaim desperately needed space in my small house.
    3) It seems the installer may be willing to replace the three bedroom wall units (9k each) with 6k units. Will this make a difference?

    Any help would be great. The installer is being reasonable, so far. They understand the situation is not acceptable. But they obviously want to spend as little time and money on this as they can. Every second they think about it, their profit margin on the sale goes down, as you guys know. Thanks in advance for your expert advice!
    1100 Sq Ft and 5 indoor units?

    I'd like to see the loaf loss calculations on that one.

  7. #7
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    Quote Originally Posted by pecmsg View Post
    1100 Sq Ft and 5 indoor units?

    I'd like to see the loaf loss calculations on that one.
    Not sure about that. And to be clear, it's 4 units for the 1100 sq.ft. the fifth is for the basement, at another 500 sq. ft. I believe he did a Manual J. Not sure if that's similar (or the same.) Most of that was focusing on heating since I am using this for my main source of heat, although we do have a nice little Valor gas fireplace on the 1st floor that can really cook the house if necessary. All I know is that the installer said I'd be very comfortable. He insisted that these things ramp down with their inverters and will be able to provide as much **or as little** heating and cooling as necessary. I thought, with the very high price, and brand name of Mitsubishi, that I couldn't go wrong. To say I feel mislead is an understatement. But as I said, I'm still working with them to get this right. I'm on this forum, however, because I don't want to get burned twice!

  8. #8
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    You can go to mr slim dot com and see what the actual turn down rate is.

    Mitisubishi is the best as far as minis but as we always say it's the installing contractor that makes the difference between a good job or not.

    Again even 4) 9000 BTU indoor units sound oversized for 1100 Sq Ft.

    I'm south of you and 12000 is enough for 750 - 1000

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  10. #9
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    I just double-checked something regarding the fan speed. In trying to stop an over-heating problem in the winter (a sad story for another day,) all the wall units had external thermostats installed. I just noticed now that the remotes that come with the unit have a "quiet" fan speed that appears to be the slowest setting, and one that is not on the mrch1 external thermostats. Is that possible? If so, I'll have to give that slower speed a try via the remote, but I can't imagine it will be the magic bullet necessary. Will update after that has been tested. Thanks, kangaroogod, for prompting me to double-check that.

  11. #10
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    Quote Originally Posted by pecmsg View Post
    You can go to mr slim dot com and see what the actual turn down rate is.

    Mitisubishi is the best as far as minis but as we always say it's the installing contractor that makes the difference between a good job or not.

    Again even 4) 9000 BTU indoor units sound oversized for 1100 Sq Ft.

    I'm south of you and 12000 is enough for 750 - 1000
    Well, if only it were 4 x 9,000 btu! There are three x 9,000 in the upstairs, one in each bedroom. It's up there that I'm considering replacing them all with 6k units. And there is an 18k on the first floor. Maybe that one should be swapped out for a 12? I do sort of need more than 9k, I think, for heating in the winter in that space. This has always been my worry: that the system is grossly oversized and no inverters on the planet will over come the issues with that despite assurances from the salesperson. Ugh.

  12. #11
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    Im not well versed on Mits outdoor unit sizes on these systems but 48k btu seams high. I would be interested to see a heat loss for this house to see if 48 is really needed. I would think if you could downsize the outdoor unit it would help the indoor units run at a lower capacity as well? Is that correct PRO's?

  13. #12
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    Quote Originally Posted by jbhenergy View Post
    Im not well versed on Mits outdoor unit sizes on these systems but 48k btu seams high. I would be interested to see a heat loss for this house to see if 48 is really needed. I would think if you could downsize the outdoor unit it would help the indoor units run at a lower capacity as well? Is that correct PRO's?
    The 48k was chosen for the coldest of New England nights. They lose capacity starting at 5 degrees and fall off dramatically at -13, I think. I don't believe a 42k or even a 36k would have made a difference. Pretty sure they all have the same low end of capacity in cooling. But yes, would love to hear a pro's take on it.

  14. #13
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    So wheres the back up heat.

    Heat pumps need something to back them up when they fail!

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