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Thread: help! new Mitsubishi 3 head 36k outdoor installed and undersized unit on upper level?

  1. #1
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    help! new Mitsubishi 3 head 36k outdoor installed and undersized unit on upper level?

    Hi guys!...just had 3 indoor units installed:
    1) upstairs high ceilings 18k
    2) master bed 9k
    3) daylight basement 12k
    Outdoor unit 36k

    The main motivation was to take care of the upper level which gets hottest due to skylights, lots of windows, french doors, etc....the other two units were mainly supplemental....I was thinking we could run our gas furnace way way less until the coldest days of the year.
    My sense is the 18k unit is undersized for the upper level space.....I had many many bids and they all said 18k for that space....but I'm testing it now, and the outdoor temp is like 65 here in Seattle...I have it running for 2 hours and it struggles to keep the temp at 70 in there when blasting at max.
    -There is a garage attached to the space the 18k is installed in, and when the outdoor temp goes up that space cooks....I haven't had a really hot day yet to test things, but I have a feeling it's going to have a hard time based on what I'm seeing already
    The bedroom unit has NO problem cooling the master bed very quickly..
    What are your thoughts? Should I try and get the contractor to replace the 18k with a 24k? am i kind of screwed because the outdoor unit is 36k?
    I was excited about the FS model 18k thinking it would do a better job of distributing the air....it seems the higher btu units don't have all the bells and whistles...

    Would love to hear your thoughts.
    Thanks!

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    Call the installers back and have them check and test the system. If something isn’t working right, now is the time to find it before the warranty runs out.

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    Quote Originally Posted by ksefan View Post
    Call the installers back and have them check and test the system. If something isn’t working right, now is the time to find it before the warranty runs out.
    what kind of check/testing would they do?
    The space has skylights which are great sources of heat....there's a kitchen right there which is a source of heat of course.....they just installed it and said "there you go" and handed me the remotes..
    After checking online calculators, it looks like it should easily have been a 24k unit there....is it do-able to swap the 18k out for a 24k unit now?.....I'd then have 24+9+12=47 total, with a 36k outdoor unit

  4. #4
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    Without a proper load calculation performed, its anyones guess. The installation company should have done a load calculation.

    loadcalc.net

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    A couple things to rule out user error:

    1. Is the fan speed on the remote set to auto?
    2. Is the remote set to the snowflake icon for cooling?
    3. Is there any cold air blowing from the wall unit?

    Either way, you may want to have your contractor come out and investigate because you might have an operational problem.
    Everything Im going to say today are my conclusions and my opinions, my opinions are based on my education, my training, my experience. Different people have different experiences, so they have different opinions and I make no claim that my opinion has its origin in the mind greatness. - Paul Harrell

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    Quote Originally Posted by CircusEnvy View Post
    A couple things to rule out user error:

    1. Is the fan speed on the remote set to auto? I ran it multiple times at "POWERFUL MODE" which supposedly blasts faster fan speed than HIGH, setting temp at 61 degrees....I've tried full speed on fan as well
    2. Is the remote set to the snowflake icon for cooling? Yes, Definitely on COOL mode
    3. Is there any cold air blowing from the wall unit? YES.....but the room won't get any cooler than 69/70........that may be ok if it can be that temp on an 80-90 degree day but I'm skeptical.....

    Either way, you may want to have your contractor come out and investigate because you might have an operational problem.

    1: I ran it multiple times at "POWERFUL MODE" which supposedly blasts faster fan speed than HIGH, setting temp at 61 degrees....I've tried full speed on fan as well
    2: Yes, Definitely on COOL mode
    3. YES.....but the room won't get any cooler than 69/70........that may be ok if it can be that temp on an 80-90 degree day but I'm skeptical.....


    ...they say wait and see how it is on a hotter day and they can come out and add refrigerant to the lines...and worst case swap the 18k with a 24k unit

  7. #7
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    Quote Originally Posted by jazzy2019 View Post
    1: I ran it multiple times at "POWERFUL MODE" which supposedly blasts faster fan speed than HIGH, setting temp at 61 degrees....I've tried full speed on fan as well
    2: Yes, Definitely on COOL mode
    3. YES.....but the room won't get any cooler than 69/70........that may be ok if it can be that temp on an 80-90 degree day but I'm skeptical.....


    ...they say wait and see how it is on a hotter day and they can come out and add refrigerant to the lines...and worst case swap the 18k with a 24k unit
    You DONT add refrigerant because its not working. When in doubt pull it out and weigh it. Compare to factory specs.

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  9. #8
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    How long are the lines to each unit?

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    Quote Originally Posted by jazzy2019 View Post
    Hi guys!...just had 3 indoor units installed:
    1) upstairs high ceilings 18k
    2) master bed 9k
    3) daylight basement 12k
    Outdoor unit 36k

    The main motivation was to take care of the upper level which gets hottest due to skylights, lots of windows, french doors, etc....the other two units were mainly supplemental....I was thinking we could run our gas furnace way way less until the coldest days of the year.
    My sense is the 18k unit is undersized for the upper level space.....I had many many bids and they all said 18k for that space....but I'm testing it now, and the outdoor temp is like 65 here in Seattle...I have it running for 2 hours and it struggles to keep the temp at 70 in there when blasting at max.
    -There is a garage attached to the space the 18k is installed in, and when the outdoor temp goes up that space cooks....I haven't had a really hot day yet to test things, but I have a feeling it's going to have a hard time based on what I'm seeing already
    The bedroom unit has NO problem cooling the master bed very quickly..
    What are your thoughts? Should I try and get the contractor to replace the 18k with a 24k? am i kind of screwed because the outdoor unit is 36k?
    I was excited about the FS model 18k thinking it would do a better job of distributing the air....it seems the higher btu units don't have all the bells and whistles...

    Would love to hear your thoughts.
    Thanks!
    Its probably not gonna go below 70*

    ill bet the thing is barely even running.

    Yes I understand you have used powerful mode, and it feels like lot of air is coming out of it. But the outdoor unit is likely barely running.

    EXTREMELY common for those larger head units to barely run when the indoor and/or the outdoor temp is low.

    6k through 12k zones in bedrooms will get down in the mid 60*s. 18k and 24k head units in larger spaces like to run in the 70* to 73* range for minimum cooling.

    WHY? Dont know. But I can promise you I ran into this issue about 5 years ago when I first started putting them in and now advise customers that below 70* is likely not realistic. Do they sometimes do it? yes. but most wont.

    When the outdoor temp rises the outdoor condenser will send more refrigerant to the indoor evaporator and you will move more heat from inside the home to outside.

    I wouldn't worry much about it until it gets hotter outside. Understand that unit has about 750 speeds. it doesn't just turn on and run at max. you cant force it. a tech can via dip switches inside the unit.

    The FS model does move less air than my preferred model of GL. But if it doesn't need a lot of throw like most upstairs/loft style settings its not gonna matter much.

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    Quote Originally Posted by jacob-k View Post
    Its probably not gonna go below 70*

    ill bet the thing is barely even running.

    Yes I understand you have used powerful mode, and it feels like lot of air is coming out of it. But the outdoor unit is likely barely running.

    EXTREMELY common for those larger head units to barely run when the indoor and/or the outdoor temp is low.

    6k through 12k zones in bedrooms will get down in the mid 60*s. 18k and 24k head units in larger spaces like to run in the 70* to 73* range for minimum cooling.

    WHY? Dont know. But I can promise you I ran into this issue about 5 years ago when I first started putting them in and now advise customers that below 70* is likely not realistic. Do they sometimes do it? yes. but most wont.

    When the outdoor temp rises the outdoor condenser will send more refrigerant to the indoor evaporator and you will move more heat from inside the home to outside.

    I wouldn't worry much about it until it gets hotter outside. Understand that unit has about 750 speeds. it doesn't just turn on and run at max. you cant force it. a tech can via dip switches inside the unit.

    The FS model does move less air than my preferred model of GL. But if it doesn't need a lot of throw like most upstairs/loft style settings its not gonna matter much.

    Thank you very much for the insights! I think I will have a real solid idea of things in a few days when the outdoor temp gets up around 75-80....I thought the FS was better with all the bells and whistles but I may end up getting them to swap it to a 24k GL if it can't keep up once the outdoor temp gets up there
    I'll know within the next week I guess.

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    Quote Originally Posted by jazzy2019 View Post
    Thank you very much for the insights! I think I will have a real solid idea of things in a few days when the outdoor temp gets up around 75-80....I thought the FS was better with all the bells and whistles but I may end up getting them to swap it to a 24k GL if it can't keep up once the outdoor temp gets up there
    I'll know within the next week I guess.
    Swapping the head unit to a 24 won't change much. And might actually make it worse.

    The outdoor unit has 36,000 btus of cooling available. You currently have 39,000 btus hooked up indoors. The system does allow you to install 130% indoor capacity to outdoor capacity. Off the top of my head I believe that's 48,000. So technically you would be at 47,000 with 24k changeout and allowed per Mitsubishi.

    However there is a trade off of robbing Peter to pay Paul. The more you over capacity the inside the more potential issues you invite.

    Post again in a few days and let us know how things are going.

    In the mean time grab some pics of the upstairs space, wall mount, etc. I'd like to see how they mounted it and what the space looks like. A lot of times it's not the unit but the location.

    I'll check in as best I can but Monday temps hit 100* here and my free time dissolves.

    Sent from my SM-G998U using Tapatalk

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  14. #12
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    I'd hold off on having them add a 24K head to the unit. The 3 ton outdoor unit can only have up to 46,000 btu connected to it. I would either have the contractor do a load calculation or do one yourself using an online calculator, then run your system through Mitsubishi's Diamond System Builder program (free) or post your info here and I'll do it. The DSB file will give you expected BTU output of each indoor unit that you can compare to the load requirements of each room.

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    Quote Originally Posted by FlyersFan View Post
    I'd hold off on having them add a 24K head to the unit. The 3 ton outdoor unit can only have up to 46,000 btu connected to it. I would either have the contractor do a load calculation or do one yourself using an online calculator, then run your system through Mitsubishi's Diamond System Builder program (free) or post your info here and I'll do it. The DSB file will give you expected BTU output of each indoor unit that you can compare to the load requirements of each room.
    12-9-24 = 45k.....not that it would make a huge difference.

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    Quote Originally Posted by Adlerberts-Protege View Post
    12-9-24 = 45k.....not that it would make a huge difference.
    Whoops, my math skills fail me sometimes. You're right though, not gonna get more than 36K out of that outdoor unit no matter how big the indoor heads are.

  18. #15
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    Quote Originally Posted by FlyersFan View Post
    Whoops, my math skills fail me sometimes. You're right though, not gonna get more than 36K out of that outdoor unit no matter how big the indoor heads are.
    Well Yes and No...

    If a 24k is swapped and ran by itself. It will provide more than 18kbtu(but less than 24kbtu). But once another zone turned on it would go to sh*t. So in rare situations such as 3 story applications, where bottom floor wouldn't need cooling when third floor does, and third floor wouldn't need heating when bottom floor does, this application does work. However I have moved away from the practice since I don't mix normals with bottom floor heating and hyperheats with top floor cooling.

  19. #16
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    Jacob,

    I have been curious why in your environment you are putting in Hyper-Heat systems.

    Does it really get that cold where you are to require that extra heating capacity?

    Just curious.
    Can someone please explain to me -
    Why is there never enough time to do it right the first time, but plenty of time to do it twice?


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    Quote Originally Posted by joemach View Post
    Jacob,

    I have been curious why in your environment you are putting in Hyper-Heat systems.

    Does it really get that cold where you are to require that extra heating capacity?

    Just curious.
    Lowest temp I've ever seen here was 17*. Most years we will see 25* as lowest and quite a few days in the low 30*s.

    Take a 3 ton 4 zone normal Mitsu. Likely gonna be in a house between 1400sqft and 1900sft.

    Look at the backend data. 36kbtu at 47* and 22kbtu at 17* out.

    22kbtu is gonna struggle to heat a home that size but probably could if all these other variables weren't involved.

    Most houses here are old and leaky with bad insulation and windows. Many are cabins.
    Half my customers are second homeowners. They want to show up and be warm.
    We can have several feet of snow.
    Non-Hyperheat models defrost much more frequently
    Non-Hyperheat models put out warm air not hot air.

    Hyperheat is only the difference of the outdoor unit. We simply charge the customer the difference in outdoor unit costs. Most when presented with a unit that will heat much better would rather pay the additional $. Except once in a while when it also has to incorporate a branch box as there is a bit of a jump price-wise.

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  22. #18
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    Quote Originally Posted by jacob-k View Post
    Lowest temp I've ever seen here was 17*. Most years we will see 25* as lowest and quite a few days in the low 30*s.

    Take a 3 ton 4 zone normal Mitsu. Likely gonna be in a house between 1400sqft and 1900sft.

    Look at the backend data. 36kbtu at 47* and 22kbtu at 17* out.

    22kbtu is gonna struggle to heat a home that size but probably could if all these other variables weren't involved.

    Most houses here are old and leaky with bad insulation and windows. Many are cabins.
    Half my customers are second homeowners. They want to show up and be warm.
    We can have several feet of snow.
    Non-Hyperheat models defrost much more frequently
    Non-Hyperheat models put out warm air not hot air.

    Hyperheat is only the difference of the outdoor unit. We simply charge the customer the difference in outdoor unit costs. Most when presented with a unit that will heat much better would rather pay the additional $. Except once in a while when it also has to incorporate a branch box as there is a bit of a jump price-wise.
    I'm the same way, although it gets much colder here.

    If I am presented with an instance where I may need a branch box, I just break it up into 2 heat pumps. Sure, they need 2 OD units, but trying to hide the branch boxes, and the additional wiring and labor rarely make sense. I have had great success with never doing more than a 3 zone HH unit.
    "Life, Liberty, and Levin!"

  23. #19
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    Quote Originally Posted by jacob-k View Post
    Lowest temp I've ever seen here was 17*. Most years we will see 25* as lowest and quite a few days in the low 30*s.

    Take a 3 ton 4 zone normal Mitsu. Likely gonna be in a house between 1400sqft and 1900sft.

    Look at the backend data. 36kbtu at 47* and 22kbtu at 17* out.

    22kbtu is gonna struggle to heat a home that size but probably could if all these other variables weren't involved.

    Most houses here are old and leaky with bad insulation and windows. Many are cabins.
    Half my customers are second homeowners. They want to show up and be warm.
    We can have several feet of snow.
    Non-Hyperheat models defrost much more frequently
    Non-Hyperheat models put out warm air not hot air.

    Hyperheat is only the difference of the outdoor unit. We simply charge the customer the difference in outdoor unit costs. Most when presented with a unit that will heat much better would rather pay the additional $. Except once in a while when it also has to incorporate a branch box as there is a bit of a jump price-wise.
    Thanks Jacob,

    Makes sense now.

    Those places must be really leaky.

    I have put in many with no "Extra Low Temp Heating" (I do Fujitsu) in an area where the low is 8 degrees and have never had an issue. But thinking about it, it may be because I put in a lot of single zone systems. Their performance is about 50% higher than a multi zone. I always push the customer towards multiple single zone systems instead of a multi zone system. Just have to be creative on design so it all looks good. As I tell my guys, "It not only has to work properly, it has to look good, too."
    Can someone please explain to me -
    Why is there never enough time to do it right the first time, but plenty of time to do it twice?


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  24. #20
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    Fujitsu doesn't lose capacity as bad as Mitsu. The normal Fujitsu is closer to a Hyperheat than a normal Mitsu.

    Sent from my SM-G998U using Tapatalk

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