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Mini Split Condenser Sizing Questions

I am looking to install mini splits throughout my entire home for air conditioning and supplemental heat for my steam boiler in fall/early spring. I have received quotes from multiple reputable Mitsubishi diamond installers but confused on the condenser sizing I’m being told I need. In total I am going to need 8 heads and his quote is split between two condensers that are 36k btu each (MXZ-4C36NA is the model) but the totals for the indoor units for each handler is 39k and 45k btus? Guy is assuring me this is ok? Said “I have a diversified load, so they'll never really ever be running at full power all at the same time? In summer I will definitely be running all of these at the same time.”

Any advice on if this sizing sounds right is appreciated. Want to be sure this is right before I move forward.



  • ratio
    ratio Member Posts: 3,406
    I can't speak to his load calcs, but it's quite common to overcommit the outdoor units. Even with all the indoor units cooling during the summer, it's unlikely that they all want 100% output at the same time; and the engineer can facilitate this by intelligent division of the locations, like putting heads from the same system on the east and west sides, so that a higher load on one side coincides with a lower load on the other.

    Note also that this is a heat pump, it will heat OR cool; not heat and cool at the same time. We've run into ...misunderstanings... regarding that before.

  • NY_Rob
    NY_Rob Member Posts: 1,370
    Take a look at the install manual for the condenser... it will tell you the "Maximum Allowable Total Indoor Unit Connected Capacity (Btu/h)".

    Looks like the current model suggests "Total connected capacity must not exceed 130% of outdoor unit capacity."
    Sounds like your guy is right in the ballpark for a 36K BTU unit.

    My 36K BTU LG condenser (LMU36CHV) has a max cooling load of 48K BTU's.

    FWIW- my 5-year old 24K BTU Panasonic minisplit has four indoor units attached with a connected load of 36K BTU's and it's never been a problem.
  • Big Ed_4
    Big Ed_4 Member Posts: 2,241
    Supplemental heat , key words for over sizing for the heat end of the unit .... Steam ,I assume you live in the North-East .
    I have enough experience to know , that I dont know it all
  • NJHomeowner
    NJHomeowner Member Posts: 66
    Thanks! He said he was using the older model, which doesn’t seem to say 130%? https://www.ecomfort.com/manuals/mxz4C36submittal.pdf. Still think I’m good?

    Fully get it in the heat or cool at once. Also I’m in norther nj.
  • NY_Rob
    NY_Rob Member Posts: 1,370

    Thanks! He said he was using the older model, which doesn’t seem to say 130%? https://www.ecomfort.com/manuals/mxz4C36submittal.pdf. Still think I’m good?

    Don't see why not.

    The 130% figure seems to be a pretty general rule that applies to most minisplits regardless of brand.

    My new 36K BTU LG mentions 48K BTU as max total connected capacity. If you do the math... that comes out to about 130% of rated capacity.

    As long as the contractor adds (by weight) extra refrigerant as specified by the manual for total measured lineset length (if) exceeding factory precharge length, you should be fine.

  • NJHomeowner
    NJHomeowner Member Posts: 66
    ok great. Do I lose any efficiency on this approach or is it negligible?
  • NY_Rob
    NY_Rob Member Posts: 1,370
    edited May 2018

    ok great. Do I lose any efficiency on this approach or is it negligible?

    If it was a commercial application where the outdoor unit would be running at or over 130% max all day long... not good... get a bigger outdoor unit or less indoor units per outdoor unit. Once you run over max capacity, the indoor units just don't provide as much cooling power.

    In a residential setting where you occasionally run at max output from time to time for short periods... no problem at all.
    Your installer is betting on the fact that you won't be running all four indoor units concurrently at max all the time... and he's probably correct, and that's why the manufacturer lets you oversize a bit. Remember that once the smaller rooms cool down... their pretty much running on fan only till the t-stat in the indoor unit calls for cold again so you just took XXX BTU cooling load off the system which makes it available for the other zones.

    JUGHNE Member Posts: 10,579
    Are you considering two 36,000 btuh units?
    How large is your house? 6 tons of AC is a lot for most residential homes.
    Even with the modulation factor of "turn down capacity" could you be oversized for the total?
  • Jack
    Jack Member Posts: 1,046
    In the northeast, I disagree with Thomas' sizing. I think his sizing and mine are VERY general, but given the fact that you need dehumidification more than absolute cooling in your area...(is that the case?) I would go 600-700 sq ft on a 12,000. 400 sq ft/12000 was the "commercial rule of thumb". Over committing the evap to condenser sizing is a safe way to go. It is rare that every unit is being called upon. If they are, you get the rated capacity of the outdoor unit split between the indoor units, so the indoors are de-rated somewhat, but you learn the system. How much direct late afternoon sun and how much glazing you have on that side of the house will definitely affect loads there. Something which may be worth looking at, and I will use my own home as an example. Originally, I was going to go with one 36 multi. At that time, '10, the efficiency of the 36 was 16 seer. When I looked at the cost of the 36 vs the cost of a 24 and another smaller unit, I chose to go with a 24 (16.5 seer) with two 12k evaps for the bedrooms upstairs. Downstairs, I went with a 12kbtu unit that was 25 seer. I use the upstairs units for cooling only. They have never been used for heating. The downstairs unit we use year round and that gives us better efficiency. Every house has a personality. Your place is much larger than mine apparently, but I would suggest that you discuss it with your preferred dealer.
  • pecmsg
    pecmsg Member Posts: 3,885
    No sizing by Sq Ft is accurate. Different solar gains, different windows and doors, insulation etc.

  • Pidge
    Pidge Member Posts: 18
    JUGHNE said:

    Even with the modulation factor of "turn down capacity" could you be oversized for the total?

    The MXZ Condenser does not fully modulate.

    The caveat is that I read about the MXZ in another forum, and I haven't confirmed the validity. But the poster, Dana Dorsett is a reliable source of information, or I wouldn't post it. But the OP should verify that the MXZ doesn't modulate. It's complicated, with related and non-related info.


    Part of Dana's 5/15/18 post in Answer 8 said:
    "MXZ compressors are not full-on VRF systems. They will support a number of different heads/cassette capacities, but as I understand it even those heads/cassettes that can modulate when married to a dedicated compressor do NOT modulate when on an MXZ compressor. That means the heads have to be reasonably sized to their room/zone loads to hit the efficiency numbers, and provide reasonable comfort.

    For very small loads like individual bedrooms you'll be better off using a mini-duct cassette sized for the combined load, whether it's on it's own modulating compressor, or an MXZ multi-split. Sizing the heads/cassettes to the room/zone loads becomes even more critical when they aren't modulating..."

    On May 25, Dana posted in the following link:

    Answer 2... "The MVZ air handler submittal sheets show only one set of capacities are only compatible with MXZ multi-split compressors, which don't fully modulate. The capacity at 47F is published, but the capacity at +17F or lower depends on the compressor it's married to. They have multiple speed settings, but the blower doesn't modulate, though it will change speeds when necessary for defrost cycles, etc.."

    I'm shopping for a minisplit that fully modulates, and thought this info might be helpful to the OP. The quotes are just a small subset of the info in the links.
  • ComfyHvac
    ComfyHvac Member Posts: 2
    edited October 2019
    A reason for oversizing the outdoor unit can also be the fact that they don’t want more than 2 outdoor units, the 36k allows 4 zones (also for additional supplemental heating ). The outdoor unit most certainly modulates but you still should not oversize the indoors in any case. You can hear the compressor speed up and slow down on the MXZ-C units. All inverter mini split I’m familiar with modulate. Mitsubishi is the way to go if you want the top of the line And they certainly maintain temperature within pretty tight tolerances which is the hallmark of inverter systems . I would definitely look in my link drive and read the spec sheets because they definitely do modulate / Engineering Manual
  • ComfyHvac
    ComfyHvac Member Posts: 2
    The hyper heat units are also nice but very pricey. They deliver excellent heating performance at very cold temperatures. We just did a 4 zone with a mix of different indoor units including the new MLZ Cassette and it works beautifully. It’s pretty hard to go wrong as long as the calculations are good and the company designing and installing the system knows what they are doing. They sound like they know what they are doing, it’s a lot of work to become a diamond contractor