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Three MXZ-5cNAHZ vs two MXZ-8cNaHZ

DavidSilva | Posted in Mechanicals on

Hi all.  We are building a “multi-generational” home for the brood here in Massachusetts.  HERS rater shows 70,000 BTUs for heating load with my intended Insulation, windows, etc.  Manual J from my own access to free software and results from a few installers shows roughly 90,000.  I will be using Mitsubishi Mini-Splits and will expand into Attic and Basement as Family continues to grow, and those numbers above are reflective of end state full usage. 

From installer, three MXZ-5cNAHZ vs two MXZ-8cNaHZ are about the same install cost with the various zones/heads we’ll have at an end state.  He says since they are variable motors, it should be a wash from an electricity usage.  When I look at the submittals per unit at the different design temperatures, its difficult for me to be confident. Is it likely that there is a minimum electrical load drawing power even in mild days that would make 2 larger units better than 3 smaller units?  I’m seeing a 24000 minimum BTU for the 5c and 27000 BTU for the 8c at 47F.

Summary of Watts listed on the Submittals are below along with efficiency stats per unit:

  nonducted /ducted nonducted/ ducted
cooling 3930 4800 3130 3890
Heating at 47°F* (rated) 4220 4800 3430 4140
Heating at 17°F* (rated) 4130 4650 3350 4230
Heating at 5°F* (Max) 7910 8790 7030 7070
COP at 47f 3.75 3.3 4.1 3.4
COP at 17 f 2.3 1.8 2.3 1.9
cop at 5f 2 1.8 2 1.9
Energy Star NO No yes no
seer 20 16 20 17
eer 12.2 10 13.4 10.8
hspf 11.5 10.1 11 10.6

Thanks in advance for advice.  Definitely learned a lot in last month or so, but still a novice.  Your help is always appreciated.

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  1. joshdurston | | #1

    I would say neither, it's important that you don't oversize a multisplit. You have some leeway on the configuration of heads within the home, but your outdoor units shouldn't exceed the load calculation by much, and possibly even be under.
    Even though outdoor units modulate, it really is just matching the nominal capacity of the running indoor units. Most multisplits do not reduce an individual heads output based on load. In other words a 9000 btu/hr head will supply that much heat until the zone gets above setpoint by a whatever the control deadband is. You can reduce the fan speed which will cause a proportional reduction in capacity, but may push the outdoor unit into short cycling.
    The outdoor units won't be happy running under 30% output.

    Listen to this podcast.

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