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Community and Q&A

Sizing Minisplit-Air Handler System

willib | Posted in General Questions on

I am replacing a multizone oil burner system with two Mitsubishi heatpumps, 2 air handlers and 3 minisplits.    I have gone through the manual J calculations and need to figure out how to size the air handlers and minisplits.  House is about 2700 sq ft, 2 floors except MBR is 1st floor with no room above it.   House is in MA.

Air handlers will connect to 1st floor duct work.  Upstairs will use minisplits.

System 1: 36K BTU singlezone – air handler for rest of 1st floor –  kitchen, dining room, family room, TV room and part of finished basement.

System 2: 36K BTU Multizone – air handler for 1st floor MBR, 3 minisplits for upstairs.

What happens if the capacity of the air handler and minisplits exceeds the heatpump capacity?   For example if the MBR air handler is 18K and the 3 minisplits are 9, 9, and 9.    18+27= 45K.  How much above rating is ok?  Other than I might not get as much heat as I want, does anything bad happen?


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  1. Expert Member
    Akos | | #1

    2700sqft with 7 tons of heat pumps doesn't sound like a proper manJ, awfully close to 500sqft/ton rule of thumb. Unless your main suite is a 1000sqft with a wall of glass, the chances of it needing a 1.5 ton head is next to zero.

    Before going much further down the road, I would get your actual heat load dialed in, if you have fuel usage from a previous heating season, follow the steps here:

  2. jwasilko | | #2

    Mitsubishi generally allows 120% indoor capacity vs outdoor capacity. However, that's not what you really need to worry about:

    Mitsu multizone heat pumps only have a ~50% turn down rate, so your 36k unit may only be able to get down to 18k at 47F. It manages this by bleeding refrigerant into other heads, which can cause overheating. You can minimize this impact by using an external control/temp sensor that will allow you to turn off the mini-split fan.

    Read this from Mitsu about this issue:

    1. user-5946022 | | #12

      Looks like the host (Mitsu?) took down this pdf. Anyone have a copy of it they can link?

      As the owner of a multi zone (1 outdoor, 1 mixing box, 3 indoor) Mitsu ducted mini system, I need all the info I can get my hands on. The Mitsu Diamond dealers around here apparently don't read and are unfamiliar with all things multi split.

      1. Expert Member
        Akos | | #13

        All the app notes are here:

        The above one is #1036

  3. willib | | #3

    The MBR section includes an office and master bath. Has cathedral ceilings, sky lights and one wall has two Anderson sliding doors and bath has windows and skylight. Manual J said about 18K heat assuming 72 degrees. Old people live here. :-)

  4. kyle_r | | #4

    I agree with Akos, your heat load sounds 2-3x too high. For comparison, I have a 2,500 sqft house in Michigan that I heat with a single 15 kbtu/hr mini split.

    I would suggest posting your Manual J and asking the community to review.

  5. willib | | #5

    Here are the Manual J calcs
    Zone 1: (air handler) Master Bedroom, Office, Bathroom - 1st floor, cathedral ceiling, door outside + 2 sliding doors, skylights. No rooms above Zone 1, exposed outside walls on 3 sides.
    Zone 2: (air handler) rest of 1st floor, kitchen, dining room, den, fireplace room, + basement (workshop, laundry, exercise room
    Zone 3,4,5 (mini splits) 2 upstairs bedrooms, bathroom, middle den room.

    Zone 3,4, 5 are above Zone 2.

    Plan is to put Zone 1, 3,4,5 on a multi zone 36K heatpump, and Zone 2 on a single zone 36K

    This recent storm provide some useful data. After power outage, current Weil-Mclain oil burner with 2 air handlers took about 4 hours to raise temperature throughout house from 55 to 63 degrees while outside temp was 13. I believe that system is about 110K BTU, but not really sure how to tell.

    Last year we used 1200 gallons of oil. That included oil for hot water which is now a electric hybrid. We had the house in the 70s all winter long due to one of the main design criteria,
    I'm not allowed to make my 92 year old mother in law cold so 72-73 degrees wherever she is. We do have a fireplace with wood insert so we can cook that room if necessary.


    1. Expert Member
      Akos | | #6

      You are probably somewhere around 6500HDD. 1200gallons of oil burned in an 80% furance is 1331therms. I would say probably %10 of that is for hot water, so you are looking at 1198Therms.

      1198therms/6500HDD=0.184therm/HDD or 767BTU/ heating degree hour.

      With a 9F design temperature so 61F delta, your loss is about 61*767=46844BTU. Not great but nowhere near the 76000BTU a pair of hyper heat 3 ton units can produce.

      I run the math with your actual HDD and usage and see where you are at then tweak your coolcalc numbers until the output matches the fuel usage results.

  6. willib | | #7

    Thanks for calculations. From a heating perspective, what happens if the system is oversized? Most of the time the outside temperature will be substantially higher than the design limits, for example 45 or 50 degrees, so by definition the system will be oversized. I imagine that it will cycle on and off. Does that damage the heat pumps? With an oil burner it just cycles more often, but while it may not be the most efficient from a cost perspective, I don't believe anything particularly bad happens. Is that also true for heat pumps?

    Part of the challenge is that the smallest Mitsubishi 4 port system is 36K, the MXZ-4C36NAHZ2. For the majority of the 1st floor and basement, the PLA-A36EA7 seems to be the only hyperheating option that is also energy star rated to qualify for rebates.

    In a nutshell I need to drive 2 air handlers and 3 minisplits. We had looked at trying to slice the problem different ways and haven't found a better way yet. Maybe there are other brands to consider?

    1. Expert Member
      Akos | | #11

      I have an oversized multi split at home with the wall mounts in the bedrooms. I only use it for cooling but even for that it barely works. The issue is that even the smallest wall mount is WAY oversized for the load (thus my earlier point about the 18k in the MBR) which makes them cycle a lot. This means when it comes to cooling, you get burst of supercold followed by long cycles of high humidity air when the unit cycles off but the fan keeps running so any moisture on the coil is re-evaporated into the room. I also find that the wallmount blows too much air even on low and can be uncomfortable to sleep with. I usually don't even run it overnight, just pre-cool the bedroom and shut it off for the night.

      The other issue is since it is oversized and cycling a lot efficiency is pretty awful. For the amount of cooling it is doing, it uses way more than comparable one to one mini split installs.

      I have tried it for heat it as well but it is even worse. Each time the outdoor unit cycles off you get a whoosh of refrigerant through the system, impossible to ignore at night time. Defrost is way louder and guaranteed to wake a light sleeper.

      If you already have ducts in the house, I would use those for heating and cooling and skip any wall mounts. If you want zoning, you can look at splitting off some of the bedrooms onto a dedicated ducted unit.

  7. greenright | | #8

    I know I sound like a broken record, but a pair of 3 ton Fujitsu airstage vrf outdoors is what you need here. Even in multi split confit they will turn down nicely and are flexible enough to satisfy your heat load requirements. The only gotcha is that you need to run external thermostats for the multi heads or your room temp will be not where you want it most of the time.

    1. Expert Member
      Akos | | #10

      Greenright, how is your setup running?

      On paper the newer Airstage does look much better than most multi splits. I'm curious if it has any of the issues of a typical multi split (temperature overshoot, short cycling, crosstalk between zones and dumping heat into zones that are off)?

  8. willib | | #9

    Greenright, what are the differences between Fujitsu and Mitsubishi? My HVAC company is recommending 2 Mitsubishi heat pumps, one 36K 4 port to drive air handler and 3 minisplits and 1 36K single zone for an air handler. I don't know much about Fujitsu. What do you mean by external thermostats? I currently have 4 Honeywell T10s that I was hoping to reuse. Would need one more to cover the 5 zones. Thanks

  9. walta100 | | #14

    Stick with the factory controls with remote sensors.

    If you connect anything else you give up the variable speed abilities of your mini splits and force them to cycle on and off.


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