GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Okay if Mini Split 24kBTU outdoor condenser capacity exceeds three 7kBTU indoor air handler capacities?

Robert Opaluch | Posted in Green Products and Materials on

Location:  Cape Cod, MA, zone 5, moderately cold winters 5700 annual heating degree days, mild humid summers 700 annual cooling degree days.

3 bedroom ranch built 1979, upgraded R-50 attic, typical 2×4 nominal R-13 walls, single pane windows with storms (see attached layout).  Existing mini split in living room area heats and cools bedrooms okay during the day with doors open, but not overnight with doors closed.  Existing oil heating boiler is failing and will be removed.  Previous whole house central air failed, and was replaced by mini split located in living room area (a central location) to provide most of the heating and cooling needed.  Now considering additional mini split with 3 heads in bedrooms, after considering various other options.

Question:  Okay if 24k condenser capacity is greater than the preferred three 7k air handlers?  What is the impact or effects of this mismatch?  Original bid included 12kBTU + two 7kBTU air handlers.

Currently have a bid for a 24kBTU Fujitsu condenser AOU24RLXFZH.
Air handlers will be in each of three bedrooms (I know, oversized…)
Two bedrooms each have
ASU07RLF1,  7kBTU cooling 7.1kBTU heating
Third ensuite bedroom could be the same ASU07RLF1, or:
ASU09RLF1  9kBTU (included in the current bid)
ASU12RLF1 12kBTU (original bid, lower kBTU requested, 9k current bid)

The bid was for 7k + 7k + 9k despite overkill.  $9,200 minus $2,300 rebate = about $7,000 net.  To get a MASS SAVE rebate for a mini split, the capacity needs to be 1 or 2 ton.

The rough calculation of heating load is 1, 2 and 3 kBTUs, respectively, per each of 3 bedrooms.  So way oversized.  Cooling not often needed during summer, run existing mini split in living area on “dry” mode to dehumidify.

Owner’s Requirements:
1.  Both heating and cooling per bedroom with an individual thermostat per room.
2.  Aesthetically pleasing (typical PTAC or window AC unit not acceptable)
3.  Will not consider electric resistance or radiant heating unit options, partly due to lack of AC. 

Also, I liked the idea of using the attractive Italian import Innova HPAC2.0 all-in-one DIY-friendly heat pump.  Anyone with experience with these??  They have DP81xxxx, a quiet 8.1kBTU 115V model (cooling & heating modes), I think $2,000 each.  However this is higher 8.1kBTU vs. 7kBTU for the bid mini split.  Wall mount or in-the-wall installable, paintable, requiring two 6″ or 8″ vents directly to outdoors.  Can plug into existing wall outlet or in-wall wiring.  Dimensions 40″w x 22″h x 6.5″d.  Haven’t got a call back, maybe not selling direct to consumer, haven’t found a dealer.  Could try one, then add more if good.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. jkumon | | #1

    I can't say I totally follow all the parts to this at quick read, but perhaps this answer will help you dial in on what may or may not be offered to you by different contractors. Can't tell by your post if you've found the magic table that outlines what combos can be done.

    The Fujitsu units do have some ability to modulate down well so having 24 and only 'needing' 21, isn't a big gap. Because you are in between the compressor unit sizing of 18 and 24, there's going to be some over or under sizing. Fujitsu has a table by which installers can couple indoor vs outdoor units. Their units do have a bit of capacity to run over their design units, so theoretically if you attached three 7s to an 18, each unit would just get slightly under served. They have accounted for which combinations they will warranty to do this or not. If this combo you want isn't on that chart, the installer won't do it even though the circumstance might make it okay (or you as the owner would not complain if at peak loads the systems run long, etc).

    Look up on Fujitsu's website for their full catalog of equipment and you'll find under the multi unit section a huge table that outlines every combo possibility. That will tell you why you are getting what you are getting. While it might be cheaper to get three single units, if the rebate is better combining them, you might come out ahead even if the sticker price is more.

    1. Robert Opaluch | | #4

      Ok thanks I'll check out the website tomorrow (getting late here).

  2. Expert Member
    Akos | | #2

    Besides the ridiculous oversize, the issue you'll have with that type of setup is that even the smallest wall mount moves a lot of air. Make sure the units are placed in such a way that they are not directly blowing at beds.

    The 7K wall mounts are above the min modulation of the outdoor unit and the 21k total capacity is within the combination range of the outdoor unit, so there is no issues with going with 3 x 7k.

    A much better setup is would be a single slim ducted unit to feed all the bedrooms. A bit more involved install, cheaper part cost, significantly better efficiency, humidity removal and comfort.

    1. Robert Opaluch | | #5

      Akos, Kyle,

      Yes we set up the location of the heads in the bedrooms so they direct air toward the bedroom door (along closets), and not over a location where you could put a bed. Good point.

      I agree about the slim duct, and discussed with the owner, but she won't go with that option. She wants a thermostat per room. You could adjust louvers in a duct grill in each bedroom to adjust airflow somewhat, which is how the previous central air worked. Unfortunately one can't try out different options then make a final decision. Maybe after a year's usage, we can report back.

      Personally I think radiant or resistance heating plus a window AC unit per room would be a more affordable and more effective solution. But she won't agree with that either, especially the noisy and not very elegant window AC units that block the window view. AC isn't really needed here very least not yet, as the climate warms.

      At least she's going with a mini split, not replacing the oil furnace, and not bringing a gas line to the house for new gas boiler to use with the existing forty-year-old whole house single zone heat distribution system. Or replacing the old central air system that had an uninsulated air handler and barely insulated ducts in the hot attic. I view that as progress, and try to push for other building envelope improvements as the house gets renovated.

      1. kyle_r | | #6

        Hi Robert, understood. Then I would explain that a thermostat that doesn’t work isn’t much better. Whenever an indoor unit calls for heating or cooling that unit will run at 6,000 btu/hr minimum and the refrigerant will flow to all three heads causing over heating or cooling issues. If she truly wants a working thermostat in each bedroom than three 1:1 systems are needed. I would look at Mitsubishi 6 or 9 ton units that modulate down to 1,800 btu/hr.

        1. Jon_R | | #8

          My understanding is that only small amounts of refrigerant may flow to multi-split indoor units that aren't calling for heat/cooling, but this only matters if loads are extremely different (like AC is off in one room). Ie, the thermostat in each room generally works.

          Ducted systems without zoning will definitely have comfort problems if room-to-room load ratio changes significantly (eg, solar gain and wind shifts effecting latent load).

  3. kyle_r | | #3

    Can’t agree with Akos more. I would spend some time reading other posts regarding multisplits and heads in each bedroom. I think you would be much happier with one ducted unit serving the bedrooms and the cheaper equipment cost would likely cancel out the ducting cost.

  4. Expert Member
    Akos | | #7

    About a decade ago before I knew better, I installed a 9k wall mount on a multi split in my bedroom, pretty close to the setup you are looking at.

    Because of the min modulation of the outdoor units, the unit runs only at full tilt or off. What happens is the unit cools down the bedroom very quickly, the compressor cycles off, the fan keeps running at a low speed re-evaporating what little moisture was removed during the on time.

    Since the unit is not modulating, it usually takes a very long time for the room to warm up to trigger the unit back on, during this time you get no cooling or dehumidification. This makes the bedroom quite uncomfortable. Eventually the unit cycles back on again and you get super freeze. Repeat. Not comfortable to sleep in.

    Because of this, I tend to only run the unit for about 2 hours to pre-cool the bedroom and for a bit to help fall asleep, off the rest of the night. My unit doesn't have a good programmable thermostat, so I have to go through all this button pressing every night. Annoying as heck.

    Technically, you get zoning and individual temperature control, but not one that works. Many times over the years, I've come pretty close to replacing it with a ducted unit.

    1. Jon_R | | #9

      It sounds like you have two equipment flaws, which don't necessarily generalize to other cases of over-sized multi-split indoor units.

      a) a temperature control with too much hysteresis allows too wide swings
      b) a fan that continues to run when the refrigerant flow is off (bad for dehumidification)

      1. Robert Opaluch | | #11

        I also recommended using a remote thermostat, rather than the standard thermostat located in the indoor air handler unit. Who cares what the temp is at the exterior wall near a corner of the room, and near the ceiling? It gets tricked when walls aren't well insulated. Or if no sleeve or insufficient caulking was done to stop air leaks when the air handler was installed.

        Would this help much? Remote thermostat is not cheap, so owner wouldn't purchase for the existing mini split in the living room area.

      2. Expert Member
        Akos | | #12


        Both of these could be solved and if that was the only problem, I would have done it a while ago.

        Unfortunately there is no way to fix the the two major issues. When the unit does run, it is uncomfortably cold. You can feel it in your bones.

        When the outdoor unit cycles off it dumps refrigerant through the indoor head causing a loud "whoosh" sound.

        When the unit cycles back on again, there is a lot of thermal contraction of the coil and indoor unit which makes a fair bit of noise.

        I've tried running it for heat in the winter but refrigerant flow reversal in defrost cycle is just way too loud.

        The problem is these sound happen randomly and very occasionally. Unless you are a comatose sleeper, it will wake you up.

        Overall any wall/ceiling/floor mount in a bedroom is a bad idea. Stick to ducted.

    2. Robert Opaluch | | #10

      Akos, this is what I've warned the owner about. The typical behavior of heating and cooling systems that are oversized. Blast of heat until the room gets a little too warm for comfort, then a gradual decline in room temperature, until the room gets a bit too cool for comfort. Or use the analogy of putting a 400 hp engine in a subcompact. Hard to control precisely. Surge then coast.

      Heat pumps work most efficiently when they are just left on, and she wants to turn it down at night and up during the day, like a typical fossil fuel heating system. Certain concepts are not intuitive when people have been trained to use systems that are on/off vs. more efficient when running at low speed.

      The analogy I try to use to explain: Compare two people who drive Boston to LA. One thinks cars use zero fuel when off, so try to keep the car not running as much as possible. The other believes that cars drive most efficiently at 45mph, so they drive with another person who keeps the car going 45 continuously across the country. Who uses the most gas? The one driving 45 continuously, or the other who drives 90mph half the time, and parks the car the other half? They both arrive at the same time. Cars vary in mpg vs. speed. Heat pumps vary with highest efficiency when they are sucking heat out of the outdoor air slowly. They are far less efficient when trying to suck lots of heat quickly out of cold outdoor air.

      1. Jon_R | | #13

        IMO, too hot/too cool can probably be avoided with the right thermostat. Directing airflow away from people is always important. Noise is consistent with my experience, but who knows how noisy any particular model is? Loss of efficiency will occur with over-sizing, but may be a not-that-significant 10%.

        I'm not in favor of over-sizing or multi-splits. Or noisy anything in a bedroom. But it looks like mostly a guess that the owner won't be happy with the Fujitsu system proposed. Even less clear are questions like "will VRF fix multi-split issues" or "are all ductless mini-splits too noisy for bedrooms?".

        1. Robert Opaluch | | #14

          I hadn't thought about the noise issue but will bring it up. She has noticed noise at startup with the existing head in the living area. Might be another plus for deciding to go with ducted for bedrooms, but probably not enough to change her decision.

  5. Mark_Nagel | | #15

    This seems to be a battle between efficiency and comfort. Might be that the efficiency part is going to have to give way. Maybe the other considerations might be reconsidered on a more comfort basis? (wondering what they were -the possibles under consideration- and why they were ruled out)

    1. Robert Opaluch | | #16

      Alternatives included:
      1. Replacing failing oil heat boiler with new oil boiler
      2. Replacing oil heat boiler with gas line and gas boiler
      3. Electric resistance or radiant panels in each bedroom, and window AC units per bedroom
      4. Ducted mini split to 3 bedrooms
      5. Mini split for ensuite bedroom, electric heating and window AC for two other bedrooms
      6. Innova HPAC2.0 all-in-one DIY-friendly heat pump per one bedroom, then add to other two bedrooms if good experience
      7. PTAC per one bedroom, then add to other bedrooms if good experience
      8. Proposed three head mini split for three bedrooms
      9. Owner duct tape boyfriend's mouth shut! ;-)
      I may have forgotten some other options discussed.

      Also proposed towel warmer, heat lamp, or other electric heating unit for ensuite bathroom.

      Also discussed impact of improving the building envelope (can lights to surface LEDs; exhaust fans; ERV; insulate and seal breezeway entry room floor; upgrade large picture window in living room; etc. Most will be done some time when I get around to it ;-) Kitchen renovation being planned, all other rooms have undergone cosmetic renovation, attic insulation upgrade and air sealing, landscaping projects too.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |