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Installing a Ductless Minisplit Head in a Bedroom

Steven T | Posted in General Questions on

My home is a 1950s Cape style house in central Massachusetts.  The 2nd floor includes two bedrooms with mostly sloped ceilings.  Air sealing and insulation have been done in the ceiling, exterior and knee walls.  These two bedrooms each have one tiny supply duct from the central furnace.  In the freezing weather, room temps can be 10F colder than the 1st floor.  In the summer these rooms get incredibly hot.  Window ACs work, but occupy the only window in each room and we need something quieter.  

Neighbors with similar houses have added central cooling along with new supply ducts to these rooms.  This delivers enough heat, but does not provide adequate cooling.  I’ve discussed adding supply + return duct runs plus creating a separate zone for the 2nd floor with HVAC installers.  Because the second zone is so small (design cooling load of 6000 BTUs) a modulating furnace and modulating AC/HP with or without a bypass damper were suggested.  I didn’t feel confident that this would be ideal for comfort or the equipment.  Could this work well?

Most installers suggest (the simple route) of adding a Mitsubishi mini split head to each of these two bedrooms.  This is appealing for the high confidence in achieving comfort without losing closet space to new ducts.  I would prefer two single-zone systems for better efficiency and comfort.  The 6000BTU high wall units would only fit in locations that would be over beds.  The floor mounted units (KJ) look to suit the rooms,  but are available no smaller than a rated 9000BTUs, with a minimum rated cooling capacity of 2300 BTUs.  This is below design load, so I imagine they would work well enough.  We also plan to leave the bedroom doors open to encourage cold air to fall down to the first floor.

Ducted mini splits haven’t been proposed, and I don’t see how a ducted system would work.  There is a low hallway ceiling and no usable attic space overhead, or behind the knee walls.

Is there another option I am missing, or are single zone ductless mini splits a good option here?  Would it be an especially terrible idea to set both heads to the 20,000 BTU multi zone outdoor unit (MXZ-2C20NAHZ2) considering its lowest cooling output is more than double the design cooling load?

Thank you

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Replies

  1. qofmiwok | | #1

    Hope you get some answers because I'm in the same boat. Keep going back and forth between 3 mini-split heads, or 1 with a small amount of ducting between the rooms. A couple companies now make 6k units. The problem is they won't let you put 3x 6k units on an 18k head, so you need a 24k head. My max design loads are about 3500 per room and average about 1500 so even 6000 worries me.

  2. Joshua Salinger | | #2

    Hi Stephen,

    This is a common question and very similar to a situation I had on my personal home. After considering all of the options (of which you laid out a number of the pros and cons) we elected to do one 6000 BTU wall mounted unit in the owners suite and a 9000 BTU unit in the hallway between the two bedrooms. We installed powered transfer ducts/grilles from the hall to each bedroom to get the conditioned air to the spaces. This was in hopes of saving the costs and extra BTU's by avoiding a head in each room. Ultimately, this approach isn't providing quite the comfort in the two bedrooms we had hoped for in the cooling season, but is better than the window units we had previously. The heating season is comfortable and no real complaints. The biggest issue is the noise created by the jump duct fan and we only use them in the cooling season because of this. If we keep the doors open and use the fans it tends to work best. The owners suite is perfectly comfortable, but due to the fact that the only place to put the head was the wall opposite the bed, it aims the airflow at us. It isn't the end of the world, but worth considering this if possible.

    I don't know if you have the option of placing a head in the hallway, but it may be another arrow in your quiver as you consider options. If I had to do it over again, I would up the BTU's in the hall and seek out some quieter fans.

  3. Peter L | | #3

    Best energy efficiency is always a ductless single head unit. Once you start going with ducts and multi-air handler units, the energy efficiency does begin to drop. Also, if the outside compressor goes on the fritz, with a multi-head unit, you lose all heating/cooling capabilities. With individual units, each indoor air handler has it's own outside compressor.

    I went with a 1 ton Mitsubishi ductless unit and it's been running for 4 years with no issues. Keeps the house at 68F-69F all year long, winter (with lows in the 10F range) or summer (pushing 95-100F). Zone 4B. With R-23 walls (ICF) and R-40 roof. Super air tight with < 0.6 [email protected]

    I would never do a ducted system again.

    1. Expert Member
      Josh Salinger | | #4

      It is true about the efficiency dropping with ducted/multi head units. That being said, it is a calculation to make and balance against performance and other issues. The energy savings on a ducted HP is still much greater than other options such as a gas furnace, etc. so it is a matter of perspective.

      One interesting thing to consider with separate heads is the amount of refrigerant and the impact it can have on the environment. See the attached presentation from PAE engineers where they discuss how multiple and longer line sets and the refrigerant they carry can have a huge impact over the course of the life of the HP, including the losses during install and the eventually end of its life. For this reason we have been spec'ing ducted units in all of our new home designs and builds.

      Just something interesting to consider...

  4. Andy CD Zone 5 - NW Ohio | | #5

    Two heads in a space with a 6kBTU/hr design load is just so .... wrong! And loud, and breezy, and unsightly. A 9 kbut/hr mini-ducted is such an elegant solution, and the duct losses are miniscule if it's mounted centrally. Can the hallway ceiling be bumped up 8" for an overhead installation, or mount it vertically behind a kneewall? You will never regret the effort (or expense, if you have to hire a creative handyman) but surely it's still cheaper than two heads.

  5. James Howison | | #6

    Issues to consider with ductless minisplits in bedrooms.

    Noise. Sure, when they run right they are fine. But get any dust on the wheels and all bets are off. Pump gets blocked. Clicking/knocking from refrigerant pipes. Start up and turn off noise changes (if you have the thermal off turned on).

    Fan speed and directing air. They are wide and can blow hard. So many hotel rooms have them mounted about the bed. When short cycling (because larger than load) they can be surprisingly cold. If you have the "thermal fan off" setting on then you have the air flow coming on and off which can also be distracting if it hits you.

    Thermal drift. The multi-headed units claim separate temperature control, but they cycling the refrigerant around. If you have a constant fan set and small loads in the room not calling then I've seen drift above/below set points like 5°F. Oh, and now you'll need remote thermostats because the internal ones report the temp of the fluid circulating, not the room. Of course the system doesn't record any of this, so you'll have to have a separate temp monitoring system to convince anyone that it's happening :)

    Cleaning. Ask about regular maintenance and costs. You might find that a cleaning is required (with a giant wet mess of water and chemicals at the unit, "managed" by a flimsy plastic bag that drains into a bucket). And that might cost $400 per unit every two years ...

    Ok, so slim ducted, right? Ok, but now with the "pizza box units" you have a unit designed to be installed in a commercial drop ceiling. This means that service is from below, which effectively means you have to be able to access below (like 50" x 36, so that's cutting out four joists) or be able to lift the unit up in the spot you put it (like 18", but 24" better), ability to disconnect the ducts to lift it and enough body space to put the body when sticking your head under it for servicing. They basically work in a dropped soffit configuration, like this (although you still have the machine in the room with you ...) https://pacificairconditioner.com/daikin_concealed_76street.JPG

    Oh, and they are thin (like 8" outlet), so regular filter housings don't fit them, requiring creative solutions (like adding a "bell" to come up to a duct large enough size to fit the filters). Or you can put filters in each and every return.

    Oh, you want it to go to two rooms? Now you need a "pair of pants" plenum in the supply. Check it out: https://www.energyvanguard.com/blog/my-duct-systems-low-static-mini-split-air-handlers/attachment/attachment-pair-of-pants-duct-tee/ That's a big pair of pants. Oh and supply is one side and return the other so any ducting will be in a H configuration (with the unit as the crossbar. Better have a lot of space for that! Something like https://www.achrnews.com/ext/resources/2017/05-2017/5-29-2017/Ducted-mini-split-2.jpg

    Like an attic space of 30 tall, by 60 wide (unit width plus access space next to it to get under it after lifting it, and something like 80" long (50" unit, plus supply and return plenum)...

    Ok, a little ranty, I know :) One vague possibility might be a vertical mount in the hallway (against a wall) with a shared return at ground level and a drop ceiling for high wall supply vents. Think a "T" with the cross bar of the T being supply to the two rooms, a return at the bottom of the downstroke and the unit sitting above that. Something like: https://uploads.disquscdn.com/images/2ffa6e108a7ded9f51130ff14126239b275b1244b7d53138beb63b4182d68f13.jpg which is documented here: https://terrylove.com/forums/index.php?threads/mini-split-three-options-and-a-lot-of-confusion.75513/ Apparently Fujitsu only? Oh, and don't forget the condensate drain and door undercuts for the return air, you didn't want noise privacy in your bedrooms too, did you? :)

  6. Expert Member
    Dana Dorsett | | #7

    >"One vague possibility might be a vertical mount in the hallway (against a wall) with a shared return at ground level and a drop ceiling for high wall supply vents"
    ----------
    >"Apparently Fujitsu only?"

    Carrier/Bryant (=Midea) slim-duct cassettes can also be mounted in an upflow configuration. The 3/4 tonner also has a minimum modulated output comparable to the Fujitsus (3160 BTU/hr vs. 3100 BTU/hr 47F):

    https://ashp.neep.org/#!/product/25310

    https://ashp.neep.org/#!/product/26241

    https://resource.carrierenterprise.com/is/content/Watscocom/carrier_40mbdq09---3_article_18203537400152152_en_ss?_ga=2.204891673.884008181.1620928420-303308979.1620158354

    The Carrier/Bryant/Midea 38MAQB09R--3 compressor unit also comes with a pan heater & controls for managing defrost water freezing up inside the pan, making it more suitable for cold climate operation than the Fujitsu competition. Fujitsu units have beefier blowers and can handle more static pressure, but in this application you're probably not looking at long & twisty duct paths. Either the 3/4 ton Midea or Fujitsu would be do-able without fat ducts in most heat-banger 1.5" story 2-3 bedroom configurations.

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