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Plan for a single minisplit head

Nathan Scaglione | Posted in General Questions on

We are in zone 6a.

Slab on grade, R20 under the slab, R23+R17 walls, R60 attic. 1400 square foot, our heat load is in the range of 17k BTU +/- a couple thousand at 0F. House is a rectangular footprint, downstairs is completely open with a central wide open U shaped stair, upstairs is 3 bedroom and a full bath.

I am picking the unit up in early Nov. Looking at the air flow diagrams in the engineering manual, it looks like I can blow warm air to the downstairs in the winter, and cool air upstairs in the summer. Or something like that.

Just wondering if anyone has tried this or has any thoughts on how effective it will be. I know that one head is not as good as two, but that’s not happening. It really needs to keep the house above freezing while no one is home in the winter, but most of the service will actually be shoulder season heat, heating on sunny winter days, dehumidification and some cooling in the summer.

I am thinking the worst case is I would need to install a separate thermostat further away from the unit if short cycles. I know for a fact if I install it downstairs I will not be able to get cool air up stairs in the summer. My fear would be that I can’t properly shoot the warm air downstairs in winter.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Nathan,
    Lots of people have tried it. For heating, this approach works great. For cooling, it simply doesn't work. Your second floor will be hot.

    That's not a deal-killer, of course, as long as you anticipate it. People have been sleeping in warm bedrooms for hundreds of years.

  2. kjmass1 | | #2

    I have one head per floor, and in the summer the second floor unit runs 95% of the time and I only turn the first floor one on during heat waves. In winter, the second floor units run probably 1/2 the time of the first floor one. My house is certainly not as tight as yours, but I would probably put a unit on each floor vs an 18k on the second floor, especially in a heating dominated climate.

  3. Nathan Scaglione | | #3

    Thanks for the reply Martin.

    I am a space cadet. I spent so much time trying to describe the house I forgot to describe where I'm planning to put the minisplit head. I want to install it above the mid-way landing in the U shaped stair. This would put it roughly 4' above the second floor. I'm attaching a picture to show where I plan on putting it.

    According to the engineering docs, with the vents facing down I should be able to shoot warm air downstairs, and in the with the vents up, I should be able to keep cool air upstairs.

    I learned this past summer that a 5k BTU AC unit could keep the house dehumidified and around 76-78F as long as the unit was upstairs.

    1. Expert Member
      Akos | | #4

      Nathan,

      I've mounted a single mini split (cooling only) in similar location for a 1 bedroom two story unit (bed bath upstairs, open concept kitchen/living downstairs). The unit is mounted near the top of the stair, so close to the 2nd floor ceiling.

      I can't speak for heat, but for cooling it works great as long as you keep the bedroom door open. With the door closed, the bedroom gets uncomfortable after a couple of hours.

      My $0.02 If you want comfort with multi bedroom setup and two stories, bite the bullet run the ducts and get a ducted unit. The one problem with a well insulated house is that having two people in a room adds enough heat to cause overheating even during the shoulder season without some form of air circulation/cooling.

      1. Nathan Scaglione | | #5

        Thanks Akos.

        Posting this got me thinking, instead of theorizing I could just test it.

        I put a space heater on the window sill upstairs to see how it heats the house. The head will be a couple feet higher, hopefully that doesn't effect things too much.

        1. Jon R | | #6

          Just keep in mind that airflow velocity from a heating device has a big effect on mixing and stratification (what you are concerned about).

          On a speculative note, it would be interesting to know if one could design a home with a ceiling fan (plenty of efficient CFM for the btu involved) blowing through a second opening between floors to get a circular flow going (without noticeable draft to occupants).

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