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Minisplit sizing

Josh_Sq | Posted in General Questions on

We have been building a house for the past year in Vermont, zip 05757.  The original plan for heating the house was using our wood boiler that we will use to heat our greenhouse as our main heat source. We guessed one ore two radiators would be enough. The greenhouse is used for our farm business and the boiler has a BTU rating that can run between 30 and 80kBtu.

We are off grid so we thought that we would be more limited on power during the winter.  It turns out we have a very good microhydro numbers during the heating months.  
This means we could potentially run a mini split as either back up heat or  main heat if we leave for a long period of time.
We just got a blower test done today.  It came out as .808ACH50. I ran the house through HeatCad 2019 and came up with a heat load of 18,499 Btu/hr. I’m no expert at modeling but I think I have done a half decent job modeling the house.

The building is 24×40 with a walk out with the 40′ being one of the fully exposed sides.  And is true rough cut 2×6 with dense pack cellulose that will have 2.5″ of polyiso on the exterior.  The basement wall is AMVIC ICF which they rate at 30R. 
The walk out level is where the two main bedrooms and family room are.  
The main floor is 8.5′ ceilings with true 6″ of dense pack cellulose with 2.5″ exterior polyiso.   It is mostly open with only two rooms a bathroom and an office/bedroom.
The attic above it is vented through the soffits and peak with 18+”  of loose fill cellulose. 

I was thinking we could get away with a  head unit in the center of walkout level near the open stairwell.   We do have a bit to much south glazing with not so great windows.  

We would also like the option of having AC for those heat waves we keep getting once or twice a summer.  

Any recommendations would be great.

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  1. Expert Member
    Dana Dorsett | | #1

    A sketch of the floor plan might be useful here, marked with the room by room heat load numbers.

  2. Josh_Sq | | #2

    Here is the drawing from HeatCad

  3. Expert Member
    Dana Dorsett | | #3

    A 1.5 ton Fujitsu 18RLFCC ceiling cassette married to an AOU18RLFC will put out more than 19,000BTU/hr @ -5F.

    Since it blows in 4 directions putting it where it would get at least some direct flow toward the door of the master bedroom would be useful. If it's an open stairway with 1/4 the flow directed at the stairway convection would take care of the rest.

    It doesn't have pan heater to automatically clear defrost ice build up during extended cold weather, so you'd have to check up on it and manually clear the drain & pan if that becomes an issue. (It usually doesn't, but if ice builds up enough to interfere with the blower the unit can be damaged.)

    The 1.5 tonner draws 2600 watts at max in heating mode. Do you have that much power?

    If not, the 1 ton version delivers about 15,000 BTU/hr @ -5F, and only draws 1670 watts max in heating mode:


    A down side to these ceiling units is the amount of joist framing modification it takes to get them in. but you'll get better heat distribution with these than with a high-wall coil in your situation.

  4. Josh_Sq | | #4

    Thanks Dana for the reply. Our estimate is a bit wide for energy production cause it depends a bit on the weather. We will likely be producing between 30 and 45kWh a day during the main heating season and into the spring. During the winter I still expect us to use the boiler most of the time with the heat pump on backup duty for a bit of additional heat. During the shoulder seasons is when we would be more likely to use the mini split. If we went away during the winter we would most likely set it around 50F to keep the house from freezing.

    Is there a good way to estimate how long the unit would run on max heat in a given hour?

    The ceiling section you are describing is actual 2x10's 24OC so it looks like these wouldn't be as much of a pain

  5. Expert Member
    Dana Dorsett | | #5

    >"Is there a good way to estimate how long the unit would run on max heat in a given hour?"

    If you know the outdoor temperature and the houses heat load at that temperature it can be estimated. Since they modulate it won't run at maximum speed unless/until the room temperature is a few degrees below the programmed set point.

    The normal minimum setpoints in heating mode for most mini-splits is only about 60F. But with both Fujitsu and Mitsubishi mini-splits there are special modes where they can be set as low as 50F. With Fujitsu it's called "MINIMUM HEAT". With Mitsubishi it has to be in "SMART SET" mode to go lower than 60F.

    The with full dimension 2x10s 24" on center you have 22" nominal space between them. Most of the square ceiling cassettes need at least 22.5" to fit them without resorting to a big sledge hammer, so there would need to be some adjustment to the framing. The Fujitsus are also 9-5/8" from the top side to the bottom of the cassette, which can be a problem for milled 2x10s, but you might just make it with full dimension 2x10s.

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