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Community and Q&A

Best HVAC system for a small inlaw addition?

Daniel McCauley | Posted in Mechanicals on

We are in the process of working out a design with a client for a small ~ 600 sq ft inlaw addition. We plan on R-24 walls and R-40 roof insulation. For various reasons we want to use a separate HVAC system, but it’s such a small space. We’ve been looking at mini-splits with two interior units fed off of one exterior (bedroom and living/dining/kitchen) but it seems the smallest such unit is rated for 1200 sq ft. My mechanical installer tells me that the units are variable speed and will compensate for the smaller space. Is he right? Is it overkill? It’s a tight budget but we are interested in maximizing efficiencies wherever we can. Has anyone come up with a decent solution for similarly small spaces?

BTW we are in the Mid-Atlantic and so need both good heating and good cooling capacity

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  1. Nathan Spriegel | | #1

    Assuming the multi-zone systems are too large why not go to two smaller single-zone minisplit systems? What are the actual calculated heating/cooling loads?

  2. Keith Gustafson | | #2

    I just happened to notice when I was looking for a cheap mini split for work that Mitsubishi now makes a 6k btu head for the multi unit compressors:


    It would appear that the smallest compressor is 20k however.

    I was going to bring it up anyway since the smallest ones they used to make [9k] really blow out a smaller room.

    Would get a lot of business in the super insulated world if they made a 1 ton multi compressor
    I still think you can get away with one for the whole place..........

  3. John Semmelhack | | #3

    Depending on the layout, solar gains, air-tightness, insulation and ventilation details, there's a good chance that you can heat and cool the space with a single 9k or 12k unit (1 outdoor - 1 indoor). Has the HVAC contractor done an accurate Manual J heating/cooling load calculation?

    I heat and cool an entire 1,450ft2 two-story, 3-bedroom house in central VA with a single 12k unit.

  4. Keith Gustafson | | #4

    As I keep looking:

    Not as efficient as I would like, but allows for ducts. There appear to be multiple brands selling off of the same picture. One might hope they will be coming out with the state of the art 26 SEER units at some point

  5. John Brooks | | #5

    Hi Keith,
    For those interested in HVAC matters.
    There are lots of good discussions at the EnergyVanguard blog.
    I think "ducted minisplits" are going to be a good solution for Not-so-Big High Performance Southern Homes.
    see David Butler's comments:
    I am posting a snip-it from a drawing that David Butler sent me for a Passivhaus project.

    Thank you David Butler.

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