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Q&A Spotlight

Do Ductless Minisplits Work With Every Floor Plan?

Or do these air-source heat pumps only make sense for open floor plans with few rooms that are closed off from the rest of the house?

Will a ductless minisplit work here? This is the first-floor plan for a house Clay Whitenack is planning to build. He wonders whether the design is suitable for a ductless minisplit heating and cooling system.

Clay Whitenack, planning a new home in central Kentucky, had assumed that a ground-source heat pump would be a “no-brainer” for heating and cooling. Then he began reading about minisplit air-source heat pumps, and suddenly the situation didn’t seem so simple.

He’s intrigued with the possibilities for minisplits, but he’s not certain he’ll have a floor plan that would be compatible with this type of system, he writes in Q&A post at Green Building Advisor.

“My question is this: Aside from getting your house as well insulated and tight as possible, do you need a certain floor plan or construction feature to make minisplits work?” he asks. “Does the foor plan need to be open without many walls and closed rooms? Will this only work with smaller square footages? In other words, can I take a traditional, 2-story, ‘normal’ house plan (3,000 to 4,000 square feet), wrap it up with R-30 walls and R-60 attic, and get good results with some minisplit units, or do I need to build a house with a specific floor plan?”

Further, Whitenack wonders whether he needs to build in a contingency plan in the event a minisplit system would leave some rooms too hot and humid in the summer or too cold in the winter. “I would be a major pain at that point to try and go back and install ductwork,” he says.

Whitenack’s questions are the topic for this Q&A Spotlight.

First things first: Good planning

It will be impossible to design an efficient heating and cooling system without performing room-by-room heating and cooling load calculations, GBA senior editor Martin Holladay tells him, and those calculations won’t be possible unless you have a floor plan.

“Any heating and cooling system needs to be designed,”…

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12 Comments

  1. Steve Knapp CZ 3A Georgia | | #1

    Always run the calculations
    I really wanted to use Daikin split minis in new house. The floor plan is fairly open, and I thought a good system would work. But when my HVAC designer ran the calculations, it did not look promising. He also looked at adding some ducting to improve air movement, but then the system's efficiency when way down. In the end, we decided to go with a conventional 17 SEER air-source heat pump.

  2. Erich Riesenberg | | #2

    venting between rooms
    I am insulating and currently plan to ventilate between rooms. Perhaps installing vents with small fans at the top of the walls between rooms.

    Replacing duct vents at the floor with vents at the ceiling. Just an idea at this point.

  3. Mark Heizer | | #3

    Multi-head mix of exposed/hidden
    Clay, I grew up in central KY. You could probably survive with a PGH using single heads up and down if you can get some sharing/distribution done. For comfort, (resale value), and the advantages of zone control you can't get with other systems, why not go with 2 multi-head systems. Probably overkill for capacity (in summer), but zonal control returns value. Downstairs with a ducted system for the central living areas and an exposed unit in the office (you could hide it if you have the $$). A ducted unit optimally located in a conditioned crawl should be able to move the air and be unobtrusive to guests. Upstairs: exposed heads in small bedrooms, ducted unit above the master closet = short run, air movement in all spaces if return is in closet. flexible control and future owners wouldn't be turned off by the system.
    Electric floor heat in the baths (you won't regret it).

  4. Expert Member
    Kohta Ueno | | #4

    BA-1407: Long-Term Monitoring of Mini-Split Ductless Heat Pumps
    BSC's research on heating and cooling using mini splits in a Northeast (Zone 5A) climate are covered in this report:

    BA-1407: Long-Term Monitoring of Mini-Split Ductless Heat Pumps in the Northeast
    http://buildingscience.com/documents/bareports/ba-1407-long-term-monitoring-mini-splits-northeast/view

    Probably easiest to look through the Executive Summary and/or Conclusions to get a "lay of the land" for what was covered in this report first. But we definitely agree with all of those points above, that closed-door operation can be a real killer for even temperatures in rooms. Using a distribution fan (e.g., Tjernlund AirShare) can help a little in edge cases, but overall, trying to heat a 65 degree room with 68 degree air requires a whole lot more airflow than those fans can provide.

    Check out sections 6.2 through 6.6 for discussions on getting even temperature distributions. Agree with the comment above on needing one on each floor. Beware rooms with exceptional loads that are different from the rest of the building--e.g., highly glazed rooms, or a bonus room over a garage.

    Also, our anecdotal conclusions (see 5.3) was that we had comfort problems when we were pushing over 1100 sf/MSHP head. Not saying that it's any type of physics rule, but that's what we experienced.

    Overall, using a mix of wall-mounted MSHP heads (e.g., a big open part of the first floor) and recessed ceiling-mounted air handlers feeding multiple rooms (e.g., hallway to bedrooms) is a pretty decent strategy. However, going to a ducted system drives up installed costs considerably.

  5. GBA Editor
    Martin Holladay | | #5

    Response to Kohta Ueno
    Kohta,
    Thanks for your useful comments. I'd like to reinforce one thing you wrote: "Using a distribution fan (e.g., Tjernlund AirShare) can help a little in edge cases, but overall, trying to heat a 65 degree room with 68 degree air requires a whole lot more airflow than those fans can provide." Many designers don't understand this.

  6. Stephen Colley | | #6

    How About in Hot Humid Climate Zones
    I live in climate zone 2A where the winters last for three weeks (maybe). Are there some additional or different planning considerations when considering mini-splits in our area where we depend highly on air conditioning?

  7. Nathan Rainbolt | | #7

    Insulation and thermal mass
    Great article. I am a little confused as to why you wouldn't recommend the possibility of an ICF build for this homeowner. You could pour an ICF foundation with a brick ledge form in it and also pour the ICF on the above grade exterior walls and even gable ends. At that point you would have a complete thermal envelope and nearly every issue you have regarding this builds insulation qualities would be resolved. Just a thought. If you want more info on ICF shoot me an email. I live in a total ICF house and have built several for clients in the southern Indiana area and they are amazingly efficient. Your biggest issue at this point is simply air quality and dehumidifying which and easy to handle.

  8. James Morgan | | #8

    You're kidding, right?
    It should be a no-brainer that extremely cellular floor plans like this in a cold/mixed climate are implicitly unsuitable for ductless units. Even ducted mini splits should only be employed in a floor plan that is designed around their optimal operating conditions, which this home clearly wasn't. Given the circumstances Mr. Knapp probably made the right decision on a conventional system. An efficient gas room heater to add radiant to the main living area might be a good supplement so that the central system setpoint could be kept lower in cold weather, but if natural gas is not available and it was necessary to run it on propane the cost benefits would probably be minimal.

    On a general space planning note, the master bath is curiously laid out and the WC compartment being so small its door should probably open out. I was told by an EMS that if anyone collapses in such a tight space (it happens, people often head for the toilet if they're feeling bad) it's a nightmare to get them out if they've fallen against the door. Also, there looks to be a laundry room on the second floor without any way to get into it. I assume this is an oversight that will be addressed in the actual build.

  9. GBA Editor
    Martin Holladay | | #9

    Response to Stephen Colley
    Stephen,
    Q. "I live in climate zone 2A where the winters last for three weeks (maybe). Are there some additional or different planning considerations when considering mini-splits in our area where we depend highly on air conditioning?"

    A. The same considerations apply: (1) Start with a room-by-room cooling load calculation; and (2) Be aware that rooms with unusual load considerations (bonus rooms over a garage, bump-out rooms, or rooms with big unshaded west-facing windows) need their own zone.

  10. Clay Whitenack | | #10

    Thanks for the reply, James.
    Thanks for the reply, James. You are right, the bathroom was funky. We have made some design tweaks since then and I think it flows better now. I have attached the new design for whoever is interested. Also, you are right, the door is missing on the laundry room. We are planning a pocket door there.

    Also, thanks for the dissenting opinion on the mini-splits. It is always good to hear varying opinions.

  11. Clay Whitenack | | #11

    Update to the plans
    Hi all,

    I thought I would stop in and post an update. We have made a few tweaks to the floor plan, and I have a Manual J calculation from our local electric co-op. I'm still going to have an HVAC pro do one, but this give me a good start.

    The heating a cooling loads were a little higher than I had hoped, and the co-op suggested a 2.5 ton heat pump for the first floor and a 1.5 ton heat pump for the 2nd floor. This puts it right in line with the "1 ton for every 1000 square feet" rule of thumb for pretty good houses, so I guess it is a good compromise between efficiency and floor plan. The 1st floor is bigger than the 2nd, and probably too sprawling for a minisplit, but I wonder if the 2nd floor could still get by with a ducted mini split.

    The 2nd floor has a heating load of 15k and a cooling load of 12.8k. It seems like that would be within the capability of a minisplit?

    See below for the updated floor plan.

    1. aditu40 | | #12

      Hello. It has nothing to do with the subject. I am interested in what program you made the sketches.
      Thanks for understanding.

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