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Building Science

Making Sense of Minisplits

Find out if this type of energy-efficient electric heating and cooling system is right for your home

Editor’s Note: This article first appeared in the Dec 2020/Jan 2021 issue of Fine Homebuilding magazine. Given the topic’s popularity and the author’s in-depth coverage, we felt it made sense to share it with GBA members too. 

Synopsis: Minisplits offer an all-electric heating and cooling solution with a range of design options. HVAC designer Jordan Goldman explains how a minisplit system works and the various options for single-zone and multizone systems, how to size a minisplit system and the hazards of doing so incorrectly, and other considerations when deciding on or installing a minisplit system.

Versatile, scalable, and all-electric, minisplit heat pumps are an increasingly attractive HVAC solution. Originally developed in Asia, where small homes are more common, minisplits made their first splash in North America in the form of single-point, wall-mounted units, offering a quiet, cost-effective way to condition a single room.

While these ductless, single-zone systems are still big sellers, their surge in popularity has ushered in an influx of new products and variations, including ducted systems that stay hidden from sight and multizone minisplits that serve an entire house.

For well-insulated houses, minisplits are beginning to dominate the market. They are especially suited for lower heating and cooling loads and providing quiet comfort with surprisingly low amounts of energy—energy that can be produced on-site or pulled from renewable sources on the grid.

For renovations and additions, today’s single-zone minisplits offer a much wider variety of configurations, including ducted versions that serve multiple rooms. Extending the reach of this exciting technology, new low-temperature units can handle all but the most frigid weather zones.

Like any building component, minisplits aren’t right for every project, and there are critical design factors to consider. While a short magazine article can’t replace a full system design by an…

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15 Comments

  1. Joe Braun | | #1

    Jordan - Thank you for taking the time to write this article. I think it will help clarify many things for a lot of people.

    I am planning on using Fujitsu XLTH version of their mini-split. I will be using three units instead of a multi-zoned one to get the best efficiency out of them and to have control over each zone as needed. Two of them will be ducted into bedrooms/bathrooms.

    My home will be in zone 5B, at 8400ft. We can have three to five feet of snow on the ground at any given time in the winter. I plan on placing all the units on the SouthEast side of the building, elevated, to get as much heat in the winter as I can.

    One question I have searched and could not find an answer to was can I put these units in some type of enclosed shed with roof vents and lower vents, maybe 4ft off the ground? I feel like it would keep the temperature higher in the shed and lessen the likelihood of the blowing snow buildup. Even if it did not keep the temperature higher, it would at least keep the snow (and wind) from the units. I could even install fans to help circulate the air if needed.

    The only things I have found online are elevated platforms (three or four feet up), attachments to the house way high up, or both of these with a small roofs that cover just the top of the unit(s).

    1. GBA Editor
      Martin Holladay | | #3

      Joe,
      Remember that in winter, your outdoor unit is lowering the temperature of the outdoor air. The exhaust air is very, very cold. So if you build a shed around the outdoor unit, you probably won't be "keeping the temperature higher in the shed" --this approach will be keeping the temperature of the shed lower than the temperature of the outdoor air, potentially hurting the unit's efficiency.

      1. Joe Braun | | #7

        Martin,

        Thanks for the reply. I knew in the back of my mind that this was the case. I was hoping you (or someone) would chime in and tell me I got it all wrong and what I need to do is lay down a super-insulated slab and North/West insulated concrete walls with black paint, with greenhouse glass on the South/East walls and louvers to control airflow (I'm making this up as I type); to create a solar slab.

        Thanks for all your help.

  2. proposterous | | #2

    Thanks for the article. Can you direct me to any info or analysis on the life cycle risks of current refrigerants regarding their high GWP? The understandable rush to embrace heat pumps seems to leave this issue largely undiscussed. I don't know how to evaluate it in terms of choosing a system for a retrofit. It seems to me that refrigerant leakage from a split system, with more connections and long line runs, would be a higher risk over the life of the device, compared to packaged units such as Chiltrix or Daikin Altherma monobloc. Also, do minisplits generally require a higher quantity refrigerant charge? Have there been any studies of how much leakage various minisplits actually experience over time? How much does it matter in the larger picture?

    Related to this, are minisplits more or less likely to be amenable to refrigerant upgrades if/when R410a is phased out, compared to packaged heat pumps?

    1. Expert Member
      Dana Dorsett | | #5

      >"Can you direct me to any info or analysis on the life cycle risks of current refrigerants regarding their high GWP?"

      In rough figures there are 2-4lbs of refrigerant per ton of compressor in a typical system. (source:
      http://www.charlottehvacguide.com/guides/how-many-pounds-of-refrigerant-does-an-ac-or-heat-pump-need/ ) R410A has a 100 year GWP of about 2100 lbs of of CO2, call it a ton per lb. (sourcer: https://www.linde-gas.com/en/products_and_supply/refrigerants/hfc_refrigerants/r410a/index.html ) That means a 3 ton multi-split system with 10 lbs of refrigerant (3.3lbs per ton) is worth about 10 tons of CO2, and that's only if it all leaks out. That happens, but it isn't predestined- it is not what happens in most installations.

      Burning natural gas releases 117lbs per million BTU(MMBTU) or 11.7 lbs per therm. (source: https://www.eia.gov/environment/emissions/co2_vol_mass.php )A pretty efficient house in New England would use that use 1000 therms per year is then releasing about 6 tons of CO2 every year, 60 tons per decade. Assuming a 20 year lifecycle that's well over the 10x the greenhouse gas emissions of a full release of refrigerant from a 3 ton heat pump using R410A.

      Clearly the CO2 footprint per kwh of the power sources will tip the analysis one way or the other, but over the lifecycle of the equipment the marginal use CO2 footprint of the heat pump is trending down, whereas a gas burner simply is what it is. The fugitive methane releases of the gas grid and gas sources also vary, but would need stricter regulation to lower those numbers.

      The paths to essentially carbon free electricity are pretty well mapped out, but will take sufficient political will to compensate owners of fossil burning generators for stranding existing assets with potential remaining lifecycle left by policies designed to accelerate a carbon-free grid. Without some sort of buy-out of the stranded assets it ends up being a fight.

      Before the Clean Power Plan got axed by the current administration the US EPA had a state by state estimate of carbon emissions per mwh online. Some napkin-math analysis of the marginalp-use carbon footprint from mini-splits using that data can be found in this bit o' bloggery from 2016:

      https://www.greenbuildingadvisor.com/article/the-carbon-footprint-of-minisplits

  3. Expert Member
    Dana Dorsett | | #4

    >"Using electricity also reduces the demand on natural-gas infrastructure, which is aged and leaky in many parts of the country—particularly in New England where I work."

    Actually that's not the case that using electricity for heating "...reduces the demand on natural-gas infrastructure..." in New England (and especially not on Massachusetts)where half the annual power is generated with natural gas, and an inordinate amount of marginal power during peak load periods is generated with gas. The effect is quite the opposite is true- heating with air source heat pumps INCREASES the load on the gas grid., especially during peak heating load periods, when the COP of air source heat pumps are under 2.

    During the heating season electric generation competes head to head with space heating for gas that has limited pipeline capacity. During cold snaps the spot price of natural gas on the New England gas grid soars to incredible levels, in no small part due to the increased demand from power generators during those cold snaps. The Polar Vortex disturbance event in 2014 brought that into stark relief, with MAJOR electricity rate hikes in the following years. One (of many) articles from that period:

    https://www.csmonitor.com/Environment/Energy-Voices/2014/0108/Polar-vortex-Stressed-about-the-cold-So-is-the-power-grid

    A combined cycle gas generator runs at about 50% thermal efficiency, and with transmission losses a COP of 2 has a net thermal efficiency from the marginal gas use lower than that of a condensing gas furnace. So when it's down in negative digits even the most optimized mini-split is using more peak gas than a home heated with condensing boiler or furnace.

    There will come a day (hopefully sooner than later) that offshore wind will be supplying a large fraction of the power grid in New England, and the constrained gas grid is almost never running near capacity even during cold snaps, but that day is clearly not today. Cold snaps in New England are highly correlated with higher wind speeds (especially in the early part of the cold snaps), which makes wind power a natural fit for covering electric heating loads, but the correlation is not perfect. For now natural gas is still picking most of the marginal load, just as it was in 2014.

  4. James Howison | | #6

    One additional thing to consider is the noise of the defrost cycle switch, both externally (at the compressors) and internally (at the heads). There can be substantial noise as the pressure is released as the system reverses temporarily. The specifications of the units do not describe those noise levels. If I understand correctly that's because the specifications are given according to an ASHRAE protocol? And that protocol takes no account of "irregular" noises? So actual noise can spike much higher than the claimed numbers. Similar issues exist with condensate pumps (as I understand it, haven't experienced that myself).

    1. Barrett Bullock | | #10

      I have no experience with the wall mounted units, but we went for Fujitsu's slim duct version with a couple of short duct runs and I've never heard the defrost cycle running. I really only notice that it's defrosting unless I happen to walk past the thermostat and see the defrost symbol on the screen. It's also completely silent in all other modes, which to me is a plus in favor of the short ducted models (where feasible).

  5. Yeldog | | #8

    I would advise not wall mounting the units unless small -- 6/9/12 ... I did a 30k multi and I can hear it. Most people don't .. but, I can and the 3T carrier on the ground near it is silent. They make more noise in the winter.

    When doing a multi head -- sizing the compressor is very important. So is understanding that the heads don't perform the same as they do when on a single setup. Too often installers just add up the heads and match the compressor ... this is not how you do it. Important to read the tables and see how the heads perform in a given configuration. Example -- My new house needs a three heads. One of the heads is a little used loft area ... it will always be off. Since multi head units always cycle some refrigerant ... that off zone is throwing off the calculations. Same with reserve capacity -- need a mini for the kitchen ... this is one area where you need some reserve. If you look at the Mitsubishi tables for a 15k connected to a multi head -- it will not have the reserve of a single unit with matching compressor. Often people assume the multi have more reserve capacity ... they tables say otherwise

  6. Vince Caruso | | #9

    Another benefit of newer mini-splits with "Dry Mode" feature is a very efficient dehumidify and cooling of a home with good energy conservation design (shading, insulation, overhangs, ...).
    The dehumidify does do some cooling but really is designed to use low energy and does a lot for less energy. If you have about 40% RH and a temp in the low 80F's most people will be very happy with the comfort and low electric bill. Some have shown about $60 bill for the summer in the midwest Zone4. And when you add solar panels and/or windmills it will cover most or all of the electricity needed.

  7. Stephen Rose | | #11

    I have been told that oversizing is not as much of a problem anymore because of variable output on newer heat pumps. I would appreciate hearing expert knowledge on this.

    1. Alex D | | #12

      Stephen, oversizing is still a problem as all units have a minimum they can modulate down to. Additionally at the bottom of their range mini splits tend to be a bit less efficient than at other modulation levels.
      Typically the bigger the unit the higher their minimum BTU is, so it is still very important to pick a correctly sized unit to ensure good efficiency.

    2. Yeldog | | #13

      This seems to be a common installer comment .... Why would you want to oversize?

      Doing a proper load calculation will give you the answers .. room by room and total load on the building.

      There are calculations for spaces were you may have occasional excess loads -- like a kitchen. Or a large living room that may hold many people for a party. You can factor for that -- look at the performance tables to see what unit fits that load the best.

  8. Jibu J | | #14

    Does anyone have insights into what is the cost delta between a zoned multi-split and 4 individual units for a 4 zoned mini-split system? Every time I ask a vendor I get looked at like I am wasting their time...

  9. Jibu J | | #15

    I am doing a multistage remodel on my 1961 built ranch home located in Northern CA (SF Bay Area) to make it "better". The goal is to add exterior insulation, add new in-wall insulation, and replace windows as I work my way around the home over the next few years. In the end, redo attic insulation; per discussions I have had, it will be tough to shift insulation plain to the roofline with exterior insulation and such due to the nature of existing framing.

    For space conditioning, I wanted to go to a heat pump system; Initially, the plan was to do it now since my existing traditional forced air system keeps breaking down. Should I wait to replace my HVAC system so they can size it to my final insulated condition or now?

    I had a couple of vendors come through and all said I would need to size a large multi-split system for my home's current condition and so concerned about losing efficiencies due to the system being too large for the ideal future state.

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