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Community and Q&A

Fresh air for minisplits

Norman Bunn | Posted in Mechanicals on

My HVAC contractor is planning to use Mitsubishi mini-splits, some ducted, some not. I am on board with this idea, but we are looking at fresh air alternatives. I was originally looking at the AirCycle g2, but it seems that the Mitsubishi wiring may be a problem, since it is not “standard”. Also, it seems the Mitsubishi fan runs continuously, albeit at a low level. Other than an ERV, which may be a more costly alternative in my climate zone, what are my alternatives for getting fresh air into the home?

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  1. Norman Bunn | | #1

    CZ 3A - Greenville SC

  2. GBA Editor
    Martin Holladay | | #2

    Start by reading this article: Designing a Good Ventilation System.

    You may be operating under a misapprehension -- namely, that it is a good idea for your ventilation system to use the same ducts as your heating and cooling system. In reality, these systems don't have anything to do with each other. It's usually a good idea to keep the systems separate.

    Your minisplits will provide heating and cooling. When it comes to providing ventilation, you have lots of options, including an exhaust-only system, Lunos fans, an HRV, or an ERV. All of these options are explained in the article I linked to.

  3. Expert Member
    Dana Dorsett | | #3

    It's almost always more effective and more efficient to separate the ventilation function from the heating & cooling function.

    When using mini-split the value of using an ERV rather than an HRV goes way down unless you're insisting on very high ventilation rates or very low humidity levels (under 40% RH year-round). Ductless ceramic core HRVs (Lunos, Twinfresh et al) are fine for most houses, but not always cheaper than a right sized central HRV/ERV. A lot depends on duct complexity, and the number of rooms that require active 24/7 ventilation.

    Exhaust-only ventilation isn't necessarily a good approach for tight new construction, but is usually going to be OK at air tightness of 5ACH/50 or higher (which is much leakier than IRC 2015 code-max). Depressuring a tight house is as likely to find all the leaks in foundation as anywhere else, and the soil gases reduce indoor air quality. See:

    The single-room ceramic core HRVs (eg Twinfresh) aren't balanced (like the Lunos), but they alternate between exhaust & intake flows, depressurizing then pressurizing the house. When pressurizing (the intake side of the cycle) the intake path is through the clean ceramic core, and it's exhausting relatively clean indoor air along the unknown air leak paths, cleaning it up a bit, reducing the contaminated path issue by a significant amount.

    1. Kent Thompson | | #8

      I believe the twinfreshes can be wired together if you have two and provide balanced ventilation.

  4. Jon R | | #4

    Building depressurization (below the neutral pressure level) occurs whether you have balanced or unbalanced-more-exhaust ventilation - less with the former, but either way it may or may not be a problem in terms of soil gases.

    Sub-slab depressurization (perhaps required anyway) can overcome any reasonable amount of home depressurization.

    I'm not promoting exhaust only ventilation in tight houses - I'm just making the point that depressurization can be a non-problem (and some is unavoidable).

  5. cmfischer | | #5

    Picking this thread back up, I'm in a related design situation and would love folks thoughts. Martin, I've read through the articles referenced above in your 2017 post.

    I'm building a tight, well insulated 6-plex in Minneapolis (Climate Zone 6). The units are small (~620 sf) 1-bed/1-bath. The building will have no gas, and based on my reading on GBA I intend to have 6 exterior mini split units, each connected to a single apartment unit (1 cassette / zone per unit).

    When going to bid with a pool of GC's, I had specified an HRV system for fresh air in the building...with the Lunos system as an alternate. We are now in the midst of value engineering and I'm meeting the harsh reality of my project budget.

    HVAC sub has suggested the following:
    - 2 speed bathroom fan for exhaust (continuous + surge when someone is showering)
    - 4 way ceiling cassette located at the nexus of the kitchen/laundry/bathroom/bedroom (see attached floor plan)
    - A continuous fan mounted above the door to the bedroom to circulate air if the bedroom door is closed
    - Fujitsu fresh air intake accessory with ducting to a hooded exterior vent for each unit

    The contractor said we need at least 35 CFM per unit (not sure where he calculated that) and the Lunos E2 only delivers up to 22 CFM (requiring two pair per unit).

    They are pushing hard for this proposed approach as a cost effective solution.

    Are there any concerns with this? Would a central HRV/ERV system be far superior and worth the added cost?

    Would love any thoughts or smart questions I can ask my build team to get to the a livable, cost-effective outcome for my future tenants!

    1. Expert Member
      Akos | | #6

      For similar size apartments I use a simgle wall mouny in the living space. The bedroom and bathroom need some small amount of heating which can be done with some electric baseboard/panel/floor heat.

      I have found the wall mounts tend to be more maintenace then expected (blower wheel cleaning, clogged drains) so I would go for something with a decent filter and condensate pump. Ducted units would be the best but floor consoles seem to be a much cheaper alternative.

      For ventilation, I like the Panasonic whispergreen ERV. Mot much more than a decent bath fan and more than enough flow for a small place. Make sure to get the tandom exterior hood for it as it makes install much easier.

      1. cmfischer | | #7

        It's my understanding that the Panasonic WhisperComfort ERV only functions as exhaust from November - March in my climate zone. Any suggestion for make-up air during those colder months?

        @Dana Dorssett -you had a nice post above on the risks of exhaust only. I'm shooting for 1ACH50, so am concerned about makeup air in those cold months.

        If I go the minisplit route - he has suggested a ceiling cassett with a fresh air accessory for makeup air. This seems less expensive than adding an HRV as well.

        Any concerns / issues seen in cold climates with this kind of accessory?

        If I go the PTAC route - those provide sufficient fresh air. So seems I would be fine to combine a PTAC with the WhisperComfort ERV. No?

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