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Minisplits, Mold, and Indoor Air Quality

marleyandbowie | Posted in General Questions on

I’m looking into ductless mini splits as part of a solution to heat and cool our 1900 sq ft midcentury home in southern NH. Indoor air quality is my highest priority, as I have struggled with various health problems over the years, but I am of course very keen on high efficiency as well. I consulted with one of the top mold inspectors in the country, and he told me to steer clear of mini splits because in his experience they frequently become infested with mold and are very cumbersome to clean. In general he advises against any type of air handler which cannot be used with at least MERV 8 level filters, and it seems like most mini splits just have a very thin and somewhat useless filter stage. Does anyone know if there is a brand/model that has better air filtration than the others? Are there any that are easier to clean than the others? How frequently would I need to clean them to keep them totally mold free? My concern is that I would have to clean them every few weeks to really protect myself against any mold/mildew that would start forming inside.

I’m really bummed because I had identified mini splits as an ideal solution to our heating/cooling challenges, but the mold specialist is highly recommending hydronic heat instead.

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  1. Jon_R | | #1

    AC equipment should periodically blow air without any cooling to dry the evaporator. This stops mold before it accumulates. Which is far better than cleaning it off later.

    In AC systems with ducts, moisture/mold on the outside can be prevented by insulation and/or mixing warm air with cooled air to keep the supply air above (or closer to) the dew point.

    Always supplement with a dehumidifier.

    Positive building pressure in Summer helps keep mold out of the walls.

    > mold specialist is highly recommending hydronic heat instead.

    What filters room air if you use hydronic heat? And what provides cooling? Only advantage I see is less air movement during the heating season (when mold typically isn't a problem).

    1. marleyandbowie | | #5

      Thanks Jon. You make a good point about cooling. In my consultation with him we didn't talk much about cooling because I told him my priority was heating (we live in NH at relatively high elevation), so I think he was acutely focused on the cleanest heating distribution. I'm thinking we'll do hydronic radiant, and then mini splits for the ~20 days a year we need AC. In fact, running the mini splits on dehumidifier mode might be enough to get our house to a comfortable temperature at the peak of summer?

  2. capecodhaus | | #2

    Mold spores exist in the air we breath normally inside and outside a house, not to mention other forms of pollution are present also. Mold can also be present in drinking water if you have a well vs municipal drinking water, and unless a person is highly sensitive it shouldn't be a problem.

    I think the main problem with moldy mini splits (yes, I have had a mini split become slightly moldy and need periodic maintenance) is that the residence lacks proper air and vapor control, which would help maintain PROPER HUMIDITY levels and reduce/eliminate the mold growth.

    A minisplit installed in an old, moldy house lacking air and vapor control layers is common because HVAC companies sell you solutions-new heating/cooling equipment-but don't bother with addressing the underlying issues. Go to the source of it and fix the problem.

  3. jberks | | #3

    consider a ducted miniplit with a custom filter box. then you can put whatever Merv filter you'd like in place.

    Wall mounted units are well and fine, but other systems need to be in place like ventilation, filtration, control layers and possibly dehumidification if you want optimal IAQ.

  4. marleyandbowie | | #4

    Thank you all for your responses. I talked to a local HVAC company that does a ton of mini splits and they said they had never even heard of mold becoming a problem in the air handlers. So I'm wondering if this is rare issue but perhaps the mold experts think it's common because any of their clients who have mini splits most likely have systemic mold problems in their house (hence their reason for consulting with the mold experts).

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