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Whole house dehumidifier – where to install?

Richard Marks | Posted in Mechanicals on

I am currently building a 2 story, 5400 sq ft home in Alabama where humidity is often high. We will install a whole house dehumidifier (“WHD”) as part of the HVAC system. We will have 3 HVAC units (2 and 3 ton down, 2 ton up with variable speed air handlers and 2 stage compressors). The installer is proposing to install one WHD (for entire house) on the larger of the downstairs unit which is also the unit that will control the kitchen and great room areas. The master bedroom/bath and another downstairs bedroom/bath will be on the other HVAC unit (no dehumidifier). No dehumidifier proposed for upstairs.

Question: Does it matter on which unit the WHD is attached to? Should it be upstairs or downstairs? Is one WHD sufficient to control the entire house? Does the humid air rise upstairs and into attic (spray foamed) and if so, does it need to be controlled?

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Richard,
    I think that it will be fine to install the dehumidifier on just one of your downstairs units.

    That said, I'm wondering if your AC equipment may be oversized. For a 5400 s.f. house, 7 tons of cooling sounds like overkill.

    -- Martin Holladay

  2. User avatar
    Dana Dorsett | | #2

    What he said: 7 tons of cooling would be extreme overkill for most 5400' houses.

    It sounds like too much equipment. When right-sized the run times and latent cooling capacity of variable speed systems would be high enough to not need a separate dehumidifier to manage the latent loads unless you're insisting on very high ventilation rates. If the cooling equipment is 2x oversized for the load (seems likely) even the low-stage output of the compressors is more than the 1% cooling load.

    Take a look at Allison Bailes' square feet per ton plot derived from Manual-Js on real houses (mostly in the Gulf Coast states), of square feet per ton plotted against house size:

    https://www.greenbuildingadvisor.com/sites/default/files/images/Bailes%20graph%20for%20Manual%20J%20blog.preview.png

    Even the worst-case of the five houses in the ~4000-6000' size range is over 1000' per ton, which means your 5400' house would likely to be under 5.4 tons. The average house in that size range came in at 1500' per ton, which would put you at ~3.5 tons, and one 5500' house that came in at ~2000'/ton or ~2.7 tons. Do you really thing you need 7 tons of cooling, in brand new foam insulated house?

    An aggressive Manual-J load calculation performed by an engineer or RESNET rater (and specifically NOT an HVAC contractor) would be in order, and have them be specific about ventilation rates (or use both an ASHRAE 62.2 & a low ventilation rate number for adjusting the load range), which will be the primary driver of whether additional dehumidification is necessary. In a tight house it's really going to be the ventilation rates that drive the latent load numbers.

  3. Richard Marks | | #3

    I will have an encapsulated crawl space with some supply and return (passive return presumably). No sure how much that adds to the load since not putting too much in there.

    HVAC said attic did not need and code didn't require a supply or return in conditioned attic.

  4. Anon3 | | #4

    If you can wait, the new Midea mini split coming out can act as a gigantic split dehumidifier.

  5. User avatar
    John Semmelhack | | #5

    Typically, a conditioned crawlspace will add next to nothing to the cooling load.

    Regarding not directly conditioning the "conditioned" attic - I'd suggest some further reading:

    https://www.greenbuildingadvisor.com/articles/dept/musings/high-humidity-unvented-conditioned-attics

    https://buildingscience.com/documents/building-science-insights/bsi-016-ping-pong-water-and-chemical-engineer

  6. User avatar
    Dana Dorsett | | #6

    If the attic is insulated with open cell foam WITHOUT a class-II interior side vapor retarder (half-perm paint on gypsum, or 2-mil nylon sheeting) there is a real risk of high daytime attic humidity during the cooling season. It may not be required by code, but a modest amount of air conditioning does a lot for mitigating that risk. The moisture that gets baked out of the roof deck and foam during the day get's moved along, replaced by much drier air, leaving much less moisture in the attic to be re-adsorbed by the roof deck & foam when the roof cools off at night.

    The additional cooling load of a foamed-roof attic that meets code minimum (or better) for R-value is small, as is the additional cooling load of an insulated encapsulate crawlspace. But a modest amount of air exchange with the fully conditioned air keeps the humidity in those locations bounded.

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