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Dehumidification duct system in a tight house

sadrian | Posted in GBA Pro Help on
I am trying to find a robust solution to ventilating and dehumidifying a tight house. I have mostly decided to have a separate ventilation duct system that uses small ducts (3″ or 4″) and has in and out registers in every room. It would be driven by something like a Zehnder HRV. This would likely provide adequate dehumidification in the winter (Zone 4). Something else is needed, however, for hot, humid summers. 
 
At first glance, it would seem to make sense to put dehumidification inline with the ventilation. For reasons I haven’t quite grasped, this is apparently harder than it seems and leads to complicated solutions like “magic boxes” that can heat, cool, dehumidify and ventilate through one duct system.
 
I came across another possible solution in an article on dehumidification in Fine Homebuilding magazine. It mentioned, almost in passing, that the best control of HVAC functions would have each function controlled in a separate duct system. I do not want large heating and cooling ducts (probably will use ductless mini-splits instead), but I did start thinking about a separate dehumidification duct system in addition to the ventilation duct system. The dehumidification duct system would have input and output registers in each room and would just recirculate air through a ducted whole house ventilating dehumidifier to extract the humidity brought in through the ventilation duct system in the summer.
 
I am hesitant to rely on mini-splits to perform the dehumidification, because it might be necessary to over-cool the house to keep the humidity down, and I also wonder if there might be humid pockets in the house since humid ventilation air will be delivered to every room.
 
My first question is whether a separate dehumidification duct system is analogous to cracking walnuts with a sledge hammer, and have I just missed the simple solution?
 
I should add that I have a family member who has debilitating reactions to mold in water damaged dwellings so robustness and humidity control are more important than expense.
 
The ventilating dehumidifiers I have looked at are designed to be connected into large heating/cooling ducts. So my second question is are there any products available that could provide a ducted dehumidification system? I would assume that the necessary ducts would be the same size (or slightly larger) than the ventilation ducts, but this might be wrong.

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Replies

  1. Expert Member
    AKOS TOTH | | #1

    Since you are in warmer climate and are looking at mini splits, I would look at the Daikin Qauternity unit:

    https://ashp.neep.org/#!/product/25236

    This unit has a special coil and valving so it can do moisture removal without any cooling. The units are not cheap, but would probably be cheaper and simpler than a ducted dehumidifier.

    Moisture freely transfer through the interior of most houses as there are no interior vapor barriers and usually a fair bit of air flow with doors open. Provided you don't have a crawlspace with exposed dirt floor, this means the dew point is the same throughout the house.

    The only time you get into trouble with moisture is if any of the areas that are too cold. For example if your interior dew point is around 50F (so 50% RH at 70F), if any rooms (or exterior closet) falls bellow 60F, it will cause the RH there to go up to 70%. This sometimes means providing a bit of supplemental electric heat in isolated rooms.

    In zone 4 with a lot of summer time humidity, I think an ERV would be the better option. Try to source one with a good moisture transfer ratio as it will bring in less moisture in the summer time.

    1. sadrian | | #2

      How does the rate of moisture transfer compare to the rate that ventilation air enters? Despite best intentions, there will invariable be times when doors are closed for an extended period, so there will be no air flow out of the room, and I would guess that the RH would increase.

      Also, I have an impression from reading that ERVs have more potential for mold growth issues than HRVs. Perhaps I should revisit this conclusion?

      1. charlie_sullivan | | #4

        It's possible that you could want to get an HRV/ERV system with swappable cores and use ERV in the seasonal extremes and HRV in the shoulder seasons.

      2. Expert Member
        AKOS TOTH | | #6

        When it is hot and humid outside an ERV will supply air that has a temperature somewhere above indoor air. You mix in there the outlet of a dehumidifier, which will have low dew point but will be pretty hot, the resulting air will be dry but too hot for comfort. If you only supply this air to a room, it will overheat it pretty quick.

        Rooms need conditioning and ventilation. Sometimes you can provide enough conditioned air to satisfy ventilation needs. Without any conditioning a closed room will never be comfortable enough.

        Generally people living in high performance houses with a single central mini split either sleep with bedroom doors open or put up with a hotter bedroom. Two people breathing in a bedroom will add a bit of extra moisture load but not enough to notice. With my bedroom mini split off, I've never noticed the RH going drastically up overnight.

        Provided you keep the interior of the house conditioned properly, I doubt you'll get mold in the ERV core. Sample of one, but when I've replaced my ERV filter, I've never noticed any signs of mold and we have some pretty hot and humid summers here. If the house is always 60%+ RH range, you'll have bigger issues than a moldy ERV.

    2. charlie_sullivan | | #5

      I've long suspected and observed that dehumidification doesn't need distribution to the extent that heating and cooling do, but I haven't run across any good documentation or references on that. If you know of any I'd be interested.

      1. Jon_R | | #7

        Getting out of the shower, I need lots of dehumidification and no heating/cooling.

        The CFM needed for dehumidification is highly variable but can be similar to heating/cooling. The biggest factor is that the cooling air often provides dehumidification too. Vapor diffusion through interior latex/drywall walls can suffice where the moisture load is low.

        It's all pretty easy to model (once you know the inputs).

  2. Jon_R | | #3

    Heating, cooling, dehumidifying and ventilating needed aren't the same ratio across rooms - so there is a large comfort advantage to independent supply and control of each.

    1. Expert Member
      NICK KEENAN | | #8

      I agree, but multiple duct systems seems like overkill. Seems like what you'd want is some sort of zoning.

      I do feel that the next wave is HVAC is going to be not just temperature control but humidity as well.

  3. Expert Member
    NICK KEENAN | | #9

    I would think the "magic boxes" are cheaper than multiple duct systems.

    1. Jon_R | | #10

      But given the varying ratios needed, I don't see them working as well as multiple ducts + zoning. Or "home-run" ducts could allow centralized custom mixing for each destination room.

      It's interesting to start with a gold standard (independent per room control of humidity, temp and fresh air) and then think about the loss of comfort/efficiency/health with various compromises.

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