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Community and Q&A

Whole-house dehumidification plan advice

EDUB6 | Posted in Mechanicals on

We are trying to develop a fairly passive house in SE Michigan. Plan on heating with wood via a boiler and radiant floor heat in concrete slab and foregoing cooling (maybe adding multi splits later if need be). We are hoping that by using a REMOTE wall system we will be able to insulate the house enough to minimize the seasonal changes in temp. However, as you would expect, we are concerned with moisture.

HRV/ERVs look to be great products, but unfortunately they do not deal with moisture. So we have more or less decided on a whole house dehumidifier with ventilation (Ultra Aire XT155H). The plan would be to draw in air from mainly the basement and a few key locations on the main floor and exhausting into bedrooms, great room, and basement at floor level. We would also have exhaust fans to the exterior for the kitchen, baths, and laundry (Ultra-Aire made it clear that drawing from these high moisture sources was bad for the unit).

Would love to hear anyones comments/concerns/confirmations.


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  1. GBA Editor
    Martin Holladay | | #1

    Dehumidifiers are energy hogs and are rarely required in your climate.

    Your first task is to address any moisture issues. If you have a damp basement, address the moisture entry issues. If you have sources of moisture inside your house, they may need to be addressed with spot exhaust fans.

    During the winter, the best way to address high indoor humidity levels is by increasing the ventilation rate. If you have an HRV, just run the HRV for more hours per day.

    During the summer, the best way to address high indoor humidity levels is to keep your windows closed and (if necessary) operate an air conditioner.

  2. Expert Member
    Dana Dorsett | | #2

    Martin has it right- whole house dehumidifiers in a SE MI location would be like going after mosquitoes with sledgehammer unless you insist on truly insane ventilation rates. While summertime dew points sometimes hit the mid or even high 70sF, they are almost always accompanied by a sensible-cooling load (unless your house is a high-mass earth berm type and you decided to have no windows. :-)

    Take the money you'd otherwise spend on the whole-house dehumidifier and apply it to a Daikin Quaternity, which is (SFAIK) the only mini-split series that will dehumidify to a RH % setpoint, with or without sensible-cooling. Even the smallest one would be able to dehumidify a decent size house unless the ventilation rates were through the roof, and would be more appropriately sized for heating a near-PassiveHouse than any wood-boiler, particularly during the shoulder seasons. As mini-splits go they're not cheap, but they are extremely well built well engineered units.

    Most US-built wood boilers are local air-pollution nightmares due to the crude control mechanism of air-starving it when the aquastat on the thermal store is reached. Net efficiency is also quite low, well below the steady-state firing rates, since they are so dramatically oversized for the heat loads of even code-min homes, let alone a high-R home. The heat load of a 1500-2500' near-PassiveHouse will usually be under 20,000BTU at +4F (Jackson/Reynold's 99% outside design temp), and even 11,000BTU/hr ( the full heating output of a pretty-good 3/4 ton mini-split @ +5F) isn't out of the question. The smallest EPA rated wood boilers out there are in the 45,000BTU/hr range, and that would still be on the high-size end of what's really appropriate. Oversizing the storage tank and always firing the under-sized boiler letting it burn to self-instinquishing is probably the best way to keep efficiencies reasonable and air pollution down.

  3. EDUB6 | | #3

    Thanks Dana,
    We will have a 2400 SQ FT ranch house with a livable walkabout basement. The main floor will be polished concrete. Maybe the minisplit would suffice and the radiant is unnecessary, however it would feel like a missed opportunity with the concrete floors.. The gassifying wood boiler I was looking at is: the smallest model (elite 100) outputs 115,000 BTU/hr... so yeah maybe a bit overkill.

    SO, if going away from gasifying wood boiler, any recommendations for water heaters?

  4. Expert Member
    Dana Dorsett | | #4

    The Elite 100 can still run a continuous not super-sooty burn at ~45,000BTU/hr firing rate, so with a sufficiently sized thermal storage tank it might not be crazy. To be sure, putting the PEX in the concrete isn't a huge expense up front, which would leave you the option of a hydronic solution. (At the risk of sounding like a Daikin rep, which I certainly am not, there's probably a Daikin Altherma hydronic output heat pump that could be sized for the load, that would also be able to handle the domestic hot water. But they're pretty expensive, and doesn't address the dehumidification load issues.) The system's distribution & standby losses of woodboilers never show up in the spec, but they're real- a tiny high-mass soapstone wood stove can often heat a place using less wood, but you don't get the hot water, or the cushy feel of radiant floors. But in very low energy buildings the additional comfort of radiant is pretty small, since most of the time the floor will be within a degree of the average room temp.

    Picking the mechanicals before you know the actual loads is bound to be pretty error prone. Terms like "...a fairly passive house..." could cover quite a range, whereas a PassiveHouse has specified BTU per square foot per year, which tightens up the likely range of peak heating loads. While still in the design process you can work it the other way- pick a peak load number as a design goal, and use energy use simulation tools (BeOpt, etc) or even crude heat load calculators to help you tweak the design until design meets your spec.

    What are your fuel options for domestic hot water?

  5. user-1072251 | | #5

    The first step is to reduce the moisture load as much as possible. In a new house, install good foundation drains, coat the footing with a capillary barrier to keep ground moisture from migrating up into the foundation walls, install a thermal break at the edge of the concrete floor, insulate under the floor and install a 6 mil poly barrier, taped, above the foam and under the slab. These measures make a big difference in keeping the space drier.

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