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Intake and exhaust air for 350 sq. ft. tiny house

milepost007 | Posted in General Questions on

Hello, another question for our shed project, which will eventually become a Tiny House.  A permanent structure, not on wheels.  Climate Zone 5B.  Please see attached FP sketches.  Only 240 sq. ft. on the main floor, plus a 110 sq. ft. loft.  The building envelope will be pretty tight.  We will be installing a small wood stove (shown in the lower right corner), and also using a 2-burner Japanese propane cooktop.  After reading this article https://www.greenbuildingadvisor.com/article/ventilation-small-home (and others), I’m leaning towards using a Lunos Air Inlet (https://foursevenfive.com/lunos-ald-r160-passive-air-inlet-kit/) for makeup air, and a Lunos exhaust fan (https://foursevenfive.com/lunos-ra-15-60-exhaust-fan-kit/).  The Air Inlet will be near the wood stove, and the exhaust fan will be near the composting toilet; above the window on the left, or next to it.  Does that sound right?

For reference, here is the wood stove that we ordered.  Overkill, I know, but we will try to make it work.
https://www.homedepot.com/p/Hi-Flame-800-sq-ft-Shetland-Extra-Small-Wood-Burning-Stove-HF-905UPB/205582659?cm_mmc=Shopping%7CG%7CBase%7CD28I%7C28-20_FIREPLACE%7CNA%7CPLA%7CFixed%7c71700000041073829%7c58700004389677714%7c92700036924273665&gclid=Cj0KCQjwhJrqBRDZARIsALhp1WRTIl4J-uq4oa8QlSRPcX1JkViI0IsMwhkJrsGWY3lNWiF1GYCGyxQaAvpCEALw_wcB&gclsrc=aw.ds

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Miles,

    Using an exhaust only ventilation strategy, even with passive intakes, will depressurize the house. I would worry about how that will affect the wo0d stove. My own inclination in such small building would be to use operable windows for ventilation, but I'll be interested to see what other posters suggest.

    I don't know where you are, but up here, if the tiny house isn't on wheels it falls under the building code, which would have consequences for a whole lot of design decisions, including ventilation, the dimensions of your toilet enclosure, and the rise of your stairs to the loft.

  2. Expert Member
    AKOS TOTH | | #2

    I don't think that inlet will provide enough flow for a wood stove plus the 8pa pressure is too high (I would check but I think it needs to be less than 5).

    I think something like a passive operable damper (workshop vacuum blast gate) and a Panasonic WhisperComfort ERV would work better.

    1. milepost007 | | #4

      Thanks for the advice, Malcolm and Akos. I'm new to this, so I don't know all the terms. When you say 8Pa, that's Pascals of pressure, right? On the Lunos website (and/or literature), where are you seeing that?

      1. Expert Member
        MALCOLM TAYLOR | | #5

        Miles,

        It's in the link you provided for the passive vent under Properties.

        1. milepost007 | | #6

          Ah, thanks. I was looking for it under the Properties for the exhaust fan. So if I'm understanding correctly, less than 5 means I want something that comes on with even a smaller pressure difference. Is that right?

          Also, I will add a link to the wood stove, in my original post.

          1. Expert Member
            MALCOLM TAYLOR | | #7

            Another reason to rely on windows. No matter how low you damp that down it will cook you. The only way to use it will be to exhaust a good half the heat it throws outside. That's not a big deal, but it precludes a mechanical ventilation system like the one you are suggesting, except when the stove is off.

          2. Expert Member
            AKOS TOTH | | #8

            Miles,

            I would check your local code. Here it is 5Pa for spillage susceptible appliance and 20Pa for sealed combustion.

            I'm not sure I would like to rely on electronics to keep the smoke out of the room. A passive P trap air inlet might also be an option. Going with a taller chimney will provide more draft, it would less susceptible to backdrafting, might help a bit.

            I'm with Malcom though, I have a similar sized stove in a 550sqft cottage with reasonable insulation and it can heat the place to 27C when it is -25C outside without issues. You will need open windows if you want to heat with that.

          3. milepost007 | | #9

            Thanks Malcolm and Akos. That's sobering news about the wood stove, but good for us to know. We had looked at much smaller stoves, but they didn't have the EPA certification required by Skamania County for Dwellings. To answer other questions, our tiny house IS a permanent structure, and our chiney/stove pipe is approx. 20' high.

  3. Expert Member
    AKOS TOTH | | #3

    P.S.

    I've done a couple of condos with 17" deep counters with 15" base cabinets. For the sink I used a narrow sink rotated 90 degrees, same for the two element stove. The couple of inches buys a lot of space in a tight area.

    For tight spaces and stairs, winders are your friend.

  4. Expert Member
    Dana Dorsett | | #10

    Even though it's a lot more money, a tiny EPA rated soapstone stove would limit the instant-sauna effect, and with intermittent high-fire burns could be more manageable without excessive pollution or creosote issues.

    An even higher-mass wood burner would have been more appropriate. Whether a built-in-place high mass burner would cut it with the local inspectors is something you'd have to investigate, but filling that corner with a pile of warm brick or stone is going to be a lot more comfortable than a point source of hot iron/steel. An extremely cursory overview:

    https://cleantechnica.com/2018/03/17/efficient-wood-burning-rocket-stoves-masonry-heaters-dakota-fire-pits/

    1. milepost007 | | #11

      Interesting. Thanks for sharing that, Dana.

      1. Expert Member
        Dana Dorsett | | #12

        There are commercially available mass heaters, but also kits for doing a mostly DIY version that won't burn your house down, some of them small enough to work in your alloted space, eg:

        http://www.dragonheaters.com/4-rocket-masonry-heater-castle-build-kit/

        http://www.dragonheaters.com/content/dragonheatercastleweblit.pdf

        https://cdn1.bigcommerce.com/server4100/lmpge/product_images/uploaded_images/arronoakley2654.jpg

        caveat: I've never built one myself, can't vouch for this vendor (or any other), but a 1 to 3 bell high mass "rocket stove" a more appropriate heat source for your tiny loads than a cast iron woodstove. Some people get creative and even build in masonry benches as part of the heater, making a relaxing spot to warm your bones when it's truly frigid outside. Firing it once or perhaps twice a day may be enough, once you've figured out how much wood to stuff in it for the current weather.

        1. milepost007 | | #13

          Hadn't seen those. Interesting idea.

  5. rockies63 | | #14

    I would second the soapstone stove. Maybe one of these award winning models.

    https://www.woodstove.com/

    If not, I would consider getting a wood stove that has it's own air intake pipe attachment right into the firebox.

    As to your design, placing the woodstove so that it is surrounded by windows means that a lot of your precious heat will radiate through that glass and be lost to the outdoors. You should have some heat absorbing mass behind the stove.

    If I were you, I'd flip the sofa and stove so that the stove is in the northeast (?) corner (without the windows around it) and the sofa is in the southeast (?) corner with windows. The kitchenette could go on the north wall in the middle, the entry door on the south wall in the middle and the toilet turned 90 degrees and remaining under the stairs.

    The loft is fine, but perhaps consider a ships ladder?

    1. Expert Member
      Peter Engle | | #15

      Scott said,

      "The loft is fine, but perhaps consider a ships ladder?"

      Or an alternate tread stair. They look really weird but they actually work pretty well once you get the hang of them.

    2. milepost007 | | #16

      Thanks Scott. Layout: From what I posted, you can't tell, but the layout is all based around views of the amazing Columbia Gorge (to the south). Ships Ladder: We would have done that for sure, but what you see is a bit of a compromise; so that the dog can go up and down, and so that the space under the stairs creates usable storage cabinets.

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