A Wood Stove in a Passive House: Feeling Warm Already!

Who would have thought that putting a wood stove into a house could be so complicated!!!  I know I didn't.  It really started when we decided to put the wood stove more centralized in the home.  Running a chimney inside the building envelope is much more efficient than running it outside the thermal envelope from many perspectives.  The chimney will develop draft much more quickly if the chimney runs inside the envelope, and there is less thermal bridging to the outside.

Have a look at the stove in question below, the Walltherm gasification boiler (www.waltherm.ca).  The picture below illustrates the back of the stove.



It has a sealed combustion chamber with an air intake at the bottom that is ducted to the outside.  It is a closed combustion system.  This is a necessity in a passive house for several reasons

1.  Draft in the chimney requires air to be pulled into the stove.  In a super tight house, the draft necessary to keep the stove going will be hard to make up if the walls are too tight, required a window to be opened while burning the stove.
2. Dangerous backdrafting could expel dangerous carbon monoxide into the space.  With normal code built homes this is dangerous enough.  If the natural air exchange rate is 10 times less, you'll be feeling sleepy pretty quickly!

Having a carbon monoxide monitor is a necessity for most homes and a passive home is no exception.

Below is a schematic of the living room.  The wood stove was meant to sit adjacent to the kitchen/living room wall.  The air intake duct was going to be poured under the slab and out to the south side of the house where it would come up from under ground.  It would work but coordinating the air intake with the chimney placement proved to be more complicated than originally planned.



In the picture above, the chimney runs in a chase in one of the upstairs bedrooms.  The chimney thimble requires a 7" offset from the kitchen/living room wall.  Somehow the chimney would have to jog across two joists and up through into the chase.  To add insult to injury, the thimble can be air tight but the attic radiation shield has to maintain a 1" clearance to the prefab chimney.  See below.   The ERSA (radiation shield) can't be caulked, or modified in order for the system to meet code.


In which case, there is no way to make the setup air tight unless you make the chase for the chimney air tight.  This is doable but complicated.

Instead we opted for the traditional approach:  Out through the side wall (with modifications to the wall) and up through the porch roof.  See Below:




The air intake is also ducted to the porch and out through the back wall of the porch.  Its labeled on the diagram as "cover for wood stove air intake".  In order to meet clearances and the fact that the wall thimble is only 12 Inches thick, we had to jog the wall on the inside of the house to a smaller thickness.  I think it will look fine. 

This setup can be air sealed nicely and will meet the specifications of the manufacturer.  I can feel the warmth already!!!!

Comments

  1. I still wonder about the efficiency of this stove, given a pellet stove that puts out about 11 kw has only a 3.5 in outlet and 2 in inlet. You have big pipes to deal with, and a bit of a problem. The appearance is not great, compared to if had had gone through your main roof. And with the wind storm recently , I watched a pole feeding my neighbours house move at the top some 3 inch back and forth. That was in Logy Bay where winds were only 118km. At Bishop`s Cove, at my cottage, I lost a 30 year old pine tree. It broke off 2 ft above the ground, but the tree was missing! I am sure 4 men would not carry this tree, with all the limbs, yet it was found about 100 ft from where it supposed to be. It is near the shore and the bank slope must have given it extra lift. Hard to imagine.Lucky I did not lose my windows, but the tree was more exposed to that wind. So your chimney will need good anchoring.
    I considered a propane stove as back up for Logy Bay, and most all have large pipes that are difficult to run, as you have encountered.
    By the way, today , at minus 8C, I was using about 200 watts (HP) for heat midday,for 1250 sq ft, getting good solar gain. My attic reached 44.8F . My place at B Cove is a bit of a hybrid.
    If a Mini-split is an option, you mentioned running wires........you should also run the copper tubing..........and especially mount it in one of your attic areas. and to do that you need a wooden chimney for airflow.I used 22 x 22 in, pressure treated plywood, split in the centre to accomodate two units, but only using one so far.
    When will you start your foundation.......
    Winston

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  2. Hey Winston,

    I had originally considered a pellet stove for the house. Pellet stoves can modulate heat based on a thermostat which is nice. There are a few european woodstoves that do the same. However, 11kW is almost 4 times the amount of heat needed. You would risk always over heating or rarely using the stove. This is the biggest "issue" with wood. As you propably know, efficient burning requires lots of oxygen, and that makes a stove burn hot, leading to less fuel consumption. Burning efficiently and having a way to capture and store that energy makes more sense. and is more versatile...you could use it to heat or for hot water. I did search for a wood pellet boiler for the living room. These systems run full on and dump a large amount of heat into a hot water tank. However, there are none in canada. They're use is fairly common-place in europe. The other issue with a wood stove is distribution. Moving heat around would depend purely on natural convection which may work but now I'll be depending on hydronic.

    I like the idea of a pellet stove, don't get me wrong but then I started thinking about pellets and decided that raw material made more sense...although I think that they are a great option for low energy homes. They may work better in single level homes....maybe.

    As for the efficiency of the stove, its all in capturing the wood gas heat after the secondary burn chamber. Allowing the wood gas to ignite allows for higher efficiency in all gasification stoves. The temperature inside the secondary chamber can be upwards of 800 C. The temperature of the flue gas after most heat is captured by water is about 120 C. combining the efficient burn with efficient capture gives high efficiency.

    I would have liked to go through the roof but the air sealing would be a mess compared to the side wall. I was thinking about chasing it in, but then thought that it will lead to more flashing, more issues, tearing it open should anything ever happen. THis set up is much more simple and the energy penatly is pretty small compared to the benefit.

    bracing shouldn't be an issue, the ICC excel chimney has stainless steel wind bracing brackets that will work with this chimney and are required every 10 ft I think so I should be good there.

    I have propane in my current house. The issue is supply. A few years ago the provice ran out! luckily enough, I had my tank filled a week before the whole dark nl thing happened. I want to get away from fossil fuels, they aren't sustainable, or reliable in terms of supply, and theres wood all around us.

    200 W is pretty impressive, If I were to guess, you are getting great efficiency because the HP is in the attic. With an attic at 44.8F you're able to capture all that heat. I do wonder...what happens to cooling efficiency in the summer with the HP in the attic...seems it would decrease?

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  3. forgot to mention, excavation is starting next month as soon at the ground is clear. I am looking into the heat pump still and will definitely follow up.

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  4. Hi David
    I had a good few days monitoring HP attic performance, and the cold sunny weather was ideal, colder than average Jan or Feb.
    I got a 24 hr COP of 4.1 with temperatures around 0C, and yesterday a daytime COP of about 4.75, with temperatures in the range of -6C to -9C. I estimate about COP of 3 at night, so a daily average of 3.87, at that colder temperature. As these figures are so high I wouldn't mind someone reviewing them, I have retained the monitoring data. Three main reasons why so high: higher attic temperature from solar gain,and part load on the compressors,eliminate most all the defrost cycles, and of course ideal weather for this. With dec and Jan, at lower sun angle , and often fog or cloud, it will not be that good. But I view them as COP 6 potential under ideal situations. My baseboard heaters are on the inside walls. If under the windows, my COP would show even higher, compared to baseboard heater consumption.
    For summer for AC, I have used it only a week or so, as I get a sea breeze, and i have patio doors that I use in summer when suitable. Attic temperature goes to about 110, and may go higher.I wondered initially if it would work for Ac. The units are rated to operate to 115F. Once they start and bring in air through the soffit, the attic cools to maybe 90. If in a location where the attic got hotter, air ducted to the back of the unit from the soffit would be needed, but not around coastal Nfld. I have not measured energy use in the summer, but the units runs at low speed apparently, and I use it sometimes to dehumidify on a muggy day.
    I wondered if your chimney could be enclosed where if you show it, and use as a dual function, for HP exhaust, in the space around the chimney and enclosure.......but adds to your cost.
    I have a pellet stove for 6 years still on the crate, never used it after calculating that the HP would be 30 percent less costly for energy...........that will change some with Muskrat rates.
    I like the hydronic, and wonder how to optimize HP with hydronic , for storage....given daytime only HP with the attic is so efficient.....for future consideration. About 1.5 percent more efficient for every 1 degree F increase in attic temperature.
    I am surprised no one seemed to have done this, (attic Mount) as far as I could see, for climates like ours and colder, as snow and defrosting often is an issue. Not so good for warm climates.
    A caution on low flow showers heads.........my wife complains a lot with low flow.......got do do with washing soap out of her hair efficiently........which is not a problem for a man with short or little hair. Really a big deal for her, and I expect most women!
    My HP is HSPF of 8.5 . Newer units are 12 or 14, and about 20-25 percent better.........but I wonder if closer spaced fins will cause more defrosts......field testing needed I guess.
    Winston

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  5. Hi David; Good to see a PH project in Nfld. I am also interested in doing the same. Will be interesting to know how the builder/GC and trades will be ready and qualified to take this on. Have you sorted this out yet? krgds, Bob

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  6. Hi Bob,

    We will see what happens with the trades. I have done the PHIUS builders course, wrote the examination and am aiming at being the first qualified PH builder in NL...we'll see how the first one goes!!

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    Replies
    1. I should add, that I completed the course knowing that there were no qualified trades/builders here for passive house. My plan is to guide the trades through all the details. It will be a win win situation for everybody!

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