woodstove in a passive house ?

[jesper]

I know, it's blasphemous even to ask such a question. Why would you install a wood stove when you'll lose heat through the chimney, and the stove will make the house feel like a sauna in 15 minutes ?

The reason that I do, is that I've come to the conclusion that in order to sell a passive house in western Maine where I live, it will be much easier with a wood stove in the house. Wood stoves are in almost every house, people cut the firewood on their own land (free heat) and I know from my own experience that I like to know that I can heat my house when the power is out (which sometimes happens for days at a time). I think that it would reassure people to have the stove. Maybe it could be installed with a metalbestos chimney so that it could possibly be removed later ?

So, although it might not seem rational to install a wood stove, I would like to at least explore the possibilities. If I built a passive house it would be a nice touch if I could actually sell it.
I realize that there will be a serious heat loss due to the air circulation in the chimney, but I also know of at least one passive house with a chimney ( check the CEPHEUS book page 94, PH in Steyr-Dietach, Austria) so obviously it is possible, although maybe not ideal.

I hope that somebody has experience with wood stoves in tight houses and that I won't get stoned to death with firewood at the next PH conference.....

[DMcEvers]

Jesper, Another option may be a "masonry heater", they are airtight, use outside air for combustion, burn cleanly, and have thermal mass for heat storage.

[jesper]

Thanks Doug. I knew that I could count on a reply from you. Do you have any experience with masonry heaters in something like a passive house ?

[Mark Siddall]

Jesper,
This paper has a little study on wood burning heating systems: Re-inventing air heating: Convenient and comfortable within the frame of the Passive House concept, Energy and Buildings Feist, 2005. In this paper Feist reported that there were no suitable wood burners for PassivHaus projects. With a wood burning stove in a PassivHaus the danger of overheating the room is quite high as it is hard to control the output. Stoves using wood chips or pellets are more suited to use in a PH as the output can be more readily controlled. Stoves have to be equipped with an room air independent air supply, and flue gas removal i.e. some sort of balanced flue.

Austrian bio-mass burners are reported to be amongst the best as I understand and Okofen are at the front of the pack. Not cheap but of high quality as I understand. Have a look at
http://www.organicenergy.co.uk/content/index.php and klick on the Okofen image.

For a review of the Okofen range that Organic Energy stock see
http://www.housebuildersupdate.co.uk/20 ... mined.html

Okofen Presnetation at European Bioenergy Business Forum 2006
http://www.aebiom.org/IMG/pdf/Ordner.pdf

There are concerns about emission from residential boilers fired with wood logs and wood pellets. For more on this see:
http://www.vtwoodsmoke.org/pdf/Johansson03.pdf
http://www.vtwoodsmoke.org/pdf/Olsson05.pdf
http://www.uku.fi/laitokset/ifk/PAVA_raportti2007.pdf

Hope this helps.
Mark

[DMcEvers]

We did install a couple of Cozy Heat in a two of the superinsulated (R-50 wall) homes I built in the 1980's but the owners did not rely on them entirely for heat, although they put out a considerable amount. My good friend Tim, southwest of Denver (Elv. 8,000) installed a Temp Cast in his new log home but this is recently so I can't vouch for the effectivness of it in this context as I have not been back to see it operate.

The "Masonry Heater Association" is worth a look, I would not discount this system in Maine in a Passive House as you will need some auxiliary heat in the winter months. The masonry heater can be sized with the appropriate thermal mass to store the right amount of heat for a given heating load, as the weather warms, you build a smaller fire. I think it is critical going forward to utilize the local (renewable) heating fuel, wood, in the best possible manner, this may be the only heating fuel available in the future. As I understand it, a masonry heater can be set up with a coil to provide domestic hot water as well. A Passive House uses roughly .6 Btu/sf/hdd, same as 15 kwh/M2 annually. Don't know the design temperature where you are but where I live, Minneapolis it is - 16F, so on the coldest days of the year a 3,000 sf passive house is going to need about 150,000 Btu's per day to maintain indoor comfort. This a therm and a half or 25 lbs. of hardwood at 75% efficiency, not trivial.

Mark, the pellet stoves do look interesting, I have considered pelletizing and burning native (prairie) grass in the future as I have this resource on my farm, has about the same energy per ton as quality firewood.

Doug

[Mark Siddall]

Pellets are interesting I admit but I had a meeting today on bio-mass boilers. It would seem that wood chip rather than pellet is preferable. The Embodied Energy of a pellet is quite high and there are few manufactures here in the UK (add transport costs/energy.) It is on this basis that chips come out on top, they to can be fed to the boiler by an ogre.

Before installing bio-mass read Butti and Perlin "A Golden Thread" and see what happended to Rome. They used to have forrests but burned them for fire wood. Today we don't have that luxury. Ensuring the long term sustainability of the source is critical. (I have a preferance for gas due to "low" CO2, in 15 years or so when the boiler needs replacing anyway upgrade to the fuel cells that should just be coming on stream for domestic use, or the new Compact Service units with a COP >6.)

Mark

[rwagner]

While I have yet to design a house that meets the Passive House standards, I do regularly put a wood stove into the very low-energy houses I'm doing. I do this for several reasons. We have 9800 heating degree days here, and many days in a row without sunshine, so often our peak heating load cannot be met through the ventilation system only. The wood stove adds an element of "energy independence." If the power grid goes down and you cannot heat the house through the ventilation system (no electricity), or if there is a disruption to supplies of natural gas (potential used for another heat source), the wood stove is there to provide heat at any time. In our cold winters this is necessary. Some clients want to heat with wood to reduce their reliance on non-renewable fossil fuels, and because it is a clean, renewable source of energy. We only use stoves with a dedicated combustion air route but it does add another hole to the envelope and so it does weaken the envelope a bit. I've decided that the trade-off is worth it. Does a wood stove overheat the house? My experience is no, not if the stove is located in an open area to promote heat distribution. There are very small stoves available, and I still prefer this to a masonry heater because the regular wood stove acts like a small radiator and you can more easily control the amount of heat in a short time. I've found that the small wood stoves work beautifully in a passive solar home. Make a small fire when you first wake up, let it die down and the sun takes over. In the evening if it it gets very cold and the passive solar gain doesn't last into the night, make another small fire and that stabilizes the house through the night. Also, another reason I use a more standard wood stove is because in our area firewood is readily available, and many people are practicing good forest stewardship as they harvest and/or sell this local, renewable resource.

A final note based on two experiences: The "Skyline House" that is calculated to need 23 kWh/m2 annually has been staying comfortable all this winter almost exclusively with the use of the wood stove and the passive solar gain. Another house recently completed has higher heating load (not as thermally aggressive in the envelope, but still has a peak load of only about 8 Btu/sf) but is very very tight - tested at 0.04 cfm/sf@50 Pa (0.4 ACH @ 50) and the wood stove with dedicated combustion air route works nicely.

[jesper]

Hi Rachel,
I was very happy to read your reply. Can you tell me what you do to keep the chimney and combustion air supply airtight where it penetrates the envelope ( i.e. what kind of chimney do you use, how do you insulate around the chimney ) ?

Thanks !

[jesper]

Hi Mark,

Thanks for your reply and the links. Did you mean to put a link for a paper by Dr. Feist ?
I'm aware that the stove would need a dedicated air supply, but I'm not sure exactly what the deal is with the flue gas removal (how is this different from a traditional house if the stove has it's own air supply) ?
The pellet stove from Okofen is interesting, but with my budget it's nothing that I even can even begin to consider. I think that one of the real benefits of burning wood logs here in Maine is that it is readily available for most people. I think people really appreciate the option of actually getting the fuel right from their own yard, and with zero transport or processing it is about as green as you can get I would think.

[Mark Siddall]

Hi Jesper,
No I didn't mean to post the paper by Feist; the reference should allow you to find it in a library though. As far as I know for air supply and exhaust to a bio-mass boiler a balanced flue, similar in principle to that on a condensing boiler, is used on the more this allows combustion to take place without unduly compromising the airtightness or the indoor air quality of the building.

Mark