Woodstove + Granite ?

Discussion in 'Homesteading Questions' started by Non Sum, Dec 20, 2004.

  1. Non Sum

    Non Sum Active Member

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    The stone stoves, such as Hearthstone, appear to carry relatively small slabs of soapstone, or granite. Yet, one hears that they appreciably prolong the heat radiation.

    I already bought an all iron woodstove, rather than a soapstone/granite one, because I wanted the quicker heat-up & cool-down through most of the heating season. BUT, would it be worth the $30 per Sq Ft cost of placing a temporary (removable) slab of granite on top of the stove and a couple larger movable slabs beside it during the all-day long log consuming weather, so as to maximize my heat mass, and minimize the wood consumption? Would the effect be negligible, or sizeable? What's your guess?
     
  2. Cabin Fever

    Cabin Fever Life NRA Member since 1976 Supporter

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    A big boulder, clay tiles, bricks, or concrete block placed on top of your stove would have the same effect. It's the "mass" (or weight density) that holds the heat and radiates it out later on...it's not all that important what the "material" is.
     

  3. Non Sum

    Non Sum Active Member

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    You're basically correct, Cabin Fever, though there are materials that absorb and hold heat better than others. Metals, such as iron, are dense and heavy (more so than most natural stone), but are quick on the uptake and just as fast on the release. Soapstone, I've been told, is about the best natural material for slow steady release; with granite a close second. It's their intrinsic resistance (though ability to absorb) to heat that makes them ideal for building stoves out of them.

    Regardless of the material, I'm wondering why it isn't a common practice to surround, or heap, absorbent/radiant 'mass' on, or around, heaters, at least during prolonged periods of heat demand? Are the returns too small to merit the effort, even when expense is non-existent (e.g. boulders)? Why isn't every space heater circled with rock? Or, if aesthetics is the barrier, with polished marble or granite? Inquiring minds want to know.
     
  4. farmmaid

    farmmaid Well-Known Member Supporter

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    Buy some old "soapstone bed warmers" place 2 on your stove then 2 on top of them. If you want extra warmth in your bed, wrap one in a towel at bedtime (careful as they are hot). You get the best both styles of stoves have to offer...Joan
     
  5. mpillow

    mpillow Well-Known Member Supporter

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    We have a steel Shanendoah rectangle stove roughly 3ft high and long and 18in across...there is a dial for air intake on the door and we have a damper in the chimney as well. It has a coal grate and removeable bottom ashpan...we have found that by freighting the stove (with wood) in the morning to get a good bed of coals and major heat in the house....we can simply pull coals to the front (by the door) and put on a good big piece of wood again close to the door it will "limp" along until we decide to heat up again. Our plain block chimney is inside and really the only mass close to the stove. We have a small electric fan that blows across it at about the height of where the stovepipe meets the chimney.

    There was an article in BHM about this. Soapstone I think is very expensive when compared with concrete or "foraged" rocks. Our stove is lined with soapstone about halfway in the firebox. We use about 5 cord of wood for a one level 1700 sq ft home in Maine that is about 25-30 years old with updates for windows and insulation. I too have wondered how much could be gained by introducing more mass but hubby is always against any project that seems like work....guess he likes cutting wood and being chewed up by the blackflies and squitos :rolleyes:

    We should all have Russian Firplace/Chimneys.....
     
  6. FrankTheTank

    FrankTheTank Well-Known Member

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    Backwoods home had something about adding cinder blocks around your stove...I'm not positive on the gains, but i would think the more mass, the more heat you can store and radiate more slowly(it also would take longer to warm)...

    MAid~I remember on a show i watched one time, it was set way back in time, but before going to bed they put a hot rock(?) inside the bed to warm it up!...That would be an excellent way to warm a sleeping bag in cold weather (camping)...
     
  7. Cyngbaeld

    Cyngbaeld In Remembrance Supporter

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    I think the main drawback to the extra mass around the stove would be that you would not notice that the fire had died down until it was too late to just throw on another log and you would have to basically start your fire from scratch. When it is really cold I like to keep the fire going continuously rather than restarting it periodically.
     
  8. ibcnya

    ibcnya Well-Known Member

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    I put 3 layers of concrete paving blocks on top of my steel box stove and painted them with black stove paint. You hardly know they are there and in the morning it feels like the oven has been on all night.
     
  9. oz in SC

    oz in SC Well-Known Member

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    Mrs oz manages a company,part of which builds stone countertops,you might be able to find some sink cutouts for free-they will be quite rough cut and will be the size of a sink(obviously) but might work....

    I tries ceramic tiles in there but eventually they all broke...once the Christmas tree is gone from in front of the woodstove we will be using it and I will try the granite and see.
     
  10. mpillow

    mpillow Well-Known Member Supporter

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    My daughter is reading the "Little House" series and they put their cast iron fry pans in the beds for warmth.....inside a sack
     
  11. Cabin Fever

    Cabin Fever Life NRA Member since 1976 Supporter

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    The ability of a material to absorb/release heat energy is measued as the materials "specific heat capacity." The specific heat capacity for any material is determined by the amount of heat it takes to raise the temperature of one pound of the material one degree compared to the amount of heat it takes to raise one pound of water one degree. (Water is used as a reference because it has the highest specific heat of any material -- which explains why it takes so long for a pot of water to boil). For example, it takes one British Thermal Unit (BTU) to raise the temperature of one pound of water one degree Fahrenheit.

    Specific heat capacity
    Water: 1
    Soapstone: 0.22
    Granite: 0.19
    Brick: 0.22
    Concrete: 0.19
    Limestone: 0.20
    Marble: 0.21
    Pyrex glass: 0.20
    Rock salt: 0.22
    Sandstone: 0.22
    (Source: http://www.engineeringtoolbox.com/24_154.html)

    So, it would appear that water can absorb (and release) almost 5 times more heat than any type of stone. The problem with water is that it will "boil" away the heat instead of storing it. Waters ability to store heat is the reason why groundwater heat pumps can be so successful.

    People who rely on thermal mass as an intregal part of their heating (and cooling) systems use many tons of mass.
     
  12. degaston

    degaston Member

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    I would refer you to the Backwoods Home Magazine article about the "Big Daddy" woodstove. The author has piles of rock and block piled all around and on top of his woodstove...but the thing works well. The presence of all that rock around the woodstove moderates the temperature extremes. It won't get hot fast and it won't cool down quickly because it is so massive. I'm sure you could set up a forced air duct to force air through the stone to get warm air into the room quickly.

    To store 250,000 BTUs of heat, enough to keep a standard house warm overnight in January, you would need 10000 to 20000 pounds of rock or concrete. That is a 4 to 5 foot cube of rock centered around your woodstove. Don't set this pile on top of regular floor joists.

    You will need to fire such a woodstove with 250000 / (8000 BTU/pound) = 30 pounds of wood (a lot more to cover inefficiencies) in order to charge the thermal mass.
     
  13. Non Sum

    Non Sum Active Member

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    Great ideas given in this thread from all!! Much appreciated.

    Your data, Cabin Fever, is correct but incomplete. "Water" for instance, is incapable of storing any temperatures over 220F at sea level; not very useful for stovetop temps. My stove often exceeds 500F. Heat storage, retention, and gradual decay are (collectively) the key elements here. Soapstone, for instance, has Twice the heat storing capacity of iron. And, more than any other natural material, including water. Some materials, such as granite and soapstone are able, due to their molecular composition, to retain heat quickly, yet radiate it slowly. You can heat them to a point where it will take >24 hours before they return to the starting temp. Some materials are ideal 'heat batteries,' and others, while having a high "specific heat capacity," are not.
     
  14. Cabin Fever

    Cabin Fever Life NRA Member since 1976 Supporter

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    The physical property that you are referring to is called “thermal conductivity” which is the quantity of heat that passes into a unit volume of material at a given temperature gradient. It is analogous to electrical conductivity. I have found the following thermal conductivities for various materials:

    Soapstone: 2 – 5.5
    Granite: 2 – 4.5
    Marble: 1.4 – 4
    Sandstone: 1.5 – 6
    Shale: 1.4 – 4
    Limestone: 1 – 4.5
    Dolomite limestone: 1.5 – 5.5
    Sand: 0.5 – 2.5
    Brick: 1.6
    Concrete: 1
    Glass: 1
    Cast iron: 55
    Steel: 46
    Water: 0.58
    (source: http://www.unileoben.ac.at/~geophwww/neu/data/chapter8thermal.pdf )
    (source: http://www.engineeringtoolbox.com/24_429.html )

    Soapstone is not that different in either specific heat capacity or thermal conductivity when compared to other types of stone.
     
  15. endinmaine

    endinmaine New Member

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    Non Sum,

    I have 22+ years experience with soap stone and would never buy another one. Reasons: take up to an hour to begin heating my house. Stones can and do crack even with TLC. It takes 6-8 logs to get heat from the stones whereas a cast iron or steel stove begin to heat after 1-3 so there is no savings. I also have a cast iron cook stove made by Royal Charm Crawford in 1901. I begins putting out heat after lighting paper and because of it's mass it radiates for several hours after the fire has gone out. I bought into the hype about the soap stones having a "heat life" but don't believe it. All stove have a heat life and if you install your stove next to a field stone chimney that extra mass will also heat up and radiate for many more hours.
     
  16. Alex

    Alex Well-Known Member

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    Thermal mass will store heat. It will not give you anymore instantaneous output.

    Actually, it will take longer to first warm up and put heat into the building with a heavy thermal mass heater, than with an instantaneous cast-iron or heavy steel airtight.

    Mainly in "shoulder" times . . . when it's not too cold . . . or at least the building heat loss is not approaching the stove output, that's when soapstone type heaters may have their place .

    Heat up a soapstone . . .then the fire can go out, and you will still get radiant, convected, and conducted heat . . . until the temperature of the soapstone is less than the room temperature. When the stored heat in the soapstone . . . or whatever type of rock . . . is used-up by heating the house on a cold day - then it will get cold or you have to make a new fire.

    You need the same BTUs to heat a house . . . to a certain temperature . . . for a certain time, using soapstone as you do for any other type of heater

    How and when to use one is very personal - like any heating source or system.

    Good luck on you choice.

    Alex

    PS, It's a "Blaze King" type catalytic heater that I love.