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Thread: O.T. firebrick VS cast Iron

  1. #1
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    Default O.T. firebrick VS cast Iron

    This is mostly off topic but does have some merit for the metallurgy guys --- here's the thing, Im an efficiency monger and want to get all I get out of my wood stove, I realize the reason for firebrick is so I dont damage the stoves metal BUT --- Im not burning coal and I also realize that fire bricks purpose is to be an insulator of sorts --- WHAT? an insulator ? im trying to get to the heat not keep it away --- I think there could be a better happy medium that allowed more heat to get to the stove without it having a meltdown and am curious if anybodys ever heard of alternative cast iron "bricks" or something of the sort rather than a firebrick which I think is better suited for something like the nose of the space shuttle.

    PS - I dont want to install a "magic heat" or some apparatus on the stove pipe or have anything funky looking as this unit is directly in my living quarters.

    Thanks.

  2. #2

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    I'm not sure it's the problem you imagine. Every bit of heat from the fire goes somewhere, and there are only two options - into the room or up the flue. If your stove is designed to transfer most of the heat except what's required to maintain draft and minimize condensation it's doing the best it can. Efficient combustion is the first principle and most stoves manufactured in the last 20 years are reasonably well designed. Heat transfer to the shell is the second principle and it's been pretty well thought out too.

    Think of the firebricks just as a time delay, they'll absorb heat at first and give it up again later. You still get their heat, just not immediately.
    .
    "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

  3. #3
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    Fire brick is not exactly an insulator, well it is but that's not it's main purpose, think of it more as a heat modulator the purpose of which is to distribute the heat more evenly. Stoves without fire brick tend to heat up rapidly and intensely but as the fuel load is consumed they also tend to cool rapidly and drastically resulting in wildly uneven heating and cooling cycles making it difficult to maintain a comfortable level. The fire brick makes it possible to sort of even out these cycles and maintain even heating trough varying degrees of fuel load.

  4. #4
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    The firebrick is there ot reflect combustion heat back into the combustion zone. It improves combustion efficiency by preheating the fuel thus allows burning of moister wood and promoting more complete combustion of the intermetate combustion products and distillates. It also reduces smoke and creosote.

    A good wood stove has several design features to improve the yield of the heat content of the fuel. The firebrick is in the conbustion zone. The heat transfer zone comes later abive the actual combustion and there's no firebrick there. Besides, any heat absorbed by the firebrick is not lost. It's re-radiated back into the firebox.
    Last edited by Forrest Addy; 01-28-2009 at 12:22 PM.

  5. #5
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    Quote Originally Posted by radkins
    Fire brick is not exactly an insulator, well it is but that's not it's main purpose, think of it more as a heat modulator the purpose of which is to distribute the heat more evenly. Stoves without fire brick tend to heat up rapidly and intensely but as the fuel load is consumed they also tend to cool rapidly and drastically resulting in wildly uneven heating and cooling cycles making it difficult to maintain a comfortable level. The fire brick makes it possible to sort of even out these cycles and maintain even heating trough varying degrees of fuel load.
    Taking this a step further, a second reason that that sort of "deep cycling" is a problem is that a warm stove (really the flue) is one that drafts well. Let it cool too much between fuelings and it takes a while for the flue to warm and drafting to reach its previous state. This drafting makes for a more efficient burn and also makes for more even heating. The cyclic nature of a wood stove is one of the down sides to wood heat, so minimizing it with some sort of thermal mass clearly is still an advantage.

    Paul
    Paul Carpenter
    Mapleton, IL

  6. #6
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    I hear what your saying but wouldnt cast iron be even better? the thermal mass has got to be twice that of the brick for the same area and it would buffer without as much insulation properties, My stove blower stays on 2 hours after the fire goes out so I know I have plenty of thermal mass, Iv seen cast iron grates in the old coal stoves I used to heat with so I would think that cast could take the heat, Maybe they would transfer a little too much to the metal box and warp it? --- Im not going to be happy till I can "hop it up a little" as its just my nature (I even modified my vacuum cleaner)

    The better the insulating properties the less that gets through to the other side --- this equates directly to the rest of the stoves area having to "milk out" the extra heat energies and if the surface area is lacking there (or all sooted up ((another fine insulator)) then the heat energies will indeed pull an "elvis" on you (leave the building).
    Cast Iron could prove to be the best of both worlds, even more thermal mass that also allows for better heat transfer --- with enough insulating properties so there's no real concern ? I bet they make them.

  7. #7
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    Default There's firebrick and then there's firebrick . . .

    You might substitute hot face firebrick for the lighter, more fragile insulating firebrick. Here in NC the Tractor Supply a mile away has both for a good price. The denser brick will have a much greater thermal mass but may, as per Forest, adversely effect the burn efficiency. I doubt it would make too much of a negative impact for a woodburner, though.

    regards,

    Jim

  8. #8

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    Quote Originally Posted by A.K. Boomer
    Cast Iron could prove to be the best of both worlds, even more thermal mass that also allows for better heat transfer --- with enough insulating properties so there's no real concern ? I bet they make them.
    I'm truly astonished that all the stove manufacturers seem to have missed such an obvious improvement.
    .
    "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

  9. #9
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    I remember being stationed at Ft. Leonard Wood, MO in the winter of 65/66. We were pretty much using WWII facilities, which meant coal fired furnaces in the uninsulated barracks (toilet bowls and butt cans would freeze), pot-bellied cast iron stoves in the mess halls and field "instructional huts".

    Those pot-bellied stoves didn't have any brick, and they threw off a tremendous amount of heat (at least to whatever side you had facing them).
    Lynn S.

  10. #10
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    I hear what your saying but wouldnt cast iron be even better? the thermal mass has got to be twice that of the brick for the same area and it would buffer without as much insulation properties


    Probably good thinking but its not area...but rather volume...so if you had a stove made of cast iron as thick as your current stove *including the fire brick* you would be all set :-) You would also need a lot of floor bracing for that. As heavy as fire brick is per unit volume, I would venture to guess that its not even close to the density and therefore mass as cast iron.

    Actually though, I would think that even if both contained as much heat energy and the fire were suddenly shut off, the fire brick would radiate it back out more slowly than cast iron...that's my gut and not a scientific analysis or "heat loss" factor calculation. I may be wrong though since fire brick is somewhat porus and therefore I would think it has more surface area.

    Realistically though there are other good reasons to use fire brick even if its a compromise....its a lot easier to work with and can be replaced in small pieces. Also it can be removed from the stove and put back in when the stove is in place, making the weight more modular. Fire brick has to be less expensive per unit of mass too.

    Paul
    Paul Carpenter
    Mapleton, IL

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