Soapstone replacement for insert firebrick

[cycloxer]

Isnt the Craftsbury cast iron? I never sold one of those, but I assumed it was

Yeah it is a full cast iron stove. Inside the firebox there are 4 soapstone bricks (two on the floor and two on back wall) roughly 4"x6"x1" that they use to increase the thermal mass of the stove. Interesting concept.

I don't see any problem with modding a stove if there is something to gain, but I don't think there is in this case.

 

[LLigetfa]

Here is a reason why NOT to mess with the CTC of a stove. In the following example the install is not to code and one has to wonder why the place hasn't burned to the ground. It looks like an insert designed to sit on a hearth, not on a thin veneer of slate and backer board. It could well be the insulating property of the firebrick that is keeping the owner from being nominated for a Darwin award.

http://i628.photobucket.com/albums/uu5/bringitbig/9fd8c24d.jpg

 

[Fsappo]

Huh, the front of that stove looks like an old Defiant. I wonder what that critter is?

 

[Martin Strand III]

"Soapstone is a unique refractory and masonry material. Compared to a pound of concrete, a pound of soapstone can store approximately 20% more heat. Its main distinctive thermal property is that it has about 4X the conductivity of concrete or about 6X the conductivity of soft clay brick. Another way of saying this is that its R value is 1/4 that of concrete. It is somewhat similar to metal in this respect. This means that a soapstone heater of equivalent mass will heat up faster on the outside surface and reach a higher surface temperature, due to the high conductivity. On the other hand, the higher rate of heat transfer to the room also means that it cools down faster than other masonry materials. Understanding the thermal properties of soapstone..... we use soapstone heat transfer plates in castable refractory bakeoven floors to even out cool spots. A nice feature of soapstone is that it can be carved quite easily."

----- Part of a presentation at All Fuels Expo 96 by Norbert Senf, Burlington Vermont, February 2, 1996

I'll put my marbles with Norbert on this soapstone thing.

Aye,
Marty

 

[Fsappo]

Nope, never trust a guy named Norbert. I rest my case.

So now, im stuck with the notion that if I heat up a red brick, a firebrick, a pumice brick and a soapstone brick all using the same amount of heat energy the soapstone will cool off quicker? I'll try it, but it may have to wait until after the busy season for me to do it right. As far as equal energy, I could just place s sterno can under each one. That should do. Wait for the cans to run out of gel, come back in an hour and shoot them with a heat gun?

So Bert here is saying a Soapstone stove will heat up and cool down faster than a steel stove? This is worse than finding out Santa wasnt real..and that ruined my 23rd christmas

 

[cycloxer]

Question for Norbert: Then why are companies like Woodstock and Hearthstone using soapstone as a heatsink in their stoves? I think he is only comparing soapstone to other masonry materials and not metals.

 

[karri0n]

So Bert here is saying a Soapstone stove will heat up and cool down faster than a steel stove? This is worse than finding out Santa wasnt real..and that ruined my 23rd christmas

No, he's saying it will heat up and cool down faster than a brick or concrete masonry heater.

You also need to take into account the mass and the conductivity of these items. A pumice brick of equal mass to a soapstone brick will be much larger, and its relative lack of thermal conductivity means it will be pretty unlikey to heat up much at all in your "can of sterno" experiment. It's likely the top of that huge piece of pumice will stay cool the whole time. The heat from the flame will be refracting and going around the pumice stone rather than into it. An oven is a much more controllable heat source for the experiment. To establish a good baseline, You would need to measure the temp of each one before heating, directly after heating, and ten minutes later. By an hour it's most likely that they will all have cooled to room temp. Even this experiment could be a bit touchy in regards to the pumice due to its porosity. For frame of reference in regards to the pumice, picture trying to test how long a piece of fiberglass insulation holds heat. It's just not easy to heat up in the first place, and as soon as air touches it, it goes back down. The low mass, light color, high refractory value, and lack of conductivity of pumice are what make it so suitable as a firebox insulator.

 

[Fsappo]

I guess I'm just not smart enough to debate this. I will revert to my ole "The Mansfield in my showroom stays hotter longer than the steel stoves" Theory. I will though try the idea of lining one side of a steel stove with soapstone instead of firebrick and compare skin temps. Any holes in that type of experiment?

 

[karri0n]

I guess I'm just not smart enough to debate this. I will revert to my ole "The Mansfield in my showroom stays hotter longer than the steel stoves" Theory. I will though try the idea of lining one side of a steel stove with soapstone instead of firebrick and compare skin temps. Any holes in that type of experiment?

Sure, try it.

I'd also like to see how different the temp of the stove is during the burn. I don't know if only doing the one side will affect the overall characteristics too much,but it would be cool to see. I think you should do one burn with the firebox fully line with firebrick, one with it fully lined with soapstone, and one with the one-side thing.

 

[Fsappo]

If I do 2 burns, I guess I'll have to weigh the wood each time to make sure we're using the same input. I just dont have enough soapstone in stock to fully line a stove. I'll keep pestering myself about it.

 

[Martin Strand III]

Yu'all:

In order to compare apples to apples, the masonry samples need to be the same MASS and roughly the same shape. Size will differ; soapstone being DENSER will be smaller than fire brick, clay brick, natural rock, etc. No need to worry about the physics.

For a heat source, consider putting the samples in an oven on the regular oven metal rack at a constant temperature (say 400* F), apart but on the same shelf, for several hours to make sure all is heated thoroughly. After 2 hours, open the oven door, record the temperature of the samples (ideally without touching them) then and then hourly leaving the door oven open, no obvious drafts, etc.. Record results and report back. Voila! Oh, yes. No cheating to favor your thesis.

Aye,
Marty

 

[Martin Strand III]

Question for Norbert: Then why are companies like Woodstock and Hearthstone using soapstone as a heatsink in their stoves? I think he is only comparing soapstone to other masonry materials and not metals.

Norbert told me ;-) adding masonry to a metal stove improves (1) combustion efficiency in the firebox (insulating, reflecting heat back into the firebox to get a higher burn temperature) and (2) moderates (lowers) the heat transfer efficiency (metal stoves typically have lower combustion efficiency and high heat transfer efficiency) of the stove.

Aye,
Marty
PS: The ''ideal" :bug: wood burning stove/heater has high combustion efficiency along with moderate heat transfer efficiency.

 

[Fsappo]

So do I determine the mass by weight? a 5lb soapstone (firebrick sized is what I have) and find a chunk of typical firebrick that weighs the same?

 

[Martin Strand III]

So do I determine the mass by weight? a 5lb soapstone (firebrick sized is what I have) and find a chunk of typical firebrick that weighs the same?

By jove, I think you've got it!

Aye,
Marty
PS 1: Which weighs more, a pound of sand or a pound of feathers?
PS 2: In order for a test to be valid (by "the scientific method"), it must be reproducible, in every detail, by others who will get the same results.

 

[Fsappo]

That should be easy enough using the oven idea. I just need to go weigh my soapstone brick and find an firebrick the same weight and relative shape. A pound of sand weighs more than a pound of feathers, silly..and a pound of gold even more.

 

[Martin Strand III]

(SNIP) A pound of sand weighs more than a pound of feathers, silly..and a pound of gold even more.

Using a pure radiant energy source, which will heat up faster (therefore cool down faster) - the sand, feathers or gold? And why?

Aye,
Marty

 

[Fsappo]

The surface temp or the core temp when heating up? The side facing the radiant heat source if it is surface temp?

 

[Martin Strand III]

The surface temp or the core temp when heating up? The side facing the radiant heat source if it is surface temp?

Not to worry. Take a surface temperature of each sample at reasonable time intervals. Radiant heat physics (sic) will not penalize you for taking a surface temperature which will be representative of the entire sample after allowing enough time for sample equilibration; i.e., you'll be close for this experiment - you'll need better experiments to inhabit Mars, find alternate fuel sources, etc...

Aye,
Marty

 

[Corriewf]

Any update with this experiment Franks? A buddy of mine was thinking about this as well.

 

[Fsappo]

Not even close. It is go time at the fireplace shop with the cold weather here. I did stash a soapstone brick aside for it. I just need to weigh it and find some other masonry type materials of the same weight. I was going to try standard firebrick and pumice, just for kicks. I could always stack brick together to get the same weight, but would the air space in between each brick cause any issues with heat transfer?

 

[karri0n]

Considering the original question was simnply replacing the firebrick, as opposed to testhing thermal properties of the stone, I think using the same dimensions rather than the same weight for the in-stove test makes more sense. As far as the test of how the different materials hold temp, this is where you will need stones of the same mass

 

[Martin Strand III]

(SNIP) would the air space in between each brick cause any issues with heat transfer?

It does the same that air space between 2 panes of glass or air space between feathers of a duck: insulate; retards heat transfer.

Aye,
Marty

 

[Fsappo]

Well, we already established duck feathers weigh the same as brick, so we're beyond that. I was wondering if the air space, being so small (brick against brick) if it would insulate to the point of changing the goal of the test.

 

[oconnor]

I don't think replacing firebrick with soapstone would result in any sort of nuclear meltdown of the stove per se, but it could change the required clearance to combustibles (CTC) enough to create a fire hazard. If CTC is not an issue, it could still move the sweet spot of the stove to where it affects the EPA rating and efficiency.

That said, with my stove I time the removal of ashes with the outdoor temps and subsequent need for heat. With fewer ashes in the stove, I get a lot more heat out of it. I would not dare change out the firebrick for fear of changing the CTC as my stove is a zero clearance and as such has no possible additional margin of safety. I run my blower when the stove is putting out max heat but in a power failure without the blower, it is pushing the CTC to the max.

Any major modification to a stove that affects the transfer of heat one way or the other could create a hazard. If you kept more heat in by lining the top/sides with soapstone it could raise flue temps. If you wick away heat by removing insulation, it can cause a CTC hazard. I'm pretty sure both the stove manufacturer and your insurance company would take a dim view of such mods.

+1. For example, in an insert stove, (like the OP has) you want the heat to reflect forward into the room vice radiate rearward into the masonary behind the stove. Also, in a stove with small rear CTC, it would totally invalidate the clearances, as you would have more heat back there than before. Could also take out the blower as a result of increased operating temps.

Experiment aside, these are the relevant points for the original post IMHO.

 

[Martin Strand III]

Well, we already established duck feathers weigh the same as brick, so we're beyond that. I was wondering if the air space, being so small (brick against brick) if it would insulate to the point of changing the goal of the test.

For an "apples to apples" test - ideally, each specimen should be the same:
1. 1 piece each, same weight, no holes or 'spaces'
2. 2 pieces each, same weight, equal 'spaces' between

:exclaim: Why not use 1/2 piece (or so) of your firebrick and whittle down a red brick (or other) to the same weight?

Aye,
Marty