Simple comparison of Thermal Conductivity in typical stove materials

[MnDave]

Cast Iron ...... 660 Btu-in/hr- sf -
Steel ..............228 Btu-in/hr- sf -
Soapstone ..... 44 Btu-in/hr- sf -

By assuming the same temperature differential and firebox inside surface area, I can drop the and sf units.

I then apply a thickness to each of stove sides based on my best guess.
For cast and steel I assume a 5/16 inch thickness. For soapstone I assume a 3/4 inch thickness.
Applying the thickness I come up with a normalized power output comparison value for each material.

Cast Iron .... 2112 Btu/hr - 100% of cast
Steel .......... 730 Btu/hr - 35% of cast
Soapstove.. 60 Btu/hr - 3% of cast, 8% of steel

One thing that surprised me was that cast iron is superior to steel. I was misled by a graph that I saw which shows that for the same heat input, the steel stove surface measured 900 , the cast iron stove surface 700, and the soapstone stove surface 400. I now think that they switched the cast iron and steel in the graph.

Bravo cast iron. You are awesome.

One thing that does not surprise me is soapstones lower power output. This is why they call it "gentle". I'm still confused about the soapstone stove mfg's claims of high efficiency. Soapstone may be able to claim high energy conservation efficiency using an EPA test. But I do not see how they can claim the high "power output" levels unless they are using something hotter than plain cordwood like coal.

I really want to understand what is going on with soapstone. I trust the members who are loving their soapstones. I just don't understand the mfg's claims on the BTU's/hr on their big box stoves.

I am waiting for an "aha" moment on the soapstone story.

MnDave

 

[jharkin]

The missing part is that most of the heat transfer to the room from the stove surface is via convection and radiation. Conduction plays only a small roll beyond moving heat from the fire through the material to the outer surface of the stove.

The benefit of soapstone is more mass to store heat and then slowly radiate it to the room over a longer time period.

 

[Ashful]

Well, I think it was pointed out the majority of that peak power was thanks to radiation thru the front glass, although I still don't understand how that puts the PH so far beyond other stoves with similar size glass.

More importantly, you're looking at steady state conduction in what's really a transient problem. Don't ignore the heat capacity and mass of each material. Maybe it's worth putting the thermal simulator (I use CST Multiphysics Studio and CFDesign flomerics solver), if I could only find some time. It would be interesting to compare transient to steady state solutions.

 

[MnDave]

Lets assume all three virtual stoves have the same amount of glass, same amount of firebrick... etc.

The conduction through the stove surface is a valid place to compare thermal power output. Once through the stove material it is convected and radiated but it has to go through the stove walls by conduction first.

MnDave

 

[MnDave]

More importantly, you're looking at steady state conduction in what's really a transient problem.

You are right. And this is just a comparison of materials.

Any quote of power output implies an instantaneous measure else it is subject to distortion, which may be the case in what stove mfg's do.

MnDave

 

[pen]

which may be the case in what stove mfg's do

What is it that stove mfg's do?

pen

 

[WhitePine]

The apparent assumption here that cast iron is best, steel is second rate, and soapstone is the worst material for a stove is fatally flawed. The fact that soapstone stoves are premium products should be a great big warning flag.

Stoves are not constructed of a single material. They are built using specific materials for specific components of the stove. The manufacturers select their materials based on what is known to work and what is known not to work for the performance and price point they are shooting for. To use an overworked term, stoves are systems, not monolithic blocks of something. If stoves really performed on a par with the thermal conductivity of their external material, as implied by the OP, steel and soapstone stove manufacturers would soon be out of business.

Not to mention that wood stoves tend to be lined with firebrick, which has a thermal conductivity of suck all.

 

[FanMan]

Couple of things: First, I don't think your values for thermal conductivity for cast iron and steel are accurate. The numbers I find are 27-46 BTU/hr-ft-F for cast iron and 31 for mild steel (324-552 and 372 in the units used above). Other sources show steel slightly higher than cast iron, but cast iron varies a lot more depending on the density of the casting. Either way, certainly not the 3X difference given above.

A steel stove will likely be made of thinner material than cast iron, as it's a stronger material and it's difficult to cast iron as thin using the methods I assume are typically used for stoves. This means more heat will transfer through, at the same temperature difference, which is probably the major difference between steel and cast iron stoves.

However, conductive heat transfer isn't the only thing here. There is also radiant heat transfer, which is a function of the emissivity of the surface (and the surface temperature, of course, which comes back to the thermal conductivity of the material), and here there is a wide variation. I didn't find values for cast iron, but for steel, there is more than a 10x difference between polished steel and oxidized steel (polished - lower emissivity and thus lower radiant heat transfer). One would expect cast iron to be close to oxidized steel than polished steel, but the black paint on steel stoves would also help. The rougher finish on cast iron would also help (more surface area), but the shiny paint often used on stoves, especially stoves that aren't black, would work the other way.

There's also convection (heat transfer via a moving fluid, i.e. air), but again like radiation that depends on surface temperature... so for the sake of this discussion the thermal conductivity is the major factor.


So: Soapstone. I didn't find conductivity values for soapstone, but I'm sure the thermal conductivity is quite a bit lower. And it's thicker. However, the thickness may be (almost certainly is) part of the issue. Heat transfer isn't just from the inside to the outside, it also moves sideways through the material, from a hot spot to a cooler spot (and we know a wood fire isn't the same temperature throughout, nor is the stove). Heat from a hot spot in the fire will move along the walls to a cooler spot, heating that spot somewhat (and cooling the hot spot). In this, a thicker material works better (the sideways cross sectional area is greater). This may explain why cast iron stoves, being thicker than steel, are better, and why soapstone is even more even.

There's also the thermal mass of the thicker material, smoothing out variations in the wood fire's heat output, though again, I don't know the difference between soapstone and iron/steel.

Finally, the lower thermal conductivity of the soapstone stove may mean a hotter fire than the iron or steel stove, but I really don't know if this is the case.

 

[BrowningBAR]

Here's a question;

Why is it my steel stove can get up to temp faster than my cast iron stoves?

 

[Waulie]

As has been pointed out, the thermal conduction of the materials has little to do with the stoves ability to put heat out into the room.

Your "aha moment" should be this: While soapstone takes longer to heat up, none of that heat is wasted. The materials used have very little to do with heating efficiency. As a thought experiment, picture two stoves that are identicle in every way, except one is a steel box and one is a soastone box. Put the exact same amount of wood in and burn it the exact same way. Overall, heating efficiency will be practically the same. The difference is that the heating output curve for the steel stove will be a big spike that will rise fast and fall fast. The output curve for the soapstone stove will rise slow, reach a lower peak, and then fall slow. The area under the curve of both stoves will be practically the same.

The benefit of soapstone to me is not really that it radiates heat after the fire dies down. It is that the heat output overall is much more stable with lower peaks and higher vallies. Some say that once you're burning 24/7 the benefits of soapstone don't matter anymore. I'd say the opposite. You get the greatest benefit from soapstone by burning 24/7. The heat output of the stove will be steadier, more consistent, and not have those super high peaks you can get from a steel or cast stove (that is why it's "gentle heat"). Actually, if I was an occasional burner or needed to heat a cold place up fast very often, I probably wouldn't go with soapstone.

My soapstone stove is basically always warm during the heating season. Therefore, the slower to warm up "problem" with soapstone doesn't affect me. I would also add that the PH does begin to thow heat quite fast due to it's high efficiency (which reduces heat up the flue during warm up) and large window with the baffle slanted to direct the heat straight out. This is night and day from the time to get heat from the old Hearthstone 1 soapstone stove I grew up with.

Also, don't forgot that mass plays a huge part in heating equations.

The biggest factor to me is simple. I've owned stoves made of all three materials. For my situation, soapstone creates a much more enjoyable heating experience. Plus, it looks pretty sweet too.

 

[MnDave]

What is it that stove mfg's do?

They claim an instantaneous power output in BTU's/hr when there is no steady state input/output in a woodstove fire.

For comparison, a pellet/corn stove (with auger feed) approaches steady state and as such any power claims in their brochures etc are more meaningful.

MnDave

 

[BrowningBAR]

They claim an instantaneous power output in BTU's/hr when there is no steady state in a woodstove fire.

I have never seen this stated in any form what so ever.

For comparison, a pellet/corn stove (with auger feed) approaches steady state and as such any power claims in their brochures etc are more meaningful.

MnDave

That I agree with as it regards wood stoves. As I have mentioned many times; BTU ratings on a wood stove offer little value to the buyer. Firebox size is the best indicator as to the stoves ability to heat an area.

 

[MnDave]

Couple of things: First, I don't think your values for thermal conductivity for cast iron and steel are accurate. The numbers I find are 27-46 BTU/hr-ft-F for cast iron and 31 for mild steel (324-552 and 372 in the units used above). Other sources show steel slightly higher than cast iron, but cast iron varies a lot more depending on the density of the casting. Either way, certainly not the 3X difference given above.

You are comparing different units.
1 BTU/hr - ft = 12 BTU - inch/hr -sf -

I will admit to not using an exhaustive seach on the thermal conductivity values I used but in all cases that I found, cast was higher than steel.

MnDave

 

[MnDave]

Here's a question;

Why is it my steel stove can get up to temp faster than my cast iron stoves?

I am guessing that the porosity of the cast iron slows the heat through the material.

I too thought that this explained the graph I was referring to where steel leads cast.

Well, I hear my wife rustling around upstairs. I better go.

If someone can find better numbers then I am all ears.

MnDave

 

[rijim]

As has been pointed out, the thermal conduction of the materials has little to do with the stoves ability to put heat out into the room.

Your "aha moment" should be this: While soapstone takes longer to heat up, none of that heat is wasted. The materials used have very little to do with heating efficiency. As a thought experiment, picture two stoves that are identicle in every way, except one is a steel box and one is a soastone box. Put the exact same amount of wood in and burn it the exact same way. Overall, heating efficiency will be practically the same. The difference is that the heating output curve for the steel stove will be a big spike that will rise fast and fall fast. The output curve for the soapstone stove will rise slow, reach a lower peak, and then fall slow. The area under the curve of both stoves will be practically the same.

The benefit of soapstone to me is not really that it radiates heat after the fire dies down. It is that the heat output overall is much more stable with lower peaks and higher vallies. Some say that once you're burning 24/7 the benefits of soapstone don't matter anymore. I'd say the opposite. You get the greatest benefit from soapstone by burning 24/7. The heat output of the stove will be steadier, more consistent, and not have those super high peaks you can get from a steel or cast stove (that is why it's "gentle heat"). Actually, if I was an occasional burner or needed to heat a cold place up fast very often, I probably wouldn't go with soapstone.

My soapstone stove is basically always warm during the heating season. Therefore, the slower to warm up "problem" with soapstone doesn't affect me. I would also add that the PH does begin to thow heat quite fast due to it's high efficiency (which reduces heat up the flue during warm up) and large window with the baffle slanted to direct the heat straight out. This is night and day from the time to get heat from the old Hearthstone 1 soapstone stove I grew up with.

Also, don't forgot that mass plays a huge part in heating equations.

The biggest factor to me is simple. I've owned stoves made of all three materials. For my situation, soapstone creates a much more enjoyable heating experience. Plus, it looks pretty sweet too.

This is a good point for someone evaluating stoves such as the thread looking for opinions on Woodstock Fireview vs Jutol Oslo.

 

[rijim]

I am guessing that the porosity of the cast iron slows the heat through the material.

I too thought that this explained the graph I was referring to where steel leads cast.

Well, I hear my wife rustling around upstairs. I better go.

If someone can find better numbers then I am all ears.

MnDave

Wouldn't porosity esentially increase surface area?

 

[Ashful]

I have never seen this stated in any form what so ever.

I'm with MnDave on this point, and its what I was debating in the Woodstock thread, which I assume spawned this one. Because we're running a cyclic load, and not continuously adding fuel second-by-second as it's consumed, there's no true steady state. The Woodstock graph was showing the peak hit during a short part of a long cycle, and I believe this is to what Dave is referring.

Questionable conductivity numbers put aside, two very big factors not considered here are the emissivity of the materials and what's lining the various fireboxes. The OP just wanted to focus in the material conductivity use, which I can appreciate... but much like the point Machria argued in the other thread, this one bit of data does little to tell the story of how the larger system works.

 

[Waulie]

I am guessing that the porosity of the cast iron slows the heat through the material.

I too thought that this explained the graph I was referring to where steel leads cast.

Well, I hear my wife rustling around upstairs. I better go.

If someone can find better numbers then I am all ears.

MnDave

But why? Those numbers don't mean anything. Consider that modern stoves that use a secondary reburn actually insulate the firebox. What do you think this does to the conductivity? Are you claiming that an old, pre-epa cast iron box is more efficient than a modern, insulated firebox because it has a higher thermal conductivity?

You're making it more diffcult than it needs to be. Yes, you can look at all the equations for conduction, radiation, convection, etc. But really, it's not conceptually difficult to understand that the heat is either going up the flue or into the room. Those are the only two places the heat can go. In turns of overall heating efficiency is doesn't really matter what the stove is made of. It just matters that it isn't sending very much heat up the flue. Edit: and that it is burning as much of the wood including smoke, particulates, etc. as possible.

 

[BrotherBart]

Tell your wife I thank her.

 

[Waulie]

Also, I believe (but am not positive) that the high burn rate determined from the EPA test is actually an average over the high burn rate. I don't believe it some measurement over a short period of time, but rather the average over the entire high rate burn. So, to score high on that test, you need to do two things. The need to burn the load very fast while still burning very efficiently (without sending a bunch of heat up the flue). Again, the material the stove is made of is not going to really affect this test.

 

[MnDave]

Tell your wife I thank her.

Help me here. What is it about searching out facts to discuss as a group of intelligent adults that would bring out sarcasm?

Again, it is a comparison of stove materials.

MnDave

 

[BrotherBart]

It is you guessing at a comparison of stove materials because you don't believe soapstone stove makers claims. And there is no way to prove them one way or the other.

.http://www.chimneysweeponline.com/btutest.htm

 

[BrowningBAR]

I'm with MnDave on this point, and its what I was debating in the Woodstock thread, which I assume spawned this one. Because we're running a cyclic load, and not continuously adding fuel second-by-second as it's consumed, there's no true steady state. The Woodstock graph was showing the peak hit during a short part of a long cycle, and I believe this is to what Dave is referring.

Questionable conductivity numbers put aside, two very big factors not considered here are the emissivity of the materials and what's lining the various fireboxes. The OP just wanted to focus in the material conductivity use, which I can appreciate... but much like the point Machria argued in the other thread, this one bit of data does little to tell the story of how the larger system works.

My point was that there is never a claim of "instantaneous power output in BTU's/hr"

 

[ddddddden]

. . .I then apply a thickness to each of stove sides based on my best guess.
For cast and steel I assume a 5/16 inch thickness.
For soapstone I assume a 3/4 inch thickness.

I think cast stoves generally use panels that are thicker than those in steel stoves, hence the increased mass typically associated with cast stoves.
I don't know of any stone stoves that use 3/4" stone. Most are at least 2x that.

. . .If someone can find better numbers then I am all ears.

Here's the most comprehensive set of numbers I've seen on soapstone. I'll leave the conversion to you.
http://www.tulikivi.com/en/fireplaces/Soapstone_Properties_of_soapstone
Thermal conductivity: 6.4 W/mK
Specific heat capacity: 0.98 J/gK
Density: 2,980 kg/m³
Flexural strength along the grain: 16.8 MN/m²
Flexural strength perpendicular to the grain: 15.7 MN/m²

Stoves are not constructed of a single material. They are built using specific materials for specific components of the stove. The manufacturers select their materials based on what is known to work and what is known not to work for the performance and price point they are shooting for. To use an overworked term, stoves are systems, not monolithic blocks of something. If stoves really performed on a par with the thermal conductivity of their external material, as implied by the OP, steel and soapstone stove manufacturers would soon be out of business.

Not to mention that wood stoves tend to be lined with firebrick, which has a thermal conductivity of suck all.

+1, except for the firebrick. . .not used much in stone stoves.

My stone stove is probably ~30% iron.

 

[BrowningBAR]

I am guessing that the porosity of the cast iron slows the heat through the material.

I too thought that this explained the graph I was referring to where steel leads cast.

But you say right here:

One thing that surprised me was that cast iron is superior to steel. I was misled by a graph that I saw which shows that for the same heat input, the steel stove surface measured 900 , the cast iron stove surface 700, and the soapstone stove surface 400. I now think that they switched the cast iron and steel in the graph.

You are implying that the cast iron stove would come up to temp faster if the internal temps are identical. I can tell you this is not the case.

Additionally:

For cast and steel I assume a 5/16 inch thickness.

This is a bad assumption as cast iron stoves castings are far thicker than the materials on a steel stove.

One thing that does not surprise me is soapstones lower power output. This is why they call it "gentle". I'm still confused about the soapstone stove mfg's claims of high efficiency.

I can say that the soapstone stoves put out heat that is on par with steel and cast iron stoves. I also say the "gentle" heat is a BS term as there was nothing gentle about the Heritage when it was cruising at 600 degrees. The usable firebox on the Heritage is smaller than that of the Encore and the output was on par with it's firebox size.

I can say for a fact, that there is little noticeable difference between the three materials. If you had three non-cat 2 cu ft firebox stoves all using the same firebox design, each using one of the three materials, you would notice very little difference in terms of heat output between the three stoves if they were all cruising at the same temperature.

The main difference is how they come up to temp. Steel reacts the quickest, then cast iron, then soapstone. There is about a 20 minute difference from first to last when it comes to this. Hardly a difference at all.