Calling all soapstone stove owners...!

[jetsam]

Make it even simpler...

Inside your house you have a box that makes a fixed number of BTUs. Some BTUs will go into the house; some will go up the flue.

If the box is wrapped in (or made of) insulation, a poor thermal conductor, the house gets less BTUs and the outside gets more.

Take the insulation off, and the house gets more and the outside gets less.

 

[WoodyIsGoody]

The amount of heat radiated at any instant is indeed dependent solely on the temperature of the object.

And what material it is. Different materials radiate heat at different rates even when they have the same surface temperature.

 

[WoodyIsGoody]

Make it even simpler...

Inside your house you have a box that makes a fixed number of BTUs. Some BTUs will go into the house; some will go up the flue.

If the box is wrapped in (or made of) insulation, a poor thermal conductor, the house gets less BTUs and the outside gets more.

Take the insulation off, and the house gets more and the outside gets less.

That's a nice simplified version. Those who want a more accurate and complete representation of the real world dynamics will have to delve into a more detailed analysis. Because it is literally impossible to accurately characterize the effect of more/less jacket insulation without looking at the chain reaction such a change can make.

For example, if the simplified explanation you provided was accurate, then the most efficient stove of all would not have any jackets on. It would be a bare, cast iron, unjacketed stove. However, the most efficient stoves always have a jacket around the firebox that effectively reduces heat transfer to the room vs. unjacketed. Yet they boast very high efficiency numbers.

The truth of the matter is that high efficiency results from a balance of design features that provides the optimum amount of heat transfer to maintain efficient combustion while keeping flue temperatures above the condensation point. This can be achieved with or without soapstone. There are many ways to skin a cat (and in this specific endeavor, some of them involve cats)!

 

[Highbeam]

They have jackets made of soapstone (with an airspace). In my example, I pointed out that two different materials, at the same temperature, can radiate different amounts of heat. It has to do with the emissivity of the material. Here is a list of the coefficients of emissivity of various materials:

http://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html

A non-contact thermometer generally assumes an emissivity of .95 unless it is adjustable and you enter the correct value for the material being measured. I have a very accurate non-contact thermometer with the ability to change the emissivity coefficient to suit the material being measured. In most cases it is more accurate than trying to obtain a surface temperature using a contact device. Those who claim non-contact thermometers are not accurate either don't understand proper operation of one or are speaking of low quality devices (perhaps even lacking adjustable emissivity).

I read your chart and I think it is BS or that you are applying it improperly here. The chart shows 10 times more emissivity for cast iron as polished steel. I surely hope that you are not trying to say that the cast iron will feel ten times hotter than a polished steel plate of the same temperature. That's how your post reads.

 

[Highbeam]

The amount of heat radiated at any instant is indeed dependent solely on the temperature of the object. However,maths temperature of the outside of the stove is dependent on the capacity of the stove's body to store and conduct heat. I don't know the relative specific heat values for steel or stone, but steel will conduct heat much more quickly. Steel stoves will get hot quicker, often getting hotter than a stone stove. Likewise, they will also lose their heat to the room after the fire goes down more quickly. This can make the room where the stove is colder in the long run.

Think about a house with a stone wall vs a house with a framed house with no insulation. After a hot day, the solid stone wall will be toasty warm all night compared to a framed wall. During the next day, as the sun heats everything up again, the stone house may feel cooler than outside,mbut the framed house will be an oven. (Not a perfect analogy, but it works)

I've had a stone stove. I hated this thermal mass "feature". Instead, if you simply buy a stove that only gets as hot as you desire, you can eliminate the need/value of thermal mass. You just need a more controllable heat source if you want to maintain the temperature of the room with more precision.

 

[BKVP]

Let's not forget the need to be able to sell the product to the consumer. If you have shielding, you can reduce clearances. Ah ha! Many consumers want a stove that can be installed closer to the wall or tighter into a corner. Few want a stove that has some large clearance requirement, regardless of efficiency.

As an example, one manufacturer had units that required R Value hearth pad protection. Retailers for decades tried to convince the mfg they simply needed to raise the legs of the units by 1" and the R value requirement could be eliminated. Doing so would remove the hurdle by the consumer to have to shell-out more money for a hearth pad with and R Value rating.

Just over a year ago, the legs were raised 1" and the R Value pad requirement was eliminated. More stoves are now being sold.

It's just marketing!

 

[WoodyIsGoody]

I read your chart and I think it is BS or that you are applying it improperly here. The chart shows 10 times more emissivity for cast iron as polished steel. I surely hope that you are not trying to say that the cast iron will feel ten times hotter than a polished steel plate of the same temperature. That's how your post reads.

No, it's real. Whether it subjectively feels exactly 10 times, probably not (due to the subjective nature of how our bodies perceive things). The best example I can give of subjectiveness vs. measured intensity has to do with light levels. Look through an SLR camera viewfinder while pushing the aperture stop down button with the aperture set one stop shy of wide open. Then compare the perceived brightness of that view with the un-stopped view. While the amount of light transmitted un-stopped will be twice as much, it will only appear a bit brighter. Such is the subjective way in which our senses perceive the world around us.

And that's why we have objective measuring devices to quantify the amount of heat, light, mass, etc. But, yes, cast iron will feel like it's radiating A LOT more heat than polished steel! Ten times? Who is to say except for an instrument capable providing an objective measurement?

 

[Highbeam]

No, it's real. Whether it subjectively feels exactly 10 times, probably not (due to the subjective nature of how our bodies perceive things). The best example I can give of subjectiveness vs. measured intensity has to do with light levels. Look through an SLR camera viewfinder while pushing the aperture stop down button with the aperture set one stop shy of wide open. Then compare the perceived brightness of that view with the un-stopped view. While the amount of light transmitted un-stopped will be twice as much, it will only appear a bit brighter. Such is the subjective way in which our senses perceive the world around us.

And that's why we have objective measuring devices to quantify the amount of heat, light, mass, etc. But, yes, cast iron will feel like it's radiating A LOT more heat than polished steel! Ten times? Who is to say except for an instrument capable providing an objective measurement?

Interesting. I wonder how an enameled cast iron stove fares here. The enamel might actually have a pretty big impact on radiated energy.

Another funny thing about radiated heat is that it is not directly related to temperature, it is change in temperature to the 4th power. Doubling temperature (absolute) results in 16 times the radiant output. So increasing the temperature of the stove by 25% from 400 to 500 results in much much more than a 25% increase in radiation output.

http://www.rumford.com/radiant/formulas.html

We know that stone stoves are typically run at lower temperatures which is, in my opinion, the real reason that people think that the stone stoves are a "softer" heat. They're just colder.

 

[WoodyIsGoody]

Interesting. I wonder how an enameled cast iron stove fares here. The enamel might actually have a pretty big impact on radiated energy.

I'm not sure. I bet it doesn't change hugely due to the fact that glass and cast iron do not have vastly different emissivity coefficients.

http://www.rumford.com/radiant/formulas.html

Very nice reference! Added to my bookmarks.

 

[bfitz3]

And what material it is. Different materials radiate heat at different rates even when they have the same surface temperature.

Yeah, but not so much. Thermal radiation is mightily independent of most material and can be approximated as black body radiation. See paragraph 1 of... https://en.m.wikipedia.org/wiki/Black-body_radiation

There really is no perfect black body though, as a body's emissivity comes into play. Note that in the chart in the following link, the emissivity for the stone and metal objects are nearly identical, aside from polished metals, aka mirrors. https://en.m.wikipedia.org/wiki/Emissivity

Stone and metal objects at the same temperature will radiate the same amount of heat at least to the extent that people standing next to the object won't know the difference.

Metal stoves heat more quickly and to a higher temp. At max temp, they will be hotter than a stone stove that heats slower and doesn't hit the same max temp.

Stone stoves, conversely, will heat more slowly, but once to temp, will retain/release the heat longer (look up specific heat and consider total mass)

Meanwhile... Does anyone remember the question asked by the OP?

Have fun, I'm out.

 

[WoodyIsGoody]

Yeah, but not so much. Thermal radiation is mightily independent of most material and can be approximated as black body radiation. See paragraph 1 of... https://en.m.wikipedia.org/wiki/Black-body_radiation

The term "black body" describes a theoretical perfect emitter which doesn't exist. The term was created for purposes of discussing/developing physics theories. It is entirely a theoretical construct which is fine if you want to approximate things but it doesn't mean the approximation becomes reality! That's why a decent infra-red non-contact thermometer will have adjustable emissivity. Otherwise your readings will be wildly inaccurate depending upon what material you are measuring. In other words, yes, different materials do radiate vastly different amounts of thermal radiation even when they both have the same surface temperature. And a materials emissivity coefficient will change with temperature. Some materials change more than others at stove temperatures.

If emissivity is something you are interested in, I recommend checking out this (big) list of emissivity coefficients at various temperatures as it is far more complete than the wiki link you cited:

http://www-eng.lbl.gov/~dw/projects/DW4229_LHC_detector_analysis/calculations/emissivity2.pdf

 

[Highbeam]

The discussion on emissivity and its effects on radiation is only valid in the context of this thread if we identify the emissivity coefficients of the various stove materials. Then we may be able to conclude that it is relevant or not.

Then of course comes the more important (much more important) realization that it's not about the stove material but about the temperature of the outer skin that determines radiant output. Some designs run cooler skin temperatures (double wall, high thermal mass) which results in less intense radiation.

Bring it back to stoves.

 

[SuperJ]

I think the arrangement of the room almost is more significant than the material. If you sit in front of the window of any stove you are going to get blasted with radiant heat from the firebox. Some more convection oriented stoves allow you to still quite close to the sides without overheating. My parents have seating within a couple feet of the side of their PE Alderlea (cast over steel = convection) and you could sit there all day. It's warm and cozy but never intense (unless the whole room overheats), but if you sit in the chair 8-10 feet from the front of the stove looking at the fire, it can be pretty intense with a decent fire going.

Don't oversize a stove in a small room, just because it's soap stone.