Is Soapstone Heat "Softer"

[timfromohio]

I'm back to fantasizing about replaceing my PE insert with a stand-alone stove.

The room in which the insert is located is difficult to arrange furniture in, consequently the sofa (leather) is pretty close to the hearth. In winter we usually drape a thick blanket over that end of the sofa to minimize radiant heat directly on the leather.

I'm wondering if I do eventually replace the insert with a stove, would a soapstone stove be a better option in terms of how it heats a room? From a thermodynamics standpoint, I know this makes little sense - btu's are btu's, but am asking nonetheless.

 

[rideau]

How close is close, and, most importantly, is the sofa in front of or beside the stove? In front, it won't make any difference, as the glass allows lots of heat to be radiated. I have a progress hybrid soapstone stove and keep a chair quite near the side of the stove, comfortably.

 

[begreen]

Yes, the heat radiated off of soapstone is "softer" or less intensively radiant than off of a straight cast iron or unjacketed steel stove. I was surprised and delighted when I found the cast iron jacketed Alderlea had this same effect.

 

[timfromohio]

begreen - is the Alderlea top vent only? Those are very nice looking stoves.

 

[begreen]

Yes, top vent only. They are good looking enough to pass the wife's critical eye and a very good heater.

 

[firecracker_77]

I cannot say as I don't own anything else. I have to move the leather swivel chairs in my avatar pic when I've got it cranking either way. I wouldn't want anything that can't handle temperature too close either way.

 

[Backwoods Savage]

At first I thought it was pure baloney about the "soft heat." Well, it takes very little time to realize that indeed the heat does feel much different. I can go right up to our stove when it is running, say, at 650 or more and the heat is not harsh. Still, when you are talking about furniture or possibly something touching the stove, there is a point where you have to be careful....

 

[rudysmallfry]

My dad used to burn a cast iron stove. That beast used to blow us out of the room it got so hot. My soapstone stove is definitely a softer, more even heat. It just doesn't have those heating highs and lows that a cast iron stove would have when newly loaded and then down to coals. The soapstones hold the heat long after the wood has burned down to coals and keeps things more steady. LOVE my soapstone.

 

[jharkin]

It is also true that different materials transmit heat via thermal radiation at different rates, so it isn't so simple as a 'btu is a btu'. The measure of a a materials ability to directly radiate is called emissivity and it mostly depends on color and surface texture. Dull black surfaces (like black iron and steel) have a nearly perfect emissivity whereas shiny polished reflective surfaces have the lowest. Same principle that causes black cars to heat in the sun more than white.


Soapstone stoves tend to be lighter colors and radiate slower. Add the larger thermal mass of the stone and they can store a lot of heat and release it slowly over longer time period. You could also soften an iron stove by painting it in bright colored gloss enamel.

 

[begreen]

Is this effect really noticeable? It seems that there is more than this going on. I have been next to a white VC stove while burning an it radiated strongly. Conversely, our neighbor has an old Hearthstone H1 with very dark stone that radiates comparatively gently with a good load of wood burning. It would seem that the materials native emissivity would be a larger factor?

 

[jharkin]

I think it does make a measurable difference, and is the reason why radiators in big old houses used to get painted with aluminum paint. Back in the Victorian age folks left their windows open in winter for health, and thus heating systems where massively oversized. In later years when they stopped leaving windows open people overheated and they started bronzing or aluminum painting the radiators to soften the heat.

 

[begreen]

But that is only one factor, right? Doesn't mass come into play here?

 

[jharkin]

I'd have to go dig my heat transfer text out of the attic and look at the formulas. Im fairly sure mass doesn't effect the rate of heat transfer by radiation, just surface area, E value and delta T. The mass does store more energy so a larger mass will keep radiating longer after the fire is out. The total amount of heat from the fire doesn't change, but a heavy slow radiating stone mass stores more and spreads the transfer out over a longer time period, whereas a thin walled black steel stove just dumps the heat to the room quick and goes cold.

Not to forget that radiant stoves are also convecting as well. Heat transfer is complex.

 

[timfromohio]

Thanks for all of the replies.

I think there are many interelated factors - I'm not sure that emissivity alone is enough to explain differences in how the heat feels. Thermal energy is absorbed from the internal side of the stove, transmitted through multiple layers, all of which will have different rates of absorption, reflection, and transmission and then we get to the soapstone. Thickness and how intimate the contact, if any, between the soapstone and the backing layer(s) will come into play. Further all of the above will vary at different temperatures as the wavelength of the radiation changes.

You guys got me thinking back to heat transfer class ....

I need to take a picture of the current hearth/insert setup and post it to see what you guys think about a potential replacement.

 

[rideau]

Most of the Woodstock stoves (except the PH) are essentially all soapstone: inside structure and outside structure, with cast iron framing to hold the soapstone together. The massive weight of soapstone absorbs a tremendous amount of heat for the small size of the stove, and radiates that heat for a considerable time, producing a comfortable, even heat.

 

[Bluerubi]

I'd have to go dig my heat transfer text out of the attic and look at the formulas. Im fairly sure mass doesn't effect the rate of heat transfer by radiation, just surface area, E value and delta T. The mass does store more energy so a larger mass will keep radiating longer after the fire is out. The total amount of heat from the fire doesn't change, but a heavy slow radiating stone mass stores more and spreads the transfer out over a longer time period, whereas a thin walled black steel stove just dumps the heat to the room quick and goes cold.

Not to forget that radiant stoves are also convecting as well. Heat transfer is complex.

You're right on with this explanation, only thing missing is the heat capacity of the overall system. For a given weight of two different materials, say wood and steel in the extreme case, the steel has the ability to store more heat. If the original poster wanted to protect just that piece of furniture, they could throw convection into the mix and have a fan blowing against the stove. The effect would be increasing the film coefficient on the stove, thus transfering more heat by convection, and dropping the surface temp a bit which really knocks down the radiant output. Radiant heat has the temperature difference to the power of four, so even small changes in surface temp can be noticable if the surface has a high emissivity.

 

[jharkin]

Oh sure, there are all kinds of factors involved here. Like Tim says there is how the stove wall materials absorb heat from the fire (via radiation conduciton and convection), how that heat moves through the stove walls to the outside surface via conduction, how it gets to the air and other materials in the room via radiation, conduction and convection.

I just focused on the radiation element because that's the factor that really makes your skin feel hot when you stand in front of the stove window and I guess is the what people perceive as "harsh" or "soft"

 

[timfromohio]

You guys must be engineers too ... we are beginning to overthink this. I was actually looking up emissivity values over the weekend and contemplated starting a spreadsheet to create a simple 1-d model.

 

[jharkin]

Guilty... I'm a BSME.

You should see some of the threads from past years, we had arguments like this that went on for pages. An annual tradition along with the cat vs noncat threads.

 

[timfromohio]

Ha! BSME as well. Went on to grad school in materials science and engineering.

I have done simple 1-D models before for work-related projects and they never agree well with actually measured temp. values, so I didn't waste the time.