Calling all soapstone stove owners...!

[YoungGirlOldHouse]

I'm deciding between a soapstone stove or a cast iron stove to heat a poorly-insulated 800 ft2 space over 3 floors. The main floor that the stove would be on is the bottom floor of three, and it's about 400 ft2. This is the living area and I'm a bit concerned that it might get too hot for comfort. My questions are:

- In your experience, how close can you comfortably sit to a soapstone stove (I'd buy a Woodstock). Is this significantly different to a cast iron one (I'd get Lopi or Jotul)? The stove would be producing about 20,000 BTU and sofa would be about 5-6 feet away from the front of the stove.
- Would a cast iron stove with convective design be better at distributing heat to a multi-story space than a soapstone stove? The Lopi specifically says it has radiant heat but also has a convection opening for air to circulate through.
- Would these stoves use approximately the same amount of fuel to heat the space to the same level?

Thanks so much for any insight! (Note: I'm actually considering a gas stove, but there seem to be a lot more users of wood here, and the general principles should be the same, I think.)

 

[bfitz3]

I sometimes need to move the dog and will lay in what he believes is "his spot" right in front of he stove while watching a movie. I'll be close enough to the hearth in summer that I can use it as a really convenient coaster for a cold beverage. In winter, I feel the heat radiating from the stove, but also the convective currents wash cooler house air over me. It makes it a nice place to lay down.

For you, in such a small space, I'd be more concerned with getting the entire room up in the 80s, making it uncomfortable if not unbearable. I think I'd go with smaller soapstone to moderate the temperature peaks/valleys.

 

[YoungGirlOldHouse]

Thanks for the reply - very helpful! Could you elaborate on what you mean by a smaller soapstone stove? I was thinking of either the medium Woodstock soapstone stove (around 18,500 BTU to 22,500 BTU input), or the large Woodstock soapstone stove (around 22,500 BTU to 34,000 BTU input). The smallest one only has around 8000 BTU input, which seems to me too small. You think the heat is likely to remain in the 400 ft2 room rather than migrating upstairs?

 

[YoungGirlOldHouse]

Also: so you're saying that you can comfortably sit within a few feet of the soapstone stove, even while it's in use, right? Do you do anything to get those convective currents or do they arise naturally? Thanks!

 

[Woody Stover]

We sit fairly close to the Keystone. I'm about 9' away, she's about 6' but we are kind of off the front corner so the radiation isn't quite as much as directly in front or square to the side. Main room is 720 sq.ft. but the door to the bedroom is open, which adds another 270. Not too well insulated and a bit leaky, but further south than you are.
The heat should rise to the upper levels, keeping the lower level tolerable, but 5' is pretty close. Anywhere within line-of-sight of the stove will benefit from the radiation, even if the actual air temp is a bit lower downstairs.The upper level might be a bit warmer than you want for sleeping. However, the Keystone can be run at lower output than the other tube stoves you mentioned, so it should be the least likely to roast you out. It will also use less wood, and if you need to let the fire mostly die out to keep room temp down, it will idle along for a while on heat stored in the stone.
I would think the gas stove's output would be totally controllable, but might not give as good a balance of heat distribution as the soapstone stove between the upper and lower level.

 

[Woody Stover]

The smallest one only has around 8000 BTU input, which seems to me too small. You think the heat is likely to remain in the 400 ft2 room rather than migrating upstairs?

The Keystone is rated 700-1300 sq.ft, and the Woodstock ratings are conservative, so I think the Keystone would handle your place even though it's not too tight. As I said above, as long as you are within sight of the stove, you'll feel the radiation.

 

[beardley]

The tolerability of the heat from the soapstone was a huge reason we went with the keystone. The close side of the couch is about 6 feet from the front of the stove, offset just a bit to the door side. Far side of the couch is about 10 feet. We sit there comfortably all winter. Our first floor is about 800 sq' with another 600 upstairs. I'm north of Syracuse NY, so a fair bit colder, but the house is new construction and insulated very well. When we get below 0F the keystone is just barely keeping up and the radiant might start to kick in.

I'd agree with the sentiment that the soapstone would feel better in the small space compared to the cast iron. If soapstone became off the table, I'd make sure I was looking at a convection model over a radiant model.

 

[YoungGirlOldHouse]

Thanks beardley, Woody Stover, and bfitz3 - really helpful insights! It sounds like the soapstone would be a good pick for my house.

One last question. It sounds like the soapstone is giving out less intense heat than a cast iron stove would. Do you think that it is giving out *less* head overall than cast iron? I.e. if I were to buy a Lopi cast iron stove, which has a convection element built into it, would that be giving out more BTUs for the same amount of energy input than a Woodstock soapstone stove? Would the heat get upstairs better with the cast iron/convection?

 

[beardley]

If that were the case, I think you would see elevated stack temps on the woodstocks, compared to other cat stoves. I think its more just a matter of the rate at which the material can retain / release the heat. The soapstone will stay hot long after the fire is out, where as the cast iron would be much more cool

As to which would move heat upstairs better, I think the more important factor is they layout of the home. In my case, the stove in right in the middle, and I have an exposed timber frame. The ceiling is 2" T&G pine, which is also the 2nd floor finished floor, so the heat just goes right though. Its generally only a couple degrees cooler upstairs. It will certainly vary based on what type of layout you have.

 

[begreen]

The Keystone sounds like a good choice. During mild weather the low output will be more comfortable. When looking at non-cats like Jotul or Lopi makes, you need to look at the different construction options. There is a big difference between their all cast iron stoves (like the Jotul F500 Oslo) and their cast-iron jacketed steel stoves like their F45 or F50. The pure cast iron stove will radiate heat directly from the sides, front and top. The cast iron jacketed stoves will be much less radiant except out of the front. Instead the cast iron jacket will act as a buffer and will store some of the radiated heat. You can comfortably sit alongside of a jacketed stove much like a soapstone and the heat is released gradually over time, much like soapstone. For the best of both worlds you might look at the Blaze King Ashford which is a catalytic, cast-iron jacketed steel stove.

 

[Highbeam]

If you close your eyes you can't tell the difference between cast, stone, or steel of the same temperature. It's a marketing scam. What does matter is if you lower the surface temperature of the stove by adding a second shell of any material. That makes the outside cooler so you can sit closer.

 

[electrathon]

One last question. It sounds like the soapstone is giving out less intense heat than a cast iron stove would. Do you think that it is giving out *less* head overall than cast iron?

Absolutely. Heat is either going to go into the room or up the chimney. The reason the stove output is so "soft" is because it is shielding the heat from the room.

If you close your eyes you can't tell the difference between cast, stone, or steel of the same temperature. It's a marketing scam. What does matter is if you lower the surface temperature of the stove by adding a second shell of any material. That makes the outside cooler so you can sit closer.

Heat is heat. Some marketing genius made up the radiant convection explanation and woodstove people bought it. The stove is in the room with you. There are no ducts or vents around your house. The stove emits heat, the room gets warmer. Slow the heat output down with soapstone and you can sit closer to the stove because the heat is being diverted up the chimney. Speed up the heat transfer with steel plate and your room heats up faster. You use less wood, you get more heat. Sometimes you do not want more heat, in that instance soapstone is awesome (it is very pretty too).

 

[begreen]

It's not all marketing. I've sat next to our pure cast iron stove and also our current cast clad stove. Blindfolded, you would note the difference right away. The Castine had much stronger heat radiating from its sides. This was also notable in wall temp. Even with a heatshield on the Castine and 4" increase in corner clearance the F400 heated up the walls significantly more than the T6 does. The buffering effect of the cast iron jacket also translates into room temp swing which is considerably less with the T6.

 

[Highbeam]

It's not all marketing. I've sat next to our pure cast iron stove and also our current cast clad stove. Blindfolded, you would note the difference right away. The Castine had much stronger heat radiating from its sides. This was also notable in wall temp. Even with a heatshield on the Castine and 4" increase in corner clearance the F400 heated up the walls significantly more than the T6 does. The buffering effect of the cast iron jacket also translates into room temp swing which is considerably less with the T6.

Don't you think that that is just the difference between shielded and unshielded. NOT a property of the material used. It is marketing. Stone/cast does not feel cooler unless it IS cooler.

Cooler outside temps will feel cooler and radiate less heat to the wall. Several things can make that outer shell cooler like gap width, actual different firebox temps, etc.

The amount of heat being radiated from a body only depends on temperature of the body. NOT what the mass is made of.

 

[begreen]

For certain it is cooler because of the convective air that flows between the firebox and jacket. There is less heat radiating from the jacket because of this. The mass helps even our the temp swings, though it does also slow the room warming a little until the mass of the stove and jacket is up to temperature. My friend's Summit is a bit more responsive in this regard.

 

[WoodyIsGoody]

Don't you think that that is just the difference between shielded and unshielded. NOT a property of the material used. It is marketing. Stone/cast does not feel cooler unless it IS cooler.

The surface temperature is not always directly proportional to the amount of heat radiated. This is a minor effect in most cases but different materials will radiate more or less heat at the same temperature. That's why you can point an infra-red non-contact thermometer (often incorrectly called a "laser thermometer") at two objects of the same temperature and get different readings.

A bigger variable will be the thermal conductivity. Soapstone has more insulation value than iron. This means you can take two identical stoves, both jacketed in soapstone, replace all the soapstone in one with cast iron of equal dimensions, and you will find the one with soapstone will have lower surface temperatures and a higher proportion of convective to radiant heat vs. the one replaced with cast iron. That's because soapstones greater resistance to thermal conduction raises the temperature between the jacket and stove and creates faster and bigger convective currents (more convective heat) while at the same time radiating less heat. More convection, less radiation. Never mind that soapstone is often marketed as being superior because it provides "gentle radiant heat". Yes, the radiation is less assuming the same surface temperature) so it's gentler. It's less radiant, more convective.

The net effect on efficiency is impossible to determine except in specific conditions and, in reality it doesn't matter because a soapstone stove is never an identical design to a cast iron jacketed stove - the stove designers design each stove around the materials used. So, a soapstone stove that naturally insulates more would likely be designed to allow more convection to compensate. Or it would have less insulation around the secondary burn. The point being, it's the specific design of the stove that determines it's subjective qualities and it's efficiency, not the material the designer used.

In general, for maximum efficiency, it's important to keep the secondary combustion chamber hot. This requires insulating materials. IMHO, those insulating materials should be around the secondary combustion chamber (not in the stoves outer jacket). I think soapstone has almost zero utility in modern stove design except as style/decor. No doubt very efficient stoves can incorporate it but the same performance and feel can be realized by a cast iron jacketed stove that was designed to accommodate the somewhat different properties of cast iron.

 

[jetsam]

Imagine the heat output of a steel stove. (Mmm!)

Now cover a lot of the outside of the same stove with a substance vastly less thermally conductive than steel- maybe Roxul insulation or soapstone.

Soapstone isn't a little less thermally conductive than steel- it's a lot less. For example, Tulikivi claims their soapstone has a thermal conductivity of 6.4 W/mK. That puts it in the neighborhood of 12% of 0.5% carbon steel.

Will the house be getting heat? Sure. Will it be getting the same amount of heat it did before the stove was insulated? I really don't see how.

 

[Highbeam]

The surface temperature is not always directly proportional to the amount of heat radiated. This is a minor effect in most cases but different materials will radiate more or less heat at the same temperature. That's why you can point an infra-red non-contact thermometer (often incorrectly called a "laser thermometer") at two objects of the same temperature and get different readings.

A bigger variable will be the thermal conductivity. Soapstone has more insulation value than iron. This means you can take two identical stoves, both jacketed in soapstone, replace all the soapstone in one with cast iron of equal dimensions, and you will find the one with soapstone will have lower surface temperatures and a higher proportion of convective to radiant heat vs. the one replaced with cast iron. That's because soapstones greater resistance to thermal conduction raises the temperature between the jacket and stove and creates faster and bigger convective currents while at the same time radiating less heat. More convection, less radiation. Never mind that soapstone is often marketed as being superior because it provides "gentle radiant heat". Yes, the radiation is less assuming the same surface temperature) so it's gentler. It's less radiant, more convective.

The net effect on efficiency is impossible to determine except in specific conditions and, in reality it doesn't matter because a soapstone stove is never an identical design to a cast iron jacketed stove - the stove designers design each stove around the materials used. So, a soapstone stove that naturally insulates more would likely be designed to allow more convection to compensate. Or it would have less insulation around the secondary burn. The point being, it's the specific design of the stove that determines it's subjective qualities and it's efficiency, not the material the designer used.

In general, for maximum efficiency, it's important to keep the secondary combustion chamber hot. This requires insulating materials. IMHO, those insulating materials should be around the secondary combustion chamber (not in the stoves outer jacket). I think soapstone has almost zero utility in modern stove design except as style/decor. No doubt very efficient stoves can incorporate it but the same performance and feel can be realized by a cast iron jacketed stove that was designed to accommodate the somewhat different properties of cast iron.

I think we're saying the same thing. You just use WAY more words.

I don't know of anybody that sells a soapstone jacketed stove. Both companies selling stone stoves use it as the primary building material with no inner welded iron firebox. You definitely see this with cast iron shells over welded steel fireboxes.

The OP wasn't asking about the radiant properties of double wall stoves vs. single wall. Specifically, they were asking about the "softness" of heat radiated by the materials which is bogus. I propose that the only way a stove feels softer is if it is cooler and since stone stoves tend to spend a lot of time heating up and cooling off they also spend a lot of time "warm" which is what the marketing department is trying to exploit.

I've owned a stone stove and it still will burn you if you are running it hot to heat your home.

 

[jetsam]

I've owned a stone stove and it still will burn you if you are running it hot to heat your home.

MOAR WORDS. (I did not include any spreadsheets in that post; you can thank me later.)

It is however safer to touch a 600 degree material with poor thermal conductivity than it is to touch a 600 degree material with good thermal conductivity because you get less heat transfer in the same time period.

 

[begreen]

Some steel stoves have a soapstone side panel option and some include a top too. The Ideal Steel and Osburn Matrix are examples as well as some European stoves like the Rais Bando. Note that if you touch the side of the T6 with a 650º stove top you will not burn your hand unless you hold it there for awhile.

 

[electrathon]

The surface temperature is not always directly proportional to the amount of heat radiated. This is a minor effect in most cases but different materials will radiate more or less heat at the same temperature. That's why you can point an infra-red non-contact thermometer (often incorrectly called a "laser thermometer") at two objects of the same temperature and get different readings.

You get different readings for one of two reasons. First is because they are different temperatures. The infra-red has no idea what material the item is, just how hot it is. Second, infra-reds are not accurate, they are just an approximation. The HVAC guy I know refuses to use one, he said he thought they were the coolest thing is the world a few years ago, until he found out haw far off the readings were he was getting. He will now use nothing but contact thermometers.

 

[Highbeam]

Some steel stoves have a soapstone side panel option and some include a top too. The Ideal Steel and Osburn Matrix are examples as well as some European stoves like the Rais Bando. Note that if you touch the side of the T6 with a 650º stove top you will not burn your hand unless you hold it there for awhile.

Do those stoves use an air gap for a double wall effect or are they just stuck on there in a weak attempt to provide extra thermal mass and beauty? Decorative?

The temps of the outside of a double wall stove will vary relative to the temperature of the firebox skin for several reasons. This has nothing to do with the felt radiation from a surface of a particular temperature.

 

[WoodyIsGoody]

Do those stoves use an air gap for a double wall effect or are they just stuck on there in a weak attempt to provide extra thermal mass and beauty? Decorative?

The temps of the outside of a double wall stove will vary relative to the temperature of the firebox skin for several reasons. This has nothing to do with the felt radiation from a surface of a particular temperature.

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).

 

[WoodyIsGoody]

Second, infra-reds are not accurate, they are just an approximation. The HVAC guy I know refuses to use one, he said he thought they were the coolest thing is the world a few years ago, until he found out haw far off the readings were he was getting.

That's not true. Infra-red thermometers are used in many critical applications when accurate temperatures are critical. It's simply a matter of using a quality device properly and knowing how to calibrate it.

Some critical applications are more suited to non-contact temperature measurements. With a contact thermometer, the simple act of taking the temperature can change the temperature of the item being measured. Regardless of the method used, you need to know how to use the instrument to get acceptable results. A contact thermometer is very good at telling you how hot the thermocouple on the device is, but not so good at telling you how hot the subject is. To get an accurate reading, you still need to know how to best use the device at hand.

I think your friend had no clue about emissivity coefficients of different materials.

 

[bfitz3]

Don't you think that that is just the difference between shielded and unshielded. NOT a property of the material used. It is marketing. Stone/cast does not feel cooler unless it IS cooler.

Cooler outside temps will feel cooler and radiate less heat to the wall. Several things can make that outer shell cooler like gap width, actual different firebox temps, etc.

The amount of heat being radiated from a body only depends on temperature of the body. NOT what the mass is made of.

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)