Lets compare soapstone, cast, and steel stoves

[sstanis]

Surprised, someone did not add to the original specifics, how is the house insulated, how many windows and what is there U-Value. The reason why I say this is that there is the new term: super-insulation. Up here in Westchester, NY there are some houses that are goign up like this: minimum of R-60 in attic and R-40 walls (spray foam insulation in walls plus 2-inch insulfoam on exterior) and high-value U windows. People have a geothermal system with heat recovery ventilator and heat there house for practically nothing, even give electricity rate of 20cents/KwH.

 

[Martin Strand III]

At the risk of opening old wounds, please consider:

The amount of the heat that is actually transferred into the room depends on a number of factors:

1. Combustion efficiency. How completely an appliance burns the wood

2. Heat transfer efficiency (thermal efficiency). How much of the heat generated in the firebox escapes through the chimney into the atmosphere (stack loss) depends on the firing strategy and the thermal mass of the heater. The combustion efficiency multiplied by the heat transfer efficiency, is the
overall efficiency, but this term still does not fully characterize the true efficiency of a heater.

3. Linearity of output. For clean combustion, wood requires a brisk, hot fire which results in shorter burn times. If the heater has no (or minimal with metal stoves) storage capacity the fire must be damped, otherwise to much heat is transferred into the room during the burn time. As a result, the room becomes overheated while the fire is burning and under heated after the fire has died down; i.e., "indoor weather", classic with metal stoves. An overheated room loses more heat to the environment because of the higher temperature differential between the room air and the outdoor air.

4. Partial charge efficiency. Many stoves do not handle small fuel charges efficiently because high temperatures are needed to obtain non-smoldering combustion.

And how efficient your wood heater operates depends on 2 more factors:

(1) Installation - location on outside v inside wall. Heater too big for house? Flue draw?
(2) Operation - Is wood dry? Wood species? % firebox load? Adequate air? Clean stove?

Conclusion: Your operating technique accounts for the largest variations in your woodstove’s heating efficiency.

Aye,
Marty

Grandma used to say, “Understanding yourself first helps avoid confusion in life.”
Dirty Harry said, "A man has got to know his limitations."
Are ya feelin' me out there?

 

[cbrodsky]

ok, so if all the stoves are identical in every way except the material they are made out of, then would the 40 lbs of white oak take the same amount of time to burn in each stove?

Wow, this thread screams "FLAME WAR!"

I think this really depends on the rate that you supply combustion air to the stove.

Burning wood in an EPA stove is essentially a controlled reaction with the limiting reagent being oxygen. If you supply oxygen at the same rate in all three stoves, then they will all burn for the same amount of time.

In a soapstone stove, if you keep the same oxygen feed rate, the fire will end up burning hotter in the stove, because the heat doesn't move to the room as easily - the stone stoves will generally slow the rate of heat transfer. In that scenario, more heat will be lost up the chimney, and it will burn out in the same time as the steel stove.

However, this is not how most people would operate a soapstone stove. Instead, a soapstone user would typically aim to have the same fire temperature inside the stove as any other stove (alternatively, you might regulate your flue temperature to the same condition with all three stoves) To accomplish this, you would have to slow the burn rate by cutting draft and feeding less oxygen with soapstone. As a result, your fire will last longer.

This is why an equally sized cast iron or steel stove will generally have higher potential peak heat output to the room, with the tradeoff being that you can run as long a burn cycle. Either will give you a similar total quantity of BTUs out of the wood - it's just how they get delivered that varies. Both have their place.

-Colin

 

[cbrodsky]

As for this specific question - assuming identical stove designs AND assuming all stoves reach temps which support efficient burning - the cast iron stove should burn the longest, steel almost the same, and soapstone slightly shorter. The reason is that soapstone has poor thermal conductivity, this results in less heat being delivered to the room, and more heat building up inside the firebox - a hotter firebox results in shorter burn times (again, the big assumption here is that even with the cast iron stove the firebox is hot enough to support full secondary combustion).

See my last post - you are missing a very key point. You are correct IF you run both at the same air feed rate. However, a soapstone stove burns with a slower supply of air, and maintains the required firebox temperature for the reason you explain. You simply cannot slow down a steel/cast iron stove to the same degree and maintain a hot clean firebox.

Personally I think there are a lot of myths that people believe when it comes to stove materials. I've seen one frustrated person comment in these forums about people ascribing "magical properties" to soapstone and I have to agree with this assessment (for example some say it absorbs all the heat like a sponge absorbs water - then gives it all back - this is junk science - yes it absorbs heat and stays warm longer - but there is a net loss).

I think I've said that on more than one occasion As a chemical engineer, the marketing stuff amuses me... but when you get down to it, it's basic heat transfer principles at work - the silly statements dilute what is sound science behind the reasons for different stove designs.

Its also "common knowledge" (probably wrong) that steel heats up faster than cast iron but doesn't hold its heat as long - I do not believe this is true - the thing is that many (most?) steel stoves are THINNER than cast stoves - which is the ONLY reason they can heat up faster and lose their heat faster when the fire goes out - however there are plenty of steel stoves today that have just as much mass as their cast iron equivalents. When you compare a 450 lb. steel stove with a 450 lb cast iron stove - these properties (heat up time and cool off time) are almost identical. On paper - cast iron should heat up SLIGHTLY faster and cool down SLIGHTLY faster than steel because it has a slightly higher thermal conductivity than steel but in reality I doubt you would even be able to tell the difference.

You are right that the mass differences are not that different. And soapstone stoves are often marketed for their "heft" and "huge storage capacity" but even there, I have posted on this forum before that the heat capacity of soapstone is only modestly higher than steel/cast iron. It's not enough to explain the different behavior of the stove. The difference comes by regulating the thermal conductivity to a slower rate than you get with the typical steel/cast stove design on the market.

As for soapstone - it has terrible thermal conductivity - which is why it takes so long to heat up and cool down after it is heated up - this should mean more wasted heat out the chimney EVEN AFTER full thermal mass has been established - this is also probably why Woodstock rates their own soapstone stoves so low as far as recommended heating area and so many reviewers have said it takes too long to heat up (yes, I know, others here don't believe the reviews). The simple fact is that anyone could turn their steel or cast iron stove into a soapstone stove just by putting some soapstone (or other rocks) on top of it!

Some of this is not correct. If you were wasting extra heat out your chimney, you'd be using the stove incorrectly. Any stove user should be controlling their flue temperature to similar ranges. Soapstone stoves let you maintain the same flue/firebox temperature profile at a lower burn rate to extend the burn for the same sized firebox.

They do take an hour to heat up - exactly why I wouldn't recommend it for a vacation cabin left cold. You could make a metal stove perform similarly, but it would take more than what you say. You'd need to enclose the entire stove in a thicker material that lowers thermal conductivity to make it behave equivalently. Essentially, you'd mimic the engineering design with equivalent materials. As I said, soapstone isn't magic - it's just convenient.

Note that I don't think soapstone is necessarily a bad choice - I think in certain situations it could work well, but from a purely "heat delivered to your living space" argument its the worst. From an "even heat" argument - again there is a cost to this - less total heat delivered to the room - if even heat is important to you then maybe you should go with soapstone. I personally would rather have my house heat up to say 75 degrees while the fire is going good, and perhaps fall to 65 after the fire dies out, than to have a stove that takes a long time (comparatively) to initially heat up the house and not deliver as much heat to the room while burning only to get a couple degrees of residual heat in the morning after the fire has died down.

Again, the overall efficiency of any of these stoves will be similar if they are burned as intended. If I overdamp a cast/steel stove to try to get the long burn time I can with soapstone, I'll lose secondary/cat combustion and ruin my chimney. If I try to run my soapstone stove as hot as a cast/steel stove, I'll waste energy with an excessively hote chimney.

If you are not a 24x7 burner, soapstone is not a great idea. If you are OK loading the stove more often, then you don't need soapstone. But if you size the stove correctly and need a long time between loads as I do, then soapstone can be a much better choice.

-Colin

 

[cbrodsky]

Well, I'm no scientist or expert on thermal conductivity, but I can honestly say my stack temps are lower with this stove than any other stove i've burned before, so there is no more heat going up my chimney than a steel/cast stove would put up there.

Not to beat a dead horse - exactly the point I just made in the other two posts. You simply extend the burn cycle with soapstone, providing a more even flat output at overall comparable efficiency, all other things being equal. You can get a lot more elaborate in the analysis, but to first order, the explanation a couple posts up should illustrate the concept.

I'm curious - surely some stove manufacturers must have tried an insulated or multiple wall system with steel to potentially deliver similar thermal conductivity results as a soapstone stove? I would think with the popularity of soapstone's heat behavior, it is only a matter of time before someone works out a steel/cast design with similarly damped response as the soapstone designs on the market.

I like the look of soapstone better than anything else, but I can see where there would be a good market for lower cost stoves that have the same ability for lower output, longer burns. Particularly in modern well-insulated houses where a steel stove can blast you out.

-Colin

 

[tradergordo]

Thanks Colin - your explanation about lower air consumption makes sense in theory at least - although I'd still have to see lab results before I believed it.

Anyway, you said "I’m curious - surely some stove manufacturers must have tried an insulated or multiple wall system with steel to potentially deliver similar thermal conductivity results as a soapstone stove"

Surely some?? They pretty much ALL have insulated/multiple wall "systems" - its called firebrick/refractory.

 

[cbrodsky]

Thanks Colin - your explanation about lower air consumption makes sense in theory at least - although I'd still have to see lab results before I believed it.

Anyway, you said "I’m curious - surely some stove manufacturers must have tried an insulated or multiple wall system with steel to potentially deliver similar thermal conductivity results as a soapstone stove"

Surely some?? They pretty much ALL have insulated/multiple wall "systems" - its called firebrick/refractory.

Well, as for lab results, combustion rate being limited by airflow is pretty clear cut. In the end, it just comes down to regulating your burn rate by watching your stack temperature no matter what kind of stove you have - in any case, you can burn it the wrong way - too hot or too cold - and screw yourself up. If I burned through a load of wood in the time a steel stove will burn through it, I'd have a meltdown inside the stove and severely overheat the chimney.

I realize most EPA stoves have multiple walls and firebrick, but a couple 1/4" plates of steel won't control heat transfer the same way a couple 1/2" slabs of stone can. As counter-intuitive as it might seem, I could see where it might be beneficial to have a steel stove with a layer of high temperature insulation around the firebox to lower the surface temperatures, keep the firebox hotter, and allow extended burns of the type achievable on soapstone stoves. Essentially, something where the composite heat transfer through the entire assembly is matched to a double-slab with air space soapstone design. It might not be right for every user, but could be for the ones who want to maximize burn time at the expense of peak BTU output. After all, if my fire never goes out, then I never have to crank up the stove to recover the house temperature Clearly there is a performance difference with the typical steel stove vs. what you can get with soapstone, but I would think somebody would have tried marketing something that closed that gap by changing the design.

-Colin

 

[begreen]

PE and Napoleon have steel multi-wall with firebrick. Maybe that's why they are popular heaters. Nice close clearances too.

 

[tradergordo]

Well, as for lab results, combustion rate being limited by airflow is pretty clear cut.

Not at all what I'm interested in. I'd want to know comparative airflow between every model stove. You claim soapstone stoves have lower airflow - this has to be proven. Every manufacturer gives various "max burntimes" for their stove models, which always have to be taken with a grain of salt - again this is something that needs to be lab tested, I'm not convinced just because its soapstone it gets a longer burntime.

If I burned through a load of wood in the time a steel stove will burn through it, I'd have a meltdown inside the stove and severely overheat the chimney.

This is a big generalization. There might be steel stoves out there capable of burntimes that are the same if not longer than your soapstone equivalent, all depends on how and with what they are lined. And like I said, they are all insulated with something.

 

[cbrodsky]

Well, as for lab results, combustion rate being limited by airflow is pretty clear cut.

Not at all what I'm interested in. I'd want to know comparative airflow between every model stove. You claim soapstone stoves have lower airflow - this has to be proven. Every manufacturer gives various "max burntimes" for their stove models, which always have to be taken with a grain of salt - again this is something that needs to be lab tested, I'm not convinced just because its soapstone it gets a longer burntime.

It's really just a basic energy balance - you don't have to look at manufacturer's numbers.

For any stove, you put fuel in, and heat comes out. It comes out the stove or it goes up the chimney.

I think we all agree that the chimney should be hot enough to prevent creosote, but not excessively hotter, because that is wasted energy.

So from that point, the main difference today is that any soapstone I've seen tends to have higher thermal resistance (as most on here agree I think... as it was pointed to as a "flaw" earlier in this thread) and any steel stove I've seen tends to have lower thermal resistance (again, most would agree - that's why they get so much hotter faster, as so many have pointed to).

To keep the same chimney temperature, there is no way to make this balance out unless you change the burn rate by slowing the oxygen feed rate to the soapstone stove. Again, I'm not saying one is better than the other - just that they are different.

I also don't see any fundamental reason a steel stove couldn't be designed to work this way, but to date, I have never seen one. I have yet to hear any steel stove user talk about their stove taking 30-45 minutes to warm up (with good wood), for example.

If I burned through a load of wood in the time a steel stove will burn through it, I'd have a meltdown inside the stove and severely overheat the chimney.

This is a big generalization. There might be steel stoves out there capable of burntimes that are the same if not longer than your soapstone equivalent, all depends on how and with what they are lined. And like I said, they are all insulated with something.

Yes, to be fair, let me clarify - I am referring to the example of the typical steel stove with high heat output, and corresponding shorter burn times, as most are. If I burned at that rate, I would melt the innards of a soapstone stove and/or overheat my chimney because the stove wouldn't conduct the heat out fast enough.

We're saying the same thing though - in theory, a steel stove design could be modified to serve one purpose or the other. But not both. And I'm just asking if there are any steel stoves that really have lower peak output/longer burn times on par with some of the current soapstone designs. So far, I haven't seen one, but I don't think that's because it's not possible. (and by seeing one, I mean not manufacturer claims, or user claims, but clear build details that explain a marked difference in thermal conductivity - which I think is the most significant differentiator) These things are fairly easy to model once you see the design in detail.

I suspect that the market just hasn't gone that way for steel, because soapstone is filling that niche, and hence soapstone is overbilled as "magical" vs. simply being an engineering design approach.

Now beyond this, I think there are other more complex differences like the radiation transfer differences from one surface to the other, but I think this entire thread does hit on some of the basic concepts that explain why today's typical soapstone stove is very different from today's typical steel stove.

-Colin

 

[stoveguy2esw]

i have personally had 12 hour burn times with loads of wood in my 24-ac, this is a steel stove , a first generation england catalytic model with a 2.4 CF firebox. cat temp measured with a condar probe thermometer was in the 800 degree range for virtually the entire burn, dropped off at about 11 hours but was in the 500 degree (active ) range at 12 hours. was burning a charge of locust firebox was full. i do not know the weight of the charge as i didnt exactly drag out the scale, i'd guess it at 35-40 lbs 4-6"splits granted locust is about as dense a wood as i can get in my area, oak charges similar in volume usually last about 9-10 hours at same temp on the cat. our 30-nc series stove have hit burn times of at least that long in our lab surface temps on the 30 were at the 550-600 degree range for most of that time during the runs. excepting startup and coaling periods when most would reload anyway. these are both single wall steel stoves with brick lining in the lower half (9") of the firebox. yes it can be done with steel, its all in how you manage your air and use your smoke(secondaries) for heat output through more complete burning

 

[Sandor]

With the soapstone, to me, it seems that since the stone is still very warm, its a quick stir of the coals or what is there - throw in a few splits, and it off and running without having to re-start the fire with kindling. Maybe it was "wasting heat" up the chimney, but the increased draft from everything being warm seems to work well for the re-starting.

This was probaly my greatest surprise when I switched to a soapstone stove. As long as the stove is still warm, and something is glowing in the firebox, chuck in some dry splits and your flamin' in no time.

 

[bceureka]

Two years later, the soapstone stove has the guts burned out of it because of overfiring because nobody wants to wait 5 hours for the room to get warm enough to take your jacket off. The heat curve is for the patient man: like steering an aircraft carrier, control changes take a LONG time to manifest themselves.
For more immediate gratification, steel or cast iron are more controllable, cast iron for those with lots of hearth space for clearance and time for radiant ( non convecting) heat dispersion (thats why they are pretty--it gives you something to look at while you wait for the room to warm up); Steel stoves with blowers for the want-it-hot-now ham fisted crowd.