Double skin in the masonry heaters

I have a question. What is the specific reason behind required double skin of the masonry heater bell in USA?
Is it for gas tightness? If this is the case, what do you think about installing 2 or 3 mm 304L stainless steel welded walls INSIDE of the bell, separated from the outside masonry bell with 5-10 mm refractory wool?
The bell above the steel lining (and few courses below the riser top) would be double skin: outside masonry and refractory bricks inside with 10 mm wool separation.


Advantages:

The gas tightness would be better than any jointed masonry inner skin.

Not only it would block potential harmful gases leaving the bell, but also would prevent transfer of colder room air into the heater through expanded joints after it was heated. I have read that the open area in the expanded joints of tightly built (2 mm joints) kafel stove of a medium size can amount even to 400 cm2 (the size of a small stove door) assuming the expansion of 0.5 mm per joint. Opening of this size causes quicker cooling of the mass by expelling warm air through the chimney. In the past it was being solved by closing the chimney that was leading to quite numerous deaths by carbon monoxide poisoning.

So steel skin has real advantages. The masonry heaters covered with steel are called armored. Second steel skin on the exterior would not be visually attractive and it would be hidden inside.

It would not affect the responsiveness of the lower single-walled part of the bell with steel thermal conductivity 15 times higher than hard bricks.



Potential disadvantages:

Warping. However measurements done by Fiedia on donkey32:

Bell temperature measurements

indicate that lower parts of the bell are much cooler than the top, not exceeding 250 C.
Coefficient of thermal expansion for 304L is 17.8 in the range 0-315 C, so a 1 meter piece would get elongated by 5 mm at 300 C.  
The structure could have two horizontal diagonal braces.

Hi Cristobal,
while I don't know why double skin is required my guess is gas tightness.

About your idea with the steel bell I have some doubts.
First of all it would be quite expensive to make and would require a lot of skill.
Second the warping you mentioned. A flat piece of steel (like a cooktop) already experiences warping, I don't know how a bell of steel would behave.
And also the bell is supposed to radiate heat out. So the insulation seems counter intuitive.

PS: you're not by any chance on the foothills of the Sierra Nevada in Spain, are you?

Double skin serves three purposes: gas tightness, heat storage, and allowance for expansion.
It is widely accepted that you only really need the double skin above the level of the heat riser exhaust though.
So I'd skip the expensive metalwork and just do a masonry bottom and double skin top (masonry outer, refractory inner.)
That's how I'm designing my own bell. It won't hold as much heat but it will be more responsive, producing heat quicker as I want it to do.

Besides that, if you used metal it should be on the outside. It wouldn't serve much purpose inside. It's not just warping to be concerned about, it's the general expansion and contraction of metal that would be a lot different than the surround masonry making it difficult to tie it all together.

Hi Cristobal;

Masonry heaters were developed long ago in Europe and Asia.
With the design of serpentine channels through a large masonry mass, it was possible to more efficiently heat homes through brutal northern winters.
The high heat at the firebox causes expansion in the firebricks that create the heat channels, with a measurable movement.  
A second skin as well as being more mass does not heat enough to have measurable movement, plus trapping any harmful gasses and directing them away from the living area.

In the US, insurance companies adopted the building rules of European Masonry Stoves, including the double skin requirement.
You can buy insurance on a home with a Masonry heater, buying fire insurance by calling your stove a Rocket Mass Heater is almost impossible.

A Rocket Mass Heater truly is a variation of a European Masonry heater.
The major difference is the use of a stratification chamber.
By following the regulations on building a Masonry heater, including a double skin it is possible to fit within the description and successfully acquire home fire insurance.

Nothing wrong with using metal as a bell and surrounding it with masonry or lining the inside with mass to hold the heat.
Everything is wrong about insulating between the metal and brick, you want the brick to absorb and radiate that heat not protect it from the heat.

Above the riser and the top of the bell need to be constructed with firebricks due to the internal temps.
Some insulate the roof of their bell when it is too close to the ceiling.
Attempting to double-skin just a portion is not realistically even possible, how would the second skin be supported?

A bell can be any material that can contain gasses, the mass is holding and slowly releasing heat.
If you want insurance,  following the regulations, including a proper double skin, is a requirement.
If insurance is not an issue, then experiment away and see what works best in your situation.

Thank you Thomas. I know it all (except the reason why such standard was adopted0, but was just curious about the opinion about potential improvement of the gas tightness in the heated bell. Especially for the incoming cold air. I'm not worried about insurance, because I don't use it and build 100% masonry structures on top of that.
A master heater mason in Europe who I know, was planning to build such a heater for that reason, but with the steel armor on the outside of course. Because it's more difficult to make steel look prettier than bricks I thought about reversing it, because I like to think about aesthetic aspect of all structures.

Benjamin, I'm in the Sierra Nevada foothills in California.