Hi @fin,
Thank you for your post, it brings up interesting questions in both material choices and overall masonry heater design.
The easier questions to address have to do with material choices. It is important to understand and distinguish between the different kinds of refractory (high temperature) brick.
Fire Brick: The standard unit for firebox construction are Fire Brick. These are dense brick, usually cream colored, designed to hold up to both high temperatures and heating and cooling cycles. They have a hard surface that stands up to the mechanical abrasion of firewood getting scraped and thrown against its surface.
Insulative Fire Brick (IFBs): Insulative Fire Brick are very light and usually white in color. They are commonly used in kiln construction due to their ability to withstand high heat without absorbing that heat. While these properties may seem ideal for firebox construction by allowing for hotter (and therefore cleaner) combustion temperatures, IFBs are more fragile and more prone to mechanical abrasion.
For residential wood-fired combustion fireboxes, firebrick (not IFBs) should be used because of their combination of refractory qualities and durability.
Firebrick also comes in several different ratings, referred to as “duties”. Medium duty firebrick is a good choice for firebox construction because it combines good refractory qualities, density, and durability. A well-made low duty fire brick is also a good choice for a firebox and especially good for the rest of the refractory core of a masonry heater since they are easier to cut on a wet saw. High duty firebrick is expensive, more suitable for industrial applications, and is very hard to cut. A specialty refractory supplier or manufacturer will offer a range of options while a local masonry yard that stocks firebrick will typically have only one option (low or medium duty) that has been selected for fireplace construction and it will usually be a good one.
Once out of the firebox…. your questions allude to some interesting divergences in masonry heater design.
The current “standard” “professional” approach to masonry heater construction in North America is referred to as Double Skin Construction where you have a clearly defined core and facing, typically with an expansion joint/plane in between. Firespeaking’s stock 5-Run with Oven Design illustrates this approach. The core for the masonry heater wraps all of the functions of combustion and gas flow in refractory materials. The facing serves as both an aesthetic skin to the core and as a significant thermal battery to store and radiate the heat generated and extracted in the core. In double-skin construction, the facing can be constructed of a whole range of “masonry” materials including red brick, cement brick, block, and stone. These units can also be further combined with surfacing such as tile and plaster. Natural building materials such as cob, adobe, and compressed earth block (CEBs) can also be used.
In double-skin construction, the core would otherwise operate as a stand-alone functional unit if it weren’t for the hardware (firebox doors and cleanouts) that is usually mounted to the facing and provide an important seal for gas flow.
The Estufa Rusa or “Russian Stove”, widely publicized by Argentina’s national agricultural technology institute (INTA), would be an example of a single skin masonry heater constructed entirely of firebrick with no additional facing. One feature of this design is that it requires little or no firebrick cutting. Because red brick set in clay mortar is substantially refractory, there are also examples of single-skin red brick heaters like those publicized by @PabloK at Hacono (website / sample plans).
Your question alludes to a third interesting approach which is a hybrid approach where the firebox serves as the more technical refractory core and is surrounded by a facing which also serves to define the heat exchange channels. This approach is probably best exemplified by the folk “Gymse” design championed by the late artist/builder Lars Helbro (Gymse overview / core documentation).
This hybrid approach was also demonstrated by Matt Hellicke and Tony DaSilva in their North American Single Skin Heater - Double Bell with Black Oven and Heated Benches workshop build at WildAcres.
Permissibility of each of these approaches in the context of the U.S. building code hinges on interpretation of:
Heat exchange channels shall be built with firebrick, soapstone, or other refractory materials laid in refractory mortar, fire clay mortar, or soapstone refractory mortar. (ASTM E1602 5.7.1)
Both a double-skinned heater and a single-skin heater constructed of firebrick laid in refractory mortar clearly meets this criteria. In the hybrid approach where facing red brick is used to also define gas channels, it depends on whether red brick is considered a refractory material. Red clay brick set in a clay-based mortar has refractory qualities. Not sufficient for fireboxes but likely sufficient for heat exchange channels. Professionals tend to favor double skin construction because it avoids this ambiguity and also reduces the likelihood of visible cracks in the facing by buffering the aesthetic exterior with an expansion joint to the core.
It’s also important to bear in mind that if the firebox walls are less than 8” thick and the heat exchange channels less than 5” thick (which basically implies double-skin construction), than the often-expressed masonry heater clearance to combustibles requirement of 4” reverts to a more generic 36” per IRC 1002.5.
I would like to follow this up with a more visual/graphic illustration of these different approaches but for now words will have to suffice. As always, curious to hear what others have to contribute!
