Masonry heater core and wall.
Today I'll be discussing construction styles, not channel layouts or firebox designs, but here I will give a brief overview of some ways that masonry stove durability has been addressed.
Masonry Heaters originate in many places around the world, and different cultures came up with different ways of overcoming the challenges faced when designing a masonry heater, which are pretty simple:
1. Effectively design a firebox to be the right size and shape to hold the right amount of fuel and optimize mixing of air with wood-gas.
2. Balance heat exchange area, firebox size, and wall thickness to absorb, store, and radiate that heat effectively while leaving enough heat in the gasses to allow chimney draft to keep the overall system under negative pressure.
3. Hold together under the stresses of expansion and contraction that are produced by the heating and cooling cycles inherent in the operation of the stove.
Today we live in many different shapes and sizes of house so we take these things into account with each stove we design, but plenty of heaters have been built so it's just a question of which approach we want to use for which situation.
Masonry Heaters in the United States
In some parts of the world, there is an outer stove wall made of a 4- to 4.5-inch thick softer, higher porosity brick than we have in the USA. These walls have the room on one side and the exhaust gas on the other, with no firebrick except the firebox and area right around it (where the highest temperatures are found). In some cases the red brick is plastered, which adds a layer of air-tightness to the stove.
In the USA, the most common construction I'll call “core and skin.” A firebox and heat exchange channels of firebrick are laid in water soluble refractory mortar with a gasket between them, and a red brick “skin,” is built around them, with a gap of 1/4- to 1/2-inch between them. This technique creates a heat exchange wall that's about 6.5-inch thick with a small air gap between the two layers. The gap allows for differential expansion between the core and skin but also acts as insulation, creating what has been called a “two-stage” heater.
Masonry Heater Kachelofen Basket Wall
Kachel Tile Masonry Heaters
In other places (Sweden and Germany, among others) a different approach has been taken. The surface of the stove is made up of a specialty tile called a Kachel (or firebrick that have shallow holes drilled into them). The tiles are thick and have a very important “doughnut” on the rear. Once laid in place, wire clips hold the units together, giving the kachels two properties: the ability to move and the ability to hold together as they do so. The wire-and-kachel-wall forms a “Basket” or “Barrel.”
The heat exchange channels are built within using clay and sand mortar, which allows for some movement of each brick. The interior bricks are laid in relation to each other in such a way that no brick can fall in to block a channel, and the Kachel-tile wall holds the assembly together. This allows for a 2.5- to 3-inch-thick heat exchange wall which is ideal for a more temperate climate or home with modern insulation and gives responsiveness impossible with a thicker wall. It also handles increased temperatures which allows a smaller stove to output more heat. For example, I build a stove on this principle that uses two 50-gallon drums as the basket, and which operates wonderfully to heat rooms or large super-insulated homes.
Similarly, soapstone heaters are built using 2- to 2.5-inch slabs of soapstone as the exterior, which are laid in a mix of soapstone dust and water-glass and are pinned together. Thinner soapstone slabs are typically placed within the stove to buffer heat and facilitate a longer storage time.
I like the basket/barrel approach the most, though as with anything of this sort, there is a cost/benefit analysis to be done, and no approach is perfect. My personal stove has some parts in which individual bricks are pinned together, and some parts where I use hard setting refractory mortars. My stove utilizes an air gap technique, though I laid my red brick on edge both to facilitate the shape I wanted and to limit the activation energy needed to charge the stove to operating temperature.
In the end, which construction method you use will have an effect on how the stove performs and each method has it's place. Thanks for reading! Please leave a comment below.
Eric Schroeder is a masonry heat expert who has been designing heaters since 2006, innovating around stove shape, size, firebox design, and heat exchange layouts. Connect with Eric at Eric Schroeder Stoves and on Twitter, and read all of his MOTHER EARTH NEWS posts here.
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