While masonry heaters have been known and used throughout cold climate countries in Europe and the former Soviet Union for hundreds of years, they have only come into regular use and popularity in North America during the last forty years. Today there is a general recognition that masonry heaters are clean burning, fuel efficient, beautiful, and easy to use. It is also agreed that of all wood burning systems available, masonry heaters give off the gentlest and healthiest heat.
They are also expensive to build.
In forty years of building, promoting and experimenting with masonry heaters, I have continuously searched for ways to build Do it Yourself and more wallet-friendly masonry heater solutions. I looked for nationally available non-exotic materials that were affordable and designs that did not require a skilled mason or heater mason to build.
To that end, I started experimenting with large fired clay flue tile liners over twenty years ago. My first experiment was with a 36” diameter by 24” tall and 3” thick round liner which is the largest flue tile liner made. The single tile became the outside shell for a large mass round cooker/heater unit with a clay mortared liner of common bricks and an inner square firebox of standard refractory mortared low duty firebrick with a steel top and baffle system to create channels and a downdraft flow within the system for maximum heat retention.
Wood was loaded through a central port in the top and preheated room air was introduced through tubes that ran into and pointed down in the front corners of the firebox. A bypass damper in the system allowed for smoke-free startups. Only two of these units were ever built but they both worked extremely well. One remains in a twenty-foot diameter insulated yurt on my land.
Our next experiment was to work with two foot by two foot by two foot two inch thick square tiles with rounded corners. These heaters we stacked two tiles tall and lined with a firebrick firebox separated from the veneer with a 1/4” sheet of mineral wool. Above the firebox, we had room for two switchback chambers made from firebrick and cast refractory slabs before the flue gases escaped from the top or rear of the heater.
We made a few of these heaters, fine-tuning the design all along, and found that they could effectively heat a space of 800 sq. feet from a brief firing in the morning and a brief firing at night. During the day, stored heat in the 6.5” thick walls did the heating. We knew from the get-go that we needed to build an expansion joint into the one piece tile, so we always cut each tile in half and then rejoined them with a high-temperature fiberglass tape ribbon glued on with high temp silicone. The halves we tightened to one another with steel strapping.
Closely fitted steel tops and bottom with lips and gasketing provided a more elegant solution to tying the two halves together. With the two tile stove experiments, we also learned the importance of putting clean outdoors for the upper channels on the seams of the two halves vs on the faces of the second tile. Soot doors become hot spots and can create differential heat stress on a full face of a tile unless the soot doors are plugged inside with a square of three-inch thick cast refractory concrete or a noncombustible structural insulation block. After the two tile heaters, we developed a design and core with more heat exchange stacked chambers for a three tile heater capable of heating spaces of 1000 sq feet and a bit more.
More recently, however, with the keen interest in Tiny House design and construction growing, I decided to create a smaller affordable mass heater appropriate for a Tiny House. With the exception of one Tiny House in which the owner was using a large model antique cylindrical footprint firebrick lined steel and enameled cast iron French Godin stove, I have not been in any wood stove comfortably heated little houses.
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Instead, I find people installing tiny stoves that have to be frequently refueled or very large stoves that quickly overheat a space and also need very frequent attention as well as significant clearances from combustible walls and materials. Sticking with the flue tile heater concept, I decided to design and build a one tile heater. I cut two tiles perfectly in half.
I used one half and turned it on its cut edges to make a foot tall support for two more tiles cut and gasketed and banded and standing on edge over the first tile. This would raise the top surface of the second pair of tiles to normal counter height for cooking and also provide a wood storage area and safety zone beneath the stove so it could be installed on a combustible floor. Between the wood box support tile and the two top tile halves, I laid down a 1/8” or 1/4” square of steel with rounded corners to match the base profile of the rejoined tiles.
On top of the steel, I laid a ribbon gasket and then mounted and gasketed and steel banded the tile halves. Reaching down into the three foot tall and two-foot deep shell, we installed a three-inch layer of non-combustible structural insulation board made of calcium silicate and manufactured by Skamol (Ska for Scandinavian mol is Danish for insulation). The insulation I covered with a layer of high duty two inch by four and a half inch by nine-inch high duty firebrick “splits”.
I then laid up eight more courses of firebrick with repeating cuts laid between full firebrick and with each course shifting ninety degrees within the core to avoid stacking the joints. The four corners of each course are trimmed slightly to allow a better fit to the inner curve of the tile shell. A 1/4” gasket of mineral wool is glued up as an expansion joint between the firebrick core and the tile shell.
To keep the coursing from exceeding the top of the tile halves, the eighth course is another two-inch split course and near the rear of the stove, the bricks are dished to start a gentle channel for the exhaust gases to follow. The last course is a full low duty firebrick course and the rear section is laid up with insulating firebrick to reduce heat stress on the back of the tiles near the smoke exhaust. The dished out area is continued in this course as well.
In the first iteration of the cooktop for this unit, I could not wait for the shop to design and make up a high temperature non warping steel top, so I set about to design and cast my own two inch thick reinforced cast refractory cooktop with a center loading port sized to a medium-sized cast iron stove lid that we had in stock.
I formed the top with tape sealed two by fours and created portholes for combustion air tubes with plastic wrapped two inch PVC tubing sections and cut out a styrofoam plug and wrapped it with gorilla tape for the narrow smoke exit towards the rear of the top. I stapled little curves of flexible plastic in the corners to mimic the shape of the tile corners. The center hole plug was made from taped cardboard and styrofoam. I mixed stainless needles with the refractory concrete and water to create a top that would be reinforced.
The concrete sets up overnight and I released it the next day, removed the plugs and trimmed off the burrs with a diamond grinder and smoothed the top surface with a forty grit disc on a random orbital sander. I put the dry one-piece cast top on the wet diamond rail saw and cut it in half to create an expansion joint and then turned each half on edge and used a dry diamond grinder to cut a shallow thin saw kerf to set a galvanized spline into the two halves of the top as a seal. The spline was then cut to fit each section of the top casting.
Opposite the two air ports in the cast top, we cut channels in the top course of firebrick to bring air tubes through the cast top and then at right angles through the top course of firebrick and then cut and welded and angled down again into the top front corners of the firebox. Our local sheet metal guru, Dave Belanger, made up a custom stainless smoke throat 2” by 8” oval to 5” round adaptor to fit my smoke hole. Our shop made up the stainless sleeves and pivoting covers to go over the air tubes.
Just as we were bringing the completed stove online hooked up to our big oven chimney with a mineral wool plug between its eight-inch diameter and our 5” pipe, my son Scott emerged with his CAD design for a steel top. We fired the unit with the cast refractory top and cleaned up the rim a little bit with a diamond barrel grinder so the cast iron loading and cook lid would have a nicer fit. Everything worked beautifully as planned.
Shortly, the beautifully engineered high temp steel top emerged with expansion rays cut into it and thin strips of steel tack welded under the rays to avoid smoking and a lovely rim under the top of a round loading plate of the same high temp steel to be mounted in the lipped top plate.
The fancy stainless throat adaptor was put back on the shelf and a simple oval welded collar on the cooktop allowed us to bend a standard 5” pipe into an oval shape at one end and secure it to the collar with sheet metal screws. The air tubes had to be reduced in length a bit, but the stainless sleeves and pivoting covers still fit perfectly, so we made the shift to the new top quite swiftly.
With a tall eight inch insulated stack, the draft was very strong so Scott welded in a little plate of stainless steel to occlude the draft opening by 70% or more and the draft calmed down. The cooktop got quite hot and everyone was thrilled. We noted that the stack temperatures had enough extra heat in them to warrant the addition of a stovepipe oven and I found one in an antique stove shop nearby. We did a six-hour temperature test and produced a graph documenting the temperatures from three points on the stove.
Jesse Cottingham bought the stove for his Tiny House. He found that his short chimney did not have an adequate draft so I urged him to remove Scott’s little temporary stainless baffle. Once Jesse did this everything worked very well. Jesse and his partner fire the stove once in the morning and once at night for a short while giving them sufficient heat to cook two meals and to charge the mass. One small armload of wood is good for each firing.
The stove easily holds heat for eight hours. It is also much safer to place closer to combustible surfaces with its six and a half inch thick mass wall that never gets nearly as hot as a metal stove. The oven is just sheet metal and only bakes during a fire so it could use some design help.
I am researching a way to find off the shelf items and adapt them for a modern higher mass stove pipe oven. I am looking at nesting stainless steel stock pots as a starting point. Both have lids. The lid of the bigger pot could be gasketed and secured to the pot with latches or screws. Collars for smoke pipe entries and exits could be easily added. The center of the lid could be cut out and the second pot and lid inserted for the oven.
The lid of the inner pot could be hinged to make the oven door. A shelf inside the pot could be welded in with a soapstone 1 1/4” slab added for extra mass. A cast refractory partial ring could be made to sleeve inside the inner pot to act as a mass wall for the sides and top of the oven.
I am also looking at non-toxic recycled 30-gallon drums as an outer shell option. I can currently buy such a drum in Maine from a recycled barrel dealer for around $20.00 The inner portion or actual oven could have a flat bottom lined with soapstone and a Quonset or square side walls and ceiling also lined with soapstone or cast refractory for extra mass and better baking.
The oven surface and extra mass will capture much of the surplus heat going up the stack. Modest legs doing down to the stove top could add any needed support for a heavier oven. I am working on it because I have another one tile stove cut out in the shop and heading to a cabin being restored in Maine’s North Woods near Katahdin. We have also provided a photos series of the construction process as well as a basic materials list and current cost estimates. Costs obviously increase every year.