Our Masonry Woodstove - Firing Up the "Macrowave"!
Many people currently cook in a microwave oven. It's called "micro" because the high-frequency radio energy used to jiggle the food's water molecules has a very short wavelength. We call our stove the macrowave because, even though it utilizes radiated infrared energy (heat) which has a much higher frequency and even shorter wavelength, the stove itself is so much larger.
Since we live in an area of North America blessed with 4 distinct seasons (or 5, if you count "mud" season), heating our home using renewable energy is a high priority. Our house is designed using passive solar principles, where sunlight shining directly on a dark-colored, massive, heat-retaining floor provides about 50% of our seasonal heating needs. The remaining 50% is obtained from burning local trees that have fallen down due to disease, wind damage, or just from thinning them for better, healthier, and straighter growth.
Heating with wood has often been discouraged, especially in highly populated areas, because of inefficiency, pollution, and non-local sourcing of firewood ("Black carbon" released by inefficient, smoky wood fires, forest and grassland fires, and diesel exhaust has about 900 times the effect on global temperature rise as carbon dioxide, from Science News, October 5, 2013). But all 3 reputed problems can be solved by burning locally obtained wood in a cleaner manner. For us this meant building a masonry, or retained-heat, stove.
To learn more about the history and technology of masonry heating, we'd suggest: "The Book of Masonry Stoves" by David Lyle.
Burning wood fast, hot, and with plenty of air, and with masonry to capture much of the heat gives efficiencies often over 80%, far fewer escaping particulates, and it is a renewable resource that breaks down into CO2 whether you burn it or let it rot on the ground. We cut up only dead fallen trees or use "thinnings" of small saplings cut to improve neighboring tree vigor, to obtain enough "kindling" wood, and to boost the potassium level in the ashes, making them a better fertilizer on low-potassium soils.
Our previous owner-built home featured 2 generations of "scratch built" stoves. The first was built primarily for baking bread. Its door was far from airtight. But it had a catalytic combustor in the upper chamber of the stove leading to a 2-hole cook-plate surface which supplied a really hot cooking area. Its simple arched shape, with recycled paving brick interior, a middle layer of insulating vermiculite cement, and a ferro-cement exterior shell worked very well and was cheap to build. But cracks in the outer skin inspired neither great faith in its longevity nor many aesthetically enraptured comments from visitors.
The second version, built on its predecessor's cement foundation, was purpose-built for heating. The recycled door framework was airtight. The inner layers of recycled paving brick harbored circuitous chambers for maximum heat transfer into the brick mass. And the outer face brick layer was chosen for appearance and isolated from the inner brick by a small airspace (created by using corrugated cardboard between masonry layers, which eventually turns to ash). This prevented any cracks in the outer brick caused by uneven expansion and contraction of the masonry. It is still in use.
Our new home needed an immediate heat source but we planned to use retained-heat principles once we were moved in. So we used a style common in Germany, the hybrid masonry stove. It uses a steel or cast-iron stove for the firebox but wraps masonry around the metal on 4 or 5 sides, excluding the loading door and, optionally, a cook-top. We added a large masonry chamber behind the stove to allow flue gases to circulate through some extra mass, boosting the total masonry weight to 2 tons. This photo shows the flue pipes in black and the air inlet pipe in silver. Check the drawing below and you will see exactly what is going on. The flue pipe is just getting buried in refractory cement and surrounded in brick at this point in construction.
In the dead of Winter, if nighttime temperatures dip below 20° F., and the Sun hasn't been abundant as a heat source that day, we will light a fire in the late afternoon. It is used to cook our evening meal, as it raises the air temperature a bit (sometimes as high as 80F if we know that it is going to be really cold at night), then heats the brick mass to provide a gentle thermal boost all night long. The fire itself only actively burns for about 2 hours, at "full throttle", getting all of the oxygen it needs for very complete, high temperature, low pollutant, high efficiency combustion. By morning, depending on outdoor temperatures, the house may be anywhere from 60F to 65F.
When the stove is first lit, combustion gases pass straight up the chimney, for maximum "draft" and a very quick start-up. When the stove pipe's external temperature reaches 500° F., we rotate the handle on a solid draft damper that forces the gases through an extra 12 feet of stove pipe surrounded by at least 8 inches of refractory cement and brick. And when the fire subsides to "coals" we usually flip an additional control on the stove that forces gases through a secondary ceramic "afterburner" designed by the stove's manufacturer, Harman Stoves in Pennsylvania.
Here's a big reason we bought this stove. It can hold a lot of cook-pots. I had rice in one, beans in another, the wok had some veggies stir-fry-steaming, behind that is some squash steaming, and the skillet in front with the circular potholder is our version of an oven for flatbreads. There is a stainless steel cake pan with dough in the cast iron skillet, with a cast iron spacer under it, a stainless steel cover over it, and the insulating potholder over that. This was enough food for easily a couple of meals, all on one brief firing of an armload of "box elder" (Manitoba Maple, or Acer Nigundum).
And the latest addition to our cooking/baking options sits right beside the masonry stove. It's a 660-watt, 12-volt electric oven, built from recycled stainless steel tanks. Details can be found near the bottom of the "Eat Local Year-Round" page. Its high temperatures, bigger capacity, heavy insulation, high efficiency (due to the round shape and infrared-reflecting stainless steel), and use of solar electricity make it a welcome addition to the household.
This is a photograph of a paper sketch showing the basic 2-dimensional smoke path. Room air enters at the base of the stove door (labeled "Room Air Intake"). The exhaust gases (red arrows) flow up a 3-foot length of 6" diameter stovepipe to a horizontal "T". If the damper is open (horizontal) the gases go to the right, then back through an "L" (90-degree bend) into the masonry. They then go into a vertical "T", turning to go up the chimney. If the damper is closed (vertical) the gases go left and back through an "L" into the masonry. Next, they go into a vertical "T" and flow downward in the left pipe. Reaching the floor, they turn right, then turn upward, eventually going up the chimney. In the process they flow through 12 feet of additional masonry-covered stovepipe, transferring the bulk of their heat to the mass. Cleaning ports (pipe caps) are included at two points so that I can run a chimney brush through the pipes once/year, removing about 1/2 gallon of "fly ash".
This 1/4" steel plate woodstove was designed with another feature that made it ideal for our tightly weather sealed home, shown in the back-facing photo but not shown in the sketch. There is a rectangular port at the base of the stove's heat exchanging unit which is intended for an external fan. The fan would normally blow air through the heat exchanger, providing extra heating efficiency and circulating warm air further from the stove site. But we used this port to pre-heat fresh air from outdoors.
In a tightly sealed home, any combustion device needs a dedicated air source. The burning gases in a stove move up the chimney and pull air into the stove to supply oxygen for combustion. In a typical home, this air would seep in through cracks in the siding, window edges, or door jambs. In our home it's an insulated 4-inch duct that starts outside our porch and ends where the fan would mount on our stove. So when the stove pulls stale, cool, room air in for combustion, the partial vacuum created by this draws fresh, cold air through this duct. The stove's heat exchanger turns that into fresh, warm air, eliminating excessive room moisture and providing oxygen for the inhabitants.
For additional details on the construction and design principles, Click Here for an Adobe .pdf file download (requires Adobe Reader).
The Solar Heat Exchanger
On sunny Winter days, when the woodstove isn't used, we exchange air using another "passive" device. On our home's South wall, we built a 24-foot-long, 1-foot tall, air pre-heater. The Sun heats a sheet of black-painted aluminum fixed behind 2 layers of recycled, patio door, "side light" glass. There are air spaces on either side of the metal that act as air ducts. The Sun also generates electricity in a small, 8-watt PV (photovoltaic) panel. This panel powers a recycled, Cray supercomputer, cooling fan that pushes air through the 2 air ducts. Cold, fresh, dry air enters the pre-heater and exits into our home as 130° F., fresh, dry air. Stale, cool, moist air exits the house via the wood-stove's chimney, completing both the air exchange and the thorough ventilation of our home.
And to make better use of the little PV panel during the summer, when we don't need it to power the air pre-heater, we have added a switch that allows us to power another 12-volt fan at the peak of our porch roof. When the sun heats up the porch it also switches the fan on, pushing hot air out of the porch and drawing cooler air in through the screened windows.
The masonry stove also supplies most of our hot water during the Winter months, augmented by excess PV (solar photovoltaic) energy on sunny days. The wood-stove portion is supplied by 7 loops of 1/2-inch copper pipe wrapped around the stove pipe, right where it first exits the stove. The radiant heat causes the water in this coil to become less dense. It rises into the top of a 10 gallon, insulated, electric water heater. Cold water at the bottom of this tank flows down into the lowest part of the heating coil, replacing the buoyant hot water with dense cold water to continue the passive "thermal-siphon". This gives us stored, pressurized, hot water whenever the stove is running. When the storage batteries in our home's solar electrical system are full, extra electricity flows to a 12-volt heating element in the water tank that supplies hot water in an alternate way.
An Update from someone who knows what they're doing:
While discussing our stove on a permaculture bulletin board, Permies.com, a real mason chimed in to let people know that the expensive refractory cement we used around the internal stovepipes may have been unnecessary. He suggested using "firebox mortar" instead. It is composed of 3 parts (by volume) sand, 1 part Portland cement, and 1 part powdered fireclay. The fireclay is primarily made from finely-milled porcelain with a little bit of added lime. He said that it is available in 50-pound bags and a common brand used by potters building a kiln is "Gladding McBean". We haven't tried it but it sounds like a less expensive option at least, whether or not it holds up as well.