Introduction:
Over ten years ago, I met Chris Hedderson from Rocky Mountain environment Technologies outside the Denver log home show. Chris had an outside booth with a masonry stove (modular refractory kit) fired up within a large tent. While it was a very cold weekend, I recall how toasty warm it was inside that tent. I kept his business card and after much research, knew that one day I would install my own stove.
I did just that last summer. This newsletter is a follow up from my previous post entitled “Wood heating options for log and timber homes“.
Within the last chapter of this newsletter, I am writing about my personal experience about heating and cooking with our masonry stove at our cabin by the lake.
Definition:
The masonry stove is a free standing heat storage fireplace.
It is a very efficient heating device that burns wood rapidly because of massive airflow intake.
A one hour fire heats the masonry mass which gently radiates the heat to the living space for 12 to 24 hours depending on how well the house is insulated, outdoor temperatures and house design. Radiant heat is absorbed by all the mass of the house. In turn, the walls, floors and furniture heat the air so there is no need to install fans, motors or duct work. No reliance on electricity is another step towards “self sufficiency”. Log homes are particularly suited to the use of masonry stoves as the log walls are huge passive mass heat storage.
Masonry stoves use only a small fraction of fuel compared to other stoves as their combustion efficiency has been rated at over 90%!
History:
Over two thousand years ago, the Romans invented the hypocaust (heat from below) to heat their masonry public baths and private house floors, channeling the smoke exhaust from a single wood fire from one side of the room to a chimney on the other side.
The birth of today’s masonry stove dates from the 17th and 18th century as a direct response to an acute energy crisis precipitated by an overuse of timber for building and heating.
European governments ordered their craftsmen to develop fuel efficient stoves. The Swedish, Finnish, Russian, German and Austrian masonry stoves all use the same idea of sending a hot, fast fire on a long path through a masonry mass that absorbs the heat. The trapped heat is then released slowly.
Today the Finnish government encourages the construction of masonry stoves for all new homes. Courtesy of generous tax incentives about 90% of new homes in Finland are heated with wood burning masonry heaters thus reducing the demand for electricity, oil and gas. This lowers the country’s dependency on foreign energy sources.
Design:
As log/timber home designers, we almost always incorporate a fireplace or wood stove in the design.
Unfortunately a common reason to add a wood burning fireplace in new homes is for resale value and as an afterthought for using on special occasions only.
Few future homeowners plan to use a wood burning heater regularly for both heating and visual enjoyment.
A masonry stove is both a heater and thanks to today’s large glass door it can become the family entertainment center. It has the potential to reintroduce the hearth as the vital center of home and family life.
For our ancestors there was no home, no family, no life without a fire.
Instead of making the fireplace an optional design item carelessly slapped on an exterior wall in our living room, we can bring back the“magic warmth and hypnotic fire dance” to the center of our main living space, thus drawing the whole family together around this eternal source of comfort and serenity.
The masonry stove can be much more than a heater. It can have a baking oven for bread, pizza or casseroles… a warm bench designed within its mass or even a bed on top, a design feature so common in Eastern Europe.
The masonry heater complex becomes the heart of the home, and the room layout of our home can flow outward from that center…
Construction:
The design and operation of the masonry stove is remarkably simple.
As the firebox is masonry, the fire burns to 1500 degrees Fahrenheit or more, insuring full combustion of the gases contained in the wood.
Heating energy produced by wood complete combustion comes from 1/3 solids and two third gases.
Combustion efficiency measures the heat energy produced to completely burn wood fuel (for example) with little or no air pollution.
Heating efficiency measures how quickly the heat generated by a fire is released into the living space.
Steel wood stoves so common in North America have a low combustion efficiency as they do not get hot enough (about 500 degrees) to burn all the gases contained in wood thus the production of creosote and the necessity to clean chimneys regularly. However they do achieve high heat transfer efficiency as metal transmits heat within minutes of the fire being lit.
My masonry stove does start to heat our cabin within 15 minutes of being lit, mainly from the front glass door. The cold thermal mass of the stove for the first season fire will start to feel warm to the touch after two hours of burn.
The masonry fireplace kit (constructed of castable recycled refractory firebrick) I bought from Chris weighs over 2000 lbs. The outer masonry layer of bricks and chimney is another 2000 lbs for a total load over 4000 lbs which requires a substantial concrete footing with rebar in the crawl space for structural support.
The air comes from outside, channeled through the crawl space by an 8” diameter steel pipe to the air damper just before it enters the base of the fireplace. I open the air damper to full during the burning and close it when the fire is out to stop cold outside air from entering and cooling the stove. The fresh air enters the firebox from the bottom through a 2” wide long slot that doubles as the ash drop. I plan to clean the small amount of ash produced by the stove yearly by accessing the ash clean out door situated in the crawl space.
It took six hours to assemble the masonry kit including the baking oven and install the butterfly smoke damper at the base of the firebox. Then I covered the inner core with cardboard before starting to lay the outer brick layer so that the outer layer is never touching or cemented to the inner core.
This is a crucial step as the inner core gets very hot and expands under heat stress. The outer brick or stone layer is never too hot to touch even during intense fires, making the masonry heater so much safer thansteel stoves.
I recycled old bricks to build the envelope around the inner core. As I had no previous masonry experience, it took me weeks to complete this job and build the chimney to 17′ high, the minimum required for a strong exhaust draft.
As part of the design a chimney clean out door is installed at the base of the chimney in line with the smoke damper so I can see past the butterfly damper all the way across the bottom of the heater for future inspection. To ensure a chimney draft is established on a first firing of a cold stove, we use this door access to the chimney to insert a crumpled ball of paper and light it to heat up the column of cold air trapped in the chimney. That creates a strong draft upward and then we light up the top of the wood pile in the stove, close the door and enjoy a fire that comes alive almost in an instant without any smoke seeping in to the living space.
Lighting the stove, The top burn fire:
I only use well seasoned wood. I collect green wood, cut it to proper length and season it for a year under cover. Then I split the wood in smaller pieces ranging from about 4” diameter to kindling size and let it season another year to make sure it is dry and ready to provide me with an intense fire. Green or wet wood is not efficient for the masonry heater! Energy is actually consumed to heat and vaporize the water contained in green wood.
To create a one hour burn fire, use the top burn method of stacking about 40 lbs of wood in the stove.
Burning one pound of wood generates close to 7000 BTU.
40 lbs will release about 280 000 BTU. As the masonry stove has a combustion efficiency over 90%, the stove can produce about 250 000 BTU from one fully loaded fire burn!
Larger 3” to 4” piece are first stacked in a crisscross pattern at the bottom of the firebox. Use smaller size wood piece as the stack gets higher. Small kindling and bark or paper complete the pile. Lighting the load from the top allows the firebox to heat up and greatly reduces smoke emissions. I only see smoke coming out of the chimney in the first seven minutes of lighting the fire. After that no visible smoke comes out during the rest of the firing.
When the fire nears the end, we start to partially close the exhaust damper while only small yellow flames are still visible.
I close the smoke damper 90 % when there are no visible flames and only red ambers left. Then I close the fresh air damper to trap all the stored heat in the masonry thermal mass. When the red coals are gone I close the exhaust damper completely.
These steps maximize the efficiency of the stove by minimizing the amount of heat escaping through the chimney.
The Masonry stove and your safety:
A- Masonry heaters do not burn to the touch except the metal and glass door at the front of the firebox. They are much safer than steel stoves around children as the brick or stone finish is always safe to touch.
B- Masonry stove fires reach such high internal temperatures that they burn off the creosote right in the firebox. There is no creosote buildup on the interior channels or chimney walls. Chimney fires are not an issue so masonry heaters are known to be a low fire hazard by insurance companies.
The Masonry stove and your health:
Each time I have started a fire in a regular steel wood stove or opened the door to reload, smoke invaded our living space, creating much indoor pollution.
This simply does not happens with the masonry stove. After many dozen firings of our stove, I have yet to see or smell any smoke when we start the fire or during operation.
Masonry stoves earn LEED points for indoor air quality in the US green building council’s LEED for home’s rating system.
Particulate emissions (outdoor pollution) are 1 to 2 grams/hour which belongs to a super low emissions category. The US environmental protection agency (EPA) does not require certification for masonry heaters.
Masonry stoves heat the home by radiation and not by convection. The indoor air is not heated so it does not lose it’s moisture content, benefiting the occupants, indoor plants and even furniture. A forced air furnaces heats and blows air, not only drying the air but actually causing a “wind chill” effect. The warmer air goes up and the cooler air rushes past your feet.
Maintenance:
As already mentioned, chimney cleaning is not needed as there is no creosote build up.
After so many firing of our stove, I am astounded to notice the walls of the firebox are as clean as the first time I lit the stove.. I understand that the high heat achieved in the firebox burns away any black marks that appear in the beginning of the firing.
Proper combustion ensures that little ash remains. This can be cleaned out every few weeks through a clean out door in a raised hearth.
The air wash door design from the RME Technologies stove keeps the door glass perfectly clean. That was not the case with any steel wood stoves that I used in the past. I had to periodically clean the glass with a specific cleaner made for that purpose if I wanted to fully view the fire.
Last words… from personal experience.
Because I built my stove using recycled bricks and ceramic flues from an old masonry fireplace, I spent $5000.00 to complete this project.
My costs include the masonry kit delivered from Rocky Mountain Environmental Technologies in Edmonton Alberta, the air damper, ash door in the crawl space and many bags of “S” type masonry cement.
If I had hired a professional mason to build it and bought bricks and ceramic flues, I estimate the cost would have probably been up to $14000.00
As the attached photos of the firing cycle shows, my stove looks rustic and amateurish. A professional mason would have done a much better job laying the bricks…
I have experimented with baking using both the firebox for high temperature cooking after the fire is out and the baking oven for slow cooking casseroles with delicious success.
