Fire is the primordial element of transformation at the heart of all our activity. Be it metabolism, propulsion, heating, or any and every industry… we humans have come to harness fire is very powerful ways. Technological advancements in the energy sector have brought coal, nuclear, hydro and fossil fuel sources into daily use with remarkable results. But at the same time, evidence all around suggests that our rampant use of fire is out of balance. For the sake of future generations we had best throttle down to more benign carbon-neutral fuel sources.
Given that roughly 50% of our energy consumption in Canada goes towards heating our homes (to heat our bodies), the manner in which we do so is well worth re-evaluation. Here in the Pacific North We(s)t we are surrounded by an abundant supply of readily renewable carbon-neutral bio-mass, but we overlook this fuel supply because we have been conditioned to believe that wood-fire = pollution. Such may often be the case with slow-burn metal box stoves – but not with masonry heaters.
During the 16th century European societies experienced a major energy crisis. A rapid growth in population and industrial activity caused a serious depletion of wood-fuel supplies. Coal and petroleum were not yet in the works, so this was indeed a major problem. In response to this crisis most every society (except the English) developed techniques to get the most out of wood fuel. This involved optimizing combustion efficiency and finding ways to harvest/utilize as much of the thermal energy as possible. From these efforts wood-fired radiant earth heaters, or ‘masonry heaters’ became commonplace.
Masonry heater with kettle deck and heated bench
Masonry (mass) heaters (including rocket mass heaters) are highly efficient wood-burning appliances that offer exceptional levels of comfort. They burn wood at very high temperatures over a short period of time, and then store the heat in their dense earthen mass before gradually releasing it into the living space. The thermal energy from a quick hot fire (usually 1 or 2 hours) is slowly emitted from the mass over the next 8 to 24 hours in gentle far-infra-red wavelengths. Soaking up warmth from the heater’s body is much like soaking up rays of sunshine, which is why some people call these heaters ‘a piece of the sun’.
Pollution-free combustion of wood is contingent upon 3 factors: time, temperature and turbulence. When wood is burned hot (over 700c) with an optimal rate of draft and fuel mixing conditions, combustion of wood gasses is over 85% complete (100% is impossible due to the presence of ballast gasses). This is a far cry above the 10% to 20% efficiency of older metal box stoves. A well-built masonry heater produces less than 0.5 grams of particulate matter per kilo of wood, with average carbon-monoxide levels well under 500 parts per million. These numbers are very close to the combustion efficiency of propane and natural gas, but without all the negative consequences associated with the oil and gas industry.
Mass heater with cooktop, oven and heated bench.
A properly built masonry/rocket mass heater requires less than half the wood of the most efficient modern metal-box stoves, and 1/10th that of the old ones. As outlandish as this claim may sound, the reasons are as follows:
Real-world combustion efficiency
Modern metal box stoves can achieve 75% to 85% efficiency, but due to their minimal heat storage capacity they must burn-on demand. When very little heat is desired (which is more and more common in modern well insulated houses) they must shrink the fire dangerously close to the point of smouldering. Under ‘real world’ firing conditions, when the wood is not always dry or well seasoned, or the operator omits to adjust the draft on reloads… the likelihood of a ‘crash smoulder’ increases. The more we try to slow a fire down, the more likely an operator error will cause a drastic reduction in combustion efficiency. In my homeland of Cowichan Valley one can see new woodstoves smouldering all the time.
By contrast, masonry heaters have a fixed high burn-rate. This means the firing phase is always very hot, and thereby optimized. For gradual heating of the living space there is no need to diminish the fire dangerously close to the point of smouldering. When only a small amount of heat is desired (eg. taking the edge off a morning chill in the shoulder season…), a smaller batch of fuel is burned, but it is always burned very fast and hot.
Heat exchange and harvesting
Another advantage of masonry heaters is that they surrender a smaller percentage of heat to the drafting function. Any naturally aspirated wood-burning heater must (usually) send a certain amount of heat up the chimney to ensure adequate draft. The smaller the fire, the lower its expansion pressure, so in order to pull the combustion process along many metal stoves have flue exit temperatures between 100c and 200c . The bigger/hotter the fire, the greater the expansion pressure, the less heat-pull it needs from the chimney. Thus, many masonry heaters can function optimally with less than 80c exiting the flue/chimney. So, a metal box stove firing at 400c surrenders a higher ratio of its heat to the drafting function than a masonry heater burning at 800c. This thermal harvest efficiency is a big factor in reducing overall wood consumption.
Some metal box stoves and pellet stoves improve harvesting ratios with electro-mechanical fans. But these are noisy, not very durable, and require another energy source to function properly. Masonry heaters are as efficient as the best pellet stoves – but without all the short-life-span gadgetry or need to purchase store-bought fuel.
J-feed Rocket mass heater with cooktop and heated bench
The thermal energy produced by masonry heaters is radiated as far-infra-red wave-lengths. This low wattage (about 500w to 700w per square metre of radiating surface at 40c to 60c) is very soothing to our senses. Also, the heater’s large surface area eliminates unwanted air convection loops created by smaller more concentrated heat sources. At the same time, heat output is regulated by feedback from the room; the greater the difference in temperatures between the mass and the surrounding objects/bodies, the greater the energy sent to them. Most comforting of all are heaters shaped with benches that allow the far infra red heat to conduct deep into our tissues. This is profoundly relaxing and very cleansing on many levels. Given that we are heating our homes to heat our bodies, to help us relax… the more we can ‘cozy up’ to our heater the better.
Masonry heaters built with unfired clay (in the cob mortars and plasters) are also very health enhancing. The clay absorbs and releases atmospheric moisture to keep indoor humidity around 50% which is ideal for our skin, lungs and mucous membranes. This ever-breathing clay absorbs positively charged ions as it hydrates and releases negatively charged ions as it dehydrates, which helps to neutralize free-radicals within our bodies. At the same time, this humidity buffering process sets up a slight electrostatic charge in the atmosphere which causes small airborne spores and particles to clump together and drop to the ground… further enhancing indoor air quality.
When weighing up the cost/benefits of various home-heating methods, we must consider not only their installation and operating costs, but also the overall embodied energy involved in their construction, use and eventual disposal. For examples: Electric base board heaters are cheap to install and last a few decades, but they rely on grid tie in, which fosters rationales for destructive hydro, coal and nuclear mega projects. Grid-tied ground-to-air heat pumps have minimal operating costs, but they tend to be noisy and last about a decade before they needing replacement. Propane and natural gas have been touted as the cleanest combustible fuels, but when we factor in the energy embodied in their gathering, processing, storage and distribution… such is far from the case – particularly when we weigh in the socio-political costs of controlling territories, and the echoing costs of environmental degradation from primary resource extraction without conscience.
Cob masonry heaters are heirloom appliances that will serve generations to come. They cost between $2000 and $16,000 to build and they last hundreds of years with minimal maintenance and repair. They are fueled by readily renewable, carbon neutral wood/bio-mass that can be locally gathered by hand, cart or vehicle. Operating cost/effort typically involves splitting less than 1 or 2 cords of firewood per year, and a commitment to make fire once or twice a day during the heating season. This small effort of chopping and burning wood is not nearly the chore it used to be. And in this day and age of high tech abstractions… such engagement with the primordial elements can evoke a reunion between the spiritual and practical aspects of life. This is good for the body, mind and soul.
In my opinion, there is no more efficient, durable and health-enhancing a way to heat our dwellings than with a simple hand-built masonry heater.
For more information about the heaters I build, call Pat at 250 748 2089, or email: email@example.com
Another good source of information is: mha-net.org
And for images of the varied aesthetic possibilities (by people way better than me at taking pictures) see: http://www.lehmundfeuer.de/