Fire is the primordial element of transformation at the heart of all our activity: metabolism, propulsion, heating, any and every industry. We humans have come to harness fire in very powerful ways. Technological advancements 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 shift 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 burning wood to generate heat is inherently polluting. Such is not necessarily the case.
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. In response to this crisis most every society (except the English) developed techniques to optimize combustion of wood fuel and – just as important – optimally harvest the thermal energy produced. 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 hour) is slowly emitted from the mass over the next 12 to 24 hours as soothing far-infra-red wavelengths of heat. 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 90% complete (100% is impossible due to the presence of ballast gasses). Somewhere around 70% wood-fire exhaust becomes smoke and odour free, which is a far cry above the clouds of blue grey coming out of most 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 similar 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 cook=top, 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 just tosses a few logs on top of coals and/or 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. We see it all the time, with clouds of blue-grey smog filling our skies and lungs.
By contrast, masonry heaters operate at 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 full blast, ensuring clean combustion.
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 send a certain amount of heat up the chimney to ensure adequate draft. (Unless it has a down drafting condensing flue, but that is another discussion). The average metal box stove with a core firing temperature of 400c typically requires flue exit temperatures of 200c or greater to ensure the secondary combustion process is fully functional. By contrast, masonry heater burning at 800c generate much greater expansion pressures that pushes the draft along. Thus, the exit temperatures can be as low as 40c without problem. So, while a thin metal box stove is surrendering 50% of its heat to the drafting function, the masonry heater is surrendering as little as 5%.
J-feed Rocket mass heater with cook-top and heated bench
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 combustion-efficient as the best pellet stoves – but without all the short-life-span gadgetry or need to purchase store-bought fuel.
The thermal energy produced by masonry heaters is radiated as far-infra-red wave-lengths from the heater’s large surface area. This low wattage (about 500w to 700w per square metre of radiating surface at 40c to 60c) is not only soothing to our senses, but also eliminates unwanted air convection loops created by smaller more concentrated heat sources. At the same time, the greater the difference in temperatures between the mass and the surrounding objects/bodies, the greater the energy sent to them. Heaters shaped with benches 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. No need to attend a spa to use an infra-red heat bench when we already have one in our home.
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.
Masonry cabin heater with cook-top and oven
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 air-ground heat pumps have minimal operating costs, but they gradually suck warmth from the earth, 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 not 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 $1000 and $12000 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 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 while tending to both spiritual and practical aspects of life can be 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 protected]
You can also check out: mha-net.org And for images of the varied aesthetic possibilities, google: cob masonry heaters.