Masonry heater with multiple benches, nearing completion
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 penchant for high-tech gadgetry is at times out of balance. When it comes to heating our homes (to heat our bodies…) there are simple, durable, efficient alternatives.
Given that roughly 50% of our energy consumption in Canada goes towards home heating, the manner in which we do so is well worth 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) refined techniques to get the most out of wood fuel. This involved optimizing combustion efficiency and finding ways to harvest and 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 rapidly at very high temperatures, 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 (1 -2 hour) fire is absorbed into the mass and slowly emitted over the next 12 – 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’.
Green house heater
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 20% to 30% efficiency of smouldering old 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 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 oil and gas industries.
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 under optimized laboratory testing conditions, but due to their minimal heat storage capacity they must always 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 we see new woodstoves smouldering all the time.
Adobe brick, 4-run heater under construction
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 hot. A hot fire is a clean fire.
Heat exchange and harvesting
Another advantage of masonry heaters is that they surrender a smaller percentage of their 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. The chimney draft is like an engine that helps to pull the combustion process along. Expansion pressure from heat generation in the firebox also helps to push the process along. The smaller/cooler the fire, the lower its expansion pressure; so a higher chimney temperature is necessary. This is why slow burning metal-box stoves, with max firebox temps @ 400c tend to have flue exit temperatures between 100c and 200c . By contrast, when the fire is roaring, expansion pressure is much higher, and less heat (draft/pull) is needed in the chimney. Thus, a masonry heater with firebox temperatures over 800c can function optimally with less than 80c exiting the flue/chimney. This comparative difference in 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 burn-chamber gadgetry or need to purchase store-bought fuel.
J-feed Rocket mass heater with cooktop and heated bench
Heat distribution
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.
Natural advantages
Masonry heaters built with unfired clay (in the cob mortars and plasters) are health enhancing in other ways too. 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.
My son Rhys, helping build a cob rocket-oven with heated bench
Cabin heater/cookstove
Embodied energy
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 that goes into in their construction, use and eventual disposal. For examples: Electric base board heaters are cheap to install and last a few decades, but their conversion of electrical energy to heat is very inefficient. Grid-tied ground-to-air heat pumps have minimal operating costs, but they usually deliver heat via convected air, which is far from optimal for bodily absorption. They also tend to be noisy with short lifespans of a decade or so before needing expensive repair or replacement. Electrical gadgetry and ‘flick of the switch’ convenience are all the rage these days… but the energy to feed such devices has to come from somewhere; and as demand grows… so also do rationales for destructive hydro, coal and nuclear mega projects.
Propane and natural gas have been touted as the cleanest combustible fuels, and they certainly are very efficient at burning slow on-demand. But their overall efficiency goes way down when we factor in the embodied energy in their gathering, processing, storage and distribution; and further still when we weigh in the impacts on wild natural environments, and the social-political costs of battling over territorial access.
But perhaps most important of all is the matter of personal and community resilience. For anyone living in a temperate or cooler climate, some kind of external (beyond body) heat source is pretty much a necessity. The extent to which we outsource the provision of such heat to external providers is the extent to which we are living on the hook of dependency. This has become more and more commonplace in many aspects of life (food… medicine…), to the point that we just assume it a given. Nevertheless, such vulnerability makes for a subtle but pervasive stressor, with an ongoing tension between thermal and financial comfort.
By contrast, masonry heaters are fueled by readily renewable, carbon neutral wood/bio-mass that can be locally gathered by hand, cart or vehicle. Even when purchasing pre-split firewood at $400/cord, the operating cost is well below any other source (except AGS passive solar – see link in website). Operating effort involves splitting kindling now and then and loading the firebox once or twice a day during the heating season. This humble ritual of preparing and lighting a daily hearth-fire is good for the body, mind and soul. It is a purposeful engagement between the prime ordeal (of keeping warm) and the primordial elements… nurturing a union between the spiritual and practical aspects of life.
Tiny home rocket heater/cooker, nearing completion
Cob masonry heaters cost between $3000 ( for smaller rockets as pictured above) and $20,000 (for ones like the one pictured at top of this page). I used to feel bad about how much more they cost than a metal box stove. But then a friend pointed out: ‘Many people are happy to shell out the same difference for extra features on a new car – which depreciates rapidly thereafter.’ I have gradually come to agree with this perspective. Masonry heaters costs more – because they are worth more. They are heirloom appliances that last 100s of years with minimal maintenance and repair; and they will serve many generations to come. When the rest of the house fades away, the hearth will remain, and a new house can be built around it.
In my opinion, a clay-masonry heater built on location by a local artisan utilizing regionally available materials and fuel supply… is the simplest. most efficient, durable, sustainable and health-enhancing a way to heat our dwellings.
For more information about the heaters I build, call Pat at 250 748 2089, or email: patamosoma@gmail.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/
Before
After
