Masonry Stove

The wood-burning masonry stove is the main source of backup heating. It is a small 700 kg high-efficiency, ultra-clean Finnish-soapstone masonry stove designed for heating approximately 50 m2 (in my central Spain climate and for a high level of insulation). It is used to heat the main living room-kitchen-dining room area; however, when the sun is able to provide most of the necessary heating needs for all of the home, the masonry stove's additional heat is able to bring up the air temperature throughout the house the necessary degrees. Nevertheless, there are five to six weeks in the winter when my small masonry stove is not enough to cover the heating gap between the energy provided by the sun (which in cloudly periods is next to nil), dump load and waste heat, and that needed to heat the whole house.

These Finnish masonry stoves burn extremely efficiently and cleanly. A very small percentage of the wood's energy is lost up the chimney, and the gases that exit out the top of the chimney are very clean. Both of these factors are due to the design of the masonry stove. The masonry stove is designed to burn the wood at extremely hot temperatures. Ultra high burning temperatures enable the wood to be burned completely and for the gases released during burning to also be burned; these ultra-high temperatures, therefore, manage to thoroughly extract the energy stored in the wood and burn the dirty gases away - efficient, clean burning. In order to achieve these ultra-high temperatures, the wood needs to be burned quickly - the faster the wood's energy is released, the hotter the fire gets. Fires in these type of masonry stoves usually last less than an hour. The trick to getting the wood to burn fast is to feed the fire lots of oxygen because fire is a chemical reaction between carbon and oxygen, a reaction that results in heat energy being released. The more oxygen, the faster the reaction can occur and the faster the wood burns away. Furthermore, it is best to spread these large quantities of oxygen over the entire exposed surfaces of the wood - the more carbon-to-oxygen surface contact, the faster the wood burns. The masonry stove has a small door that opens to extract the ashes from the bottom of the masonry stove; on this door is a small vent slide that allows for control of oxygen levels into the burning fire. Fully opened this vent allows large quantities of air into the stove. As the air enters from below the wood (which sits on a thick steel grate through which ashes fall into a metal box below) and travels up through it, the oxygen is forced to spread uniformly throughout all the surface areas of the wood. By splitting the wood into relatively thin slices, this creates more surface area for carbon-to-oxygen reaction to take place and allows for faster, hotter burns. Moreover, hot air moving over the wood helps it to burn even faster. Since the air travels a little within the interior of the hot masonry stove before reaching up into the wood, it gets heated and therefore facilitates fast burns. The fires in these stoves are very powerful, not like the slow, tame fires in old-fashioned chimneys - they are extremely hot, large, fast-moving flames. For this reason one has to be careful when opening the steel-and-glass door and adding extra wood to the fire.

The problem with extremely hot fast burns is that the huge amounts of heat released in a short period of time would make a room get extremely hot for an hour and then be cold for the rest of the day. This is where the soapstone thermal mass comes in. Soapstone is a unique kind of stone with exceptional heat absorption and retention qualities. Soapstone is able to absorb very large quantities of heat in a very short period of time and then release that heat slowly over many hours. The soapstone therefore absorbs the vast majority of the heat put out by the fire. This heat is released over many hours (in my model it is over 12 hours, other bigger models allow for heat to be released over a 24 hour period so that only one burning a day is necessary) predominantly in the form of radiant heat but also through direct conduction to air in contact with the surface of the soapstone. In a traditional chimney, the fire is made directly under the chimney exhaust and the hot air rises straight up and out, heating very little of anything except for the air over the roof of the house. These Finnish masonry stoves avoid wasting heat like this and ensure that heat gets absorbed by the soapstone by creating up-down air channels within the mass of the soapstone. As hot air rises, and refuses to fall, the hot air gets 'trapped' and 'concentrated' in these up-down exhaust air passageways; the heat of this superhot air gets absorbed by the soapstone, raising the temperatures of the stones to very hot levels. Because these stones get so hot, the thermal mass of the soapstone is divided into two parts with a thin layer dividing them; in essence, two structures are built one within the other separated by a layer of fireproof insulation. This ensures that the exterior soapstone does not get so hot as to burn at the touch. Furthermore, the insulation helps in releasing the stored heat into the room slowly over the day. The soapstone, unfortunately, is not capable of absorbing unlimited quantities of heat energy; at some point, the increasingly hot temperatures will cause the atoms of the stones to move fast enough to create expansion, at which point the stone block cracks. This is not good for the masonry stove and needs to be avoided. For this reason there is a maximum amount of wood (based on weight) that can be burned over a 24 hour period; for my masonry stove, I am limited to about 9 kg per day - I burn six in the morning and three in the evening. If I were to burn this maximum amount every day of the heating season (which I don't), I would end up burning about 1000kg of wood.

There are a number of advantages of using masonry stoves for heating. In comparison to most other types of biomass-burning (and fossil-fuel burning) space heating devices, the wood (other types of biomass could be burned as well) is burned extremely efficiently thereby thoroughly releasing all of the wood's stored energy. This results in less wood needed. Since the released gases are also thoroughly burned, and their stored energy also released, the exhaust emissions are extremely clean and almost completely limited to CO2 - with biomass-generated CO2 not being a net contributor to global-warming IF the biomass is sustainably harvested. Because the fire lasts such a short time (from 20 minutes to 1 hour in mine, depending on quantity of wood burned - from 3 to 7kg), the amount of cold exterior air coming into the house throughout the day to replace the air consumed by the fire and up the chimney exhaust is relatively small in comparison with most other continuously burning space heaters; this results in energy-savings (less wood needed) by reducing these large infiltrations of cold air. And, as mentioned above, most traditional biomass-burning devices simply exhaust most of the heat generated straight up and out of the house in the form of hot air; most masonry stoves greately limit this loss of heat up the chimney exhaust, thereby saving more wood. Another benefit is that the complete burning of the wood generates much less ashes than most other biomass-burning devices, reducing cleanup work; I only have to empty my ash box about once a week. And the thorough burning of the gases virtually eliminates creosote buildup, which practically eliminates this cleanup maintenance task common in most other biomass-buring devices. Another benefit in relation to forced-air and air heating systems is that the heat from the soapstone is released into the room primarily in the form of radiant heat, just like that of the sun, heating objects directly as opposed to first heating the air through conduction and then the air heating the objects in the room, including humans, through further conduction. This results in greater energy-efficiency as the masonry stove can keep a person warm without needing to maintain high indoor air temperatures. Radiant heating is also said to be more comfortable than forced-air heating.

I cut my own wood. Most of my three hectares of land is forested. The land all around mine is forested. Much of the land to the north half of my property is public and unwisely unkempt and uncared-for, and the lands to the south half are abandoned private lots even more unkempt and uncared for. There are numerous dead or diseased trees on and around my property - due to fires, drought, disease, and infestations (particularly processionary moth infestations) - that need to be taken away. Many other trees need to be pruned - to cut away dead branches, to make them more capable of withstanding wild brushfires, to better withstand infestations, to grow healthier and taller, etc. There is no shortage of either dead wood or live wood that should be cut away. Actually, there is way too much. There is simply too much for me to clean up. Even though by law private land owners have a legal (and moral) responsibility to keep their properties clean of dead material, this obligation goes ignored, unfulfilled and unenforced. Public authorities also have an obligation to keep public lands clean, but they too quit this responsibility. I asked the local forest rangers last year when they planned to take away the dead trees on public land that had been burned by the forest fire of three years ago, and their response was that they would never do it because the local municipality was broke and didn't have any money for such a low priority. They told me that if I was worried about this dead, burnt wood making another fire more likely and more dangerous, then I was free to cut it away. Easier said than done. Whenever a dead pine tree falls, I try to cut most of it away, but this requires time, sweat, and patience - and I have to work with a small electric chainsaw. Anyway, I have firewood piling up all around my house. Why do I go to the trouble to mention all this? For two reasons. One, to highlight that there is a difference between responsible, sustainable harvesting of biomass that helps the environment and irresponsible, unsustainable harvesting of renewable biomass like wood. Cutting away dead and diseased trees to help the remaining trees to grow stronger and healthier and to lessen the impacts of the frequent forest fires is responsible and sustainable; cutting away perfectly good trees without this having a net positive impact on the health of the surrounding trees and forest for reasons such as illegal housing developments or golf courses - or for inappropriate firewood use - is irresponsible and unsustainable. And second, to highlight that there is an abundance of biomass out there that can be harvested in a responsible and sustainable way.