Because we live in a northern climate with long, cold winters, we designed our house to take maximum advantage of solar gain by orienting it towards the sun. In addition, we gave careful attention to the insulation to protect against heat loss.
For a heating fuel, we chose wood because it is abundant in Vermont, and it is a local, renewable resource. Unfortunately, most wood stoves pollute both the inside and outside environment, are not very efficient, and provide uneven heat with great temperature fluctuations. To overcome these problems, we installed a masonry heater.
Unlike conventional American wood stoves, masonry heaters burn wood very efficiently (>90% vs. 70-80% for a good modern stove) because the burn takes place at a very high temperature (1500°F vs. 400—900°F). The combination of efficient burning coupled with good insulation and passive solar heating means that we can heat our 2500 square foot home with less than 4 cords of wood per winter, despite living in such a cold climate.
Another advantage of this high temperature, high efficiency burn is that it virtually eliminates smoke (typically a source of indoor air quality problems with wood burning) and particulate emissions (typically a source of outdoor air pollution from wood burning).
Instead of heating the air directly around it (convection heat), masonry stoves work by radiating heat; more than 80% of the stove’s mass is thermal mass that absorbs heat from the fire and then radiates it slowly. Although the masonry stove may feel warm to the touch, the heat-absorbing properties of the stove are so great that the surface can still be comfortably touched even when a fire is burning. Straw bale construction works well with masonry heaters, because the plaster on the straw bale walls has considerable thermal mass; the walls can absorb large quantities of heat and then slowly radiate back into the house. As a result of this radiant heat, the ambient air temperature is more stable, in contrast to the sharp fluctuations with conventional American wood stoves.
Because masonry heaters (and the thermal mass of the house) release the heat so slowly, we could leave for several days in the wintertime and still have our house remain relatively warm. However, if we left for an extended period of time in the winter the house would become rather cold, a problem for our pets while we’re gone, and also a problem for us when return, since the masonry stove is slow to reheat the house. Therefore we decided to install a backup hydronic (connected to our hot water system-see below) baseboard heating system.
Our new building also uses a masonry heater as its main source of heat. This time the heater is considerably smaller and will use, we hope, only 1-2 cords of wood to heat the entire space (about 1700 square feet). The masonry heater is located in the 750 square foot apartment of the new building. The heat from the masonry heater, and from its plastered chimney, rises up into the attic space above the apartment which houses our new office. At the time of this writing we have been heating the entire new structure with wood for about two months. As expected the apartment floor is comfortable and the office above is too hot. Once walls and doorways are put in to block the free flow of heat, we think we will have ideal temperatures in both spaces. In the event that the apartment dwellers choose NOT to use the masonry heater, there is a hydronic system for heat with baseboard heaters on all three floors of the new structure (basement, apartment and office).