A fire always burning

How we designed and built the original thermal mass stove at Autosufficienza Farm

“It heats even when turned off” is one of the first comments received upon completion of construction: the original thermal mass stove you see in these pages is part of the natural building renovation of the first nucleus in local stone and natural lime of Autosufficienza Farm.

The unusual double flame shape immediately strikes, specifically wanted as a representative and connotative symbol of the Farm.

“The flame,” located in the large common hall, immediately becomes a focal point of attraction and gathering—a centerpiece for meetings, educational workshops, and social moments held during the many activities at the Autosufficienza farm.

In designing the stove, great care was taken in the challenging task of giving a sense of lightness and softness to this massive structure, which stands 3.4 meters tall and weighs over 3,000 kg in total.

What it is and how it works

Also known as a Stube, Tyrolean stove, Kachelofen, or more accurately, a Kachelgrundofen, it is a thermal mass stove built entirely from high-quality refractory material using the most advanced masonry heater techniques.

Utilizing the ancient principle of the Roman hypocaust, it features a very thick refractory core where combustion takes place. From here, an intricate network of channels known as the “smoke flue” (extending 8 meters in this case) guides the hot gases, which progressively release their heat to the refractory bricks.

Enclosing all of this, without ever touching and at a distance of at least 2 cm, is an additional layer of refractory material that creates the final shape of the stove.

The two “chambers” work in synergy to store heat, which is then released slowly over the following 24 hours without the need to add more wood.

The advantages of the thermal mass stove

The benefits of this type of heating are manifold, making it particularly suitable for the Autosufficienza Farm, as well as for stone house renovations and new low-consumption ecological buildings. It stands out for:

living well-being: operating primarily through radiation, it provides the sensation of having “the sun inside the walls of your home.” In this specific case, the stone walls further contribute to heat storage;
health: unlike other heating systems, radiant heat does not create convective air currents, meaning it doesn’t stir up dust or mites, nor does it dry out or diminish the quality of indoor air. It is the gold standard for those suffering from asthma or respiratory issues. Furthermore, like sunbeams, the radiation penetrates deep into the body, providing a pleasant sense of well-being and stress reduction during dark, cold winter days. It is no coincidence that this type of heating is increasingly used in new hospitals and nursing homes in Austria, and not just in wellness centres;
savings: wood has been the most cost-effective energy source for centuries, and this stove’s wood consumption is extremely low—the lowest among wood-burning systems relative to the volume of the space being heated. In this specific case, only one load per day of 14 to 29 kg is required, depending on the outdoor temperature;
ecology: it is an eco-compatible heating system; wood is currently the only renewable heat source at our disposal;
efficiency: it achieves nearly 90% efficiency over a complete thermal cycle. Therefore, it heats not only when the fire is lit—as is the case with cast iron or pellet stoves—but throughout the long cycle between one wood load and the next, which generally lasts between 12 and 24 hours;
low tech & autonomy: the total absence of electrical components ensures the stove always functions, even during power outages. Furthermore, it is not subject to the typical malfunctions of electronic components over time, as it has none. This makes it particularly suited to the spirit of the Autosufficienza Farm and to off-grid situations in general;
low-maintenance: ash only needs to be removed from the combustion chamber a couple of times per season. The smoke flues and the chimney are generally cleaned every 15 years, so maintenance time and costs are almost zero;
design: this is an artistic, custom-built stove created after a careful design and calculation phase in accordance with current Italian regulations and the strictest Austrian UNI standards. The “custom-made” nature of these stoves makes them particularly suitable for solving any situation, even the most difficult ones, which conventional prefabricated stoves generally cannot guarantee. The design is unique and enhances the beauty of the space, becoming its centerpiece: in this particular case, it is intentionally organic;
durability: if used correctly, they function for many years and are passed down from generation to generation; in Alpine regions, there are still thermal mass stoves in operation dating back to the 18th century;
cost: the quality-price ratio makes it highly advantageous when considering that, in addition to all the benefits listed above, it can entirely replace boiler heating systems and wall or underfloor systems.