Distribution of Radiant Heat
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Radiant Heating with a Tulikivi

We are often asked, how can the warmth of a Tulikivi reach beyond the room in which it is located? The assumption of the person asking this question usually is that, unless air is being forced from one room to the next, even heating of adjoining rooms will not take place and that the room with the Tulikivi will be very warm or even hot in comparison to the surrounding rooms. This is, in fact, what happens when a wood stove (rather than a mass storage heater) is used for heating. Since most people in this country are familiar with wood stoves, and very few familiar with mass storage heaters like the Tulikivi fireplace, it is not surprising that people are skeptical when told that the air temperature, between the room with the Tulikivi and the adjoining rooms, will be quite even if not the same temperature.

Radiant Heat Distribution

So why is a Tulikivi fireplace able to heat more than one room relatively evenly? The answer lies in the way heat (BTU) is transferred to the living area by a Tulikivi versus a traditional wood stove (or for that matter any forced air system). With a Tulikivi most of the heat is transferred to the living area by radiation rather than convection. Instead of heating the air directly the radiation from the Tulikivi passes through the air in the room, and warms all of the solid surfaces in the room that it "shines" on. In effect, the Tulikivi becomes a miniature "sun" in your living area, warming those surfaces evenly to, for example, 75 degrees. The surface areas warmed, and now effectively heating the room, can add up to many hundreds of square feet. The amount of mass warmed, and now effectively acting as a thermal flywheel stabilizing the room temperature, can equal many thousands of pounds.

Heating Adjoining Rooms

So what causes heat from a Tulikivi to migrate to adjoining rooms?... Don't you need fans to move heat from the Tulikivi to other rooms? Would it be a good idea to place the cold air return near the Tulikivi in order to circulate its heat throughout the house? The answer to these questions, which assume that heated air must be moved mechanically, is actually no. The secret lies in the fact that the surface temperatures in the adjoining rooms (i.e. walls and floors) are cooler than in the room directly heated by the Tulikivi. The laws of thermal dynamics dictate that heat flows from warm surfaces to cool surfaces. This occurs through convection of air between rooms. As long as there is an opening between rooms the air temperature will equalize naturally. The natural convection process will continue until the wall surfaces in both rooms are equal. Convection will not stop until the Tulikivi stops radiating to the surfaces of the room in which it is located (normally between 12 to 24 hours). When you walk from one room to the next it will be similar to walking from the direct ray of the sun in to the shade. When you walk into an adjoining room you will loose the direct warmth of the Tulikivi "sun" but the air temperature will change very little if at all.
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