The weight, or thermal mass, of the masonry heater determines the amount of heat it can store. This equals weight times stone temperature. The radiation surface determines how much heat transfers into the room. This equals area times surface temperature.
We call the difference between room temperature (also called ambient temperature) and the soapstone surface temperature Delta T. The lower the Delta T value is the more comfortable, healthful and safer the heat. Soapstone masonry heaters operate with an average surface temperature of about 170° F, or a Delta T of 100. Contrariwise, cast iron wood stoves can have a surface temperature of well over 400° F, or over double the Delta T.
A soapstone masonry heater has three operating phases:
- Warm-up phase: The surface temperature rises from 130°F to 170°F.
- Equilibrium phase: The hot core stones maintain the surface temperature nearly constant at 170°F.
- Cool-down phase: The surface temperature starts to decline gradually.
The heater surface has the effect of acting as a stone-air heat exchanger. Delta T, the difference between the soapstone surface temperature and ambient air, determines how much heat can physically exchange between these two means. For a Delta T of 100°F, calculate the required soapstone heater surface as follows:
We do not include the top surface of the soapstone masonry heater in calculating the radiation surface if it is insulated. We also do not include the bottom of the heater.