Size and Output

     Heater output is determined essentially by three factors, surface area, temperature, and emissivity.  It is also effected by room temperature, but we're leaving that for another day.  Emissivity is a way of stating the amount of energy a given material puts out per degree per unit of surface area.  It's a way of comparing materials.  

     For example, Soapstone has a higher emissivity than brick.  German Kachels are engineered to maximize emissivity and durability.  Some are more emissive that even soapstone.  

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    A given material at a given temperature gives off a given amount of heat (which I might express as watts per square meter or BTUs per hour per square foot). 

     Whether that surface area is a simple box seven feet tall in the middle of the room or a bench running around the perimeter of a room, each square foot that's exposed and radiating into the space being warmed will put out a fixed quantity of heat energy per degree.       

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     It's a bit oversimlified, but it's a good place to start.  If you need output X you need a certain square footage of skin that hits said output after you burn a fire in it.

     

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Sizing

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     One way to come in is to say "I've built this heater before in a house like this and it worked pretty good, so let's do that."  That's actually a great way to go if that heater works for you. I use this approach in some situations.  

     Another way is to start from surface area (SA).  Once SA is figured, we can figure how much wood we'll need to burn which gives us firebox size.  We then design as long a channel into the stove as possible.  

     Another approach goes from heating need to firebox size and channel length.  This approach assumes certain wall thickness and so on, and it works really well if those assumptions are met.   

     A final approach happens when the aesthetic desires call for a heater larger than would be required; When we want a big heater or have a large aesthetic space to fill.  We then design the outside and give it an appropriate firebox for it's size.

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Conclusion

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     Sometimes it makes sense to follow different pathways to get there, and what's great is that there's enough room at the top of the performance curve for a cozy warmth to be created no matter how we approach it.   We vary wall thickness, structure, surface area, and channel length (or bell size) to refine what we're doing for a specific customer. 

      At the end of the day Output = Area x Temp. x Emissivity.  Our job is to get the right Surface to the right temp for you!