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Help understanding thermodynamics/BTUs

I am trying to design a heating element to heat a greenhouse. I found a very accurate calculator that helped me find the BTU/hr I need to heat this area to my desired night time max temp, that figure is 116,556.8BTU/hr. I need to figure out the dimensions of the stove I am building so I make it large enough to heat the space I am trying to heat. I also would like to make the fuel chamber large enough for the heater to last several hours (but this is less important and easier to figure out) I would like to know how different dimensions would affect the intensity and duration of my heat. I imagine the wider the spread of a flame, the more heat is produced, where height is not so much a factor.

Now, let's talk about the heater:

How it works: It is very very simple. The stove works by igniting combustible wood gas that is constantly released from underneath the top lit flame. The top lit flame begins to burn the top layer of fuel (usually wood pellets or wood pieces) and then the continuous up draft air flow (provided by the fan at the bottom of this device) pushes this gas up for the flame to burn. This gas is "preferred" and the process leaves solid chunks of carbon behind. This type of stove is not regulated by thermostat. It is a live fire which is controlled by the fan. The exhaust is clean and does not require an exhaust system for leaving the greenhouse. This is why it can be so small and have a high BTU/hr rating - because it is the exhaust is most of the heat.

Luckily, I have some head start actual figures. There is a company selling one of these such woodgas stoves and the specs are: (This is a completely cylindrical stove. The fuel chamber rests inside of an outer chamber (which in the dimensions listed below is the "total").
Total height: 11.5"
Total diameter: 14"
Fuel chamber height: 8.5"
Fuel chamber diameter: 10"
weight: 15lbs
material: stainless steel
Heat output: 50,000 BTU/hr
Power usage: 240mA @ 9v

Final details/thoughts needed for answering this question:
If we made fuel type and amount of air-flow into constants, we are left with diameter and depth of the cylinder to reflect the BTU/hr rating of this stove. Obviously, the stove needs some amount of depth in order to work, but after a few inches in height....I'm not sure making the fuel chamber taller (I'm using depth and height interchangeably now) would affect the heat output. As far as I can tell, it will be the diameter of this cylinder which will change the amount of heat this puts out. To calculate the BTUs of the device two types of heat must be measured; gaseous heat and radiant heat: 1. heat from the exhaust 2. the heat which is radiating from the metal fuel chamber. The example stove I have given unfortunately doesn't tell us if they took the BTU/hr measurement from the exhaust only, or if the radiant heat from the device itself was also included. Any thoughts?

Thanks for taking a look, any feed back is appreciated.

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