Combustion Wall Heat Loss
 

I am attempting to model combustion within a heater but am hoping to factor in for heat loss due to the walls. I have changed wall temperature, wall thickness, shell conduction, thermal conditions, radiation models, and wall material and have not been able to achieve an average temperature under 3000 F in my outlet when I should be getting something around 1800. Is there some way to model heat loss that I am unaware of?