The stack loss of a combustion system is basically the amount of heat that travels up the stack without doing any useful work. The stack loss hence depends on the amount of air travelling through the burner and the temperatures of inlet and outlet. There are also some fuel-specific factors that will affect stack loss. Naturally, this is not a true measure of all the losses that will occur in a combustion system, but stack loss is a handy number that shows factors that can be changed when tuning a combustion system. As can be seen from the formula, the temperature difference between inlet and stack is important, which is why an accurate measurement of ambient temperature at the inlet is essential for correct calculation of stack loss.