Question

How to calculate stack heat losses for furnace?

Answer 1

by clinical or digital thermometer

Answer 2

/// hellooo///

write a combustion reaction with stoichiometry of react and product finding out the product gases released .
you have to find out the adiabatic flame temperature.
the temperature of the products is referred to as the adiabatic flame temperature. 
This is the maximum temperature that you can achieve for a given reactant.

hope this helps you...!!