Heat (Q) is the transfer of thermal energy between two bodies which are at different temperatures. The SI unit for heat is joule.
Heat is the thermal energy that flows between regions or objects that are not in thermal equilibrium. When two bodies of different temperature come into thermal contact, they will exchange their internal energy until the temperature is equalized.
In a boiler we find heat transfer coming from the combustion fire to the air flow and then through the wall of the water steam pipes into the working medium.