Boiler efficiency is defined in two ways:
Input minus stack (chimney) loss, divided by input. This ranges from 75 to 94 percent.
Combustion efficiency = (Input- Stack loss)/Input
Overall efficiency in gross output divided by input.
Overall efficiency = Output/Input
Input is the amount of energy used by boilers.
Output is the amount of energy boilers produce for heating systems.
Steam boilers are operated by boiler mounted, pressure-actuated controls which vary the input of fuel to the boiler. Hot-water boilers are operated by temperature-actuated controls that are usually mounted on the boiler.
Hot water temperatures for heating are usually 180℉ - 200℉ for supply and 140℉ - 160℉ for return. The temperature difference varies between 35℉ - 40℉.