The efficiency is a ratio of benefits to effort which is based on the energetic outputs. In the case of a steam boiler, the efficiency is therefore the quotient of the thermal output released in the form of steam to the thermal output supplied in the form of fuel. The efficiency is a measure of how effectively the energy is converted in the boiler. The efficiency of steam boiler systems is determined by the combustion efficiency and the heat losses due to radiation and conduction on the boiler surface. In this case, it is important for the efficiency to be defined only for a new system at its nominal steam output or for specific partial load steam outputs when the system is in equilibrium.
Losses due to starting up and shutting down, surface blowdown and bottom blowdown, contamination of heating surfaces, feed water treatment and heat losses in further pipework are disregarded.