Boilers with heat outputs of 300 000 Btu/hr to 2 500 000 Btu/hr are rated by Thermal Efficiency.
Thermal Efficiency Equation
We are interested in the steady-state Thermal Efficiency – i.e., after the flue gas temperature has warmed up and reached equilibrium. Many combustion systems do not operate in steady-state equilibrium: they cycle up and down, taking a significant time to reach equilibrium, if at all. Nearly all transient systems are significantly less efficient than ones that operate in the steady state.
Thermal Efficiency is a steady-state measure only and does not include the effects of heat loss caused by on-off cycling or transient operation. This measure is different from the Annual Fuel Utilization Efficiency (AFUE) rating, which measures the average efficiency of a system over a year. The AFUE rating takes into account the cyclic on/off operation and associated energy losses of the heating unit as it responds to changes in the load, which in turn is affected by changes in weather and occupant controls.
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