Heating System
Combustion Efficiency

The efficiency rating of a gas or oil heating system is a measure of how effectively it converts fuel into useful heat. There are three common types of efficiency:

Combustion Efficiency tells the system's efficiency while it is running. Combustion Efficiency is like the miles per gallon a car gets when it is cruising a steady speed on the highway.

A more accurate estimation of fuel use, is the seasonal efficiency rating known as the Annual Fuel Utilization Efficiency or AFUE. This is a measure of the system efficiency and accounts for start-up, cool-down and other operating losses that occur under normal operating conditions. AFUE is like your car's mileage over all kinds of driving conditions from stop-and-go traffic to highway driving.

The third common efficiency measure is Steady-State Efficiency. After the heating system has been operating for a period of time, it is said to be operating at a steady state. The Steady-State Efficiency is the ratio of the heat actually available to the distribution system to the amount of heat potentially available in the fuel. Since Steady-State Efficiency takes into consideration the stack losses, it's lower than the Combustion Efficiency, but higher than AFUE which accounts for start-up and cool-down losses. Steady-State Efficiency can be calculated by a flue gas analysis.