How Does a High Efficiency Gas Furnace Work?

Efficiency Ratings

Furnace and boilers are given efficiency ratings by the Federal Trade Commission. Efficiency is measured by something called the annual fuel utilization (shortened to AFUE). The AFUE measures how much energy a furnace uses compared to how much heat it puts out in one year.

For example, a furnace that has a rating of 80 percent means that 20 percent of the energy in a furnace's fuel is wasted, usually through leakage. High-efficiency gas furnaces have an AFUE of between 90 to 97 percent.

Summary

All gas furnaces work in much the same manner. Cool air enters the furnace though either the bottom or the top of the unit. The air travels through a filter to remove dust and allergens, then mixes with natural gas, which is then ignited by the pilot light in the combustion chamber. The thermostat triggers the gas valve to open or close, allowing the gas to enter the chamber or alternately, stopping the flow of gas, which turns off the furnace.

The ignited air heats up the heat exchanger, which is made of metal tubing. The metal tubes retain heat. As the air flows around the tubes, it heats up. The warmed air is then sent through ducts to the rooms of the house.

High Efficiency

High-efficiency gas furnaces work in the same basic manner as described in Section 2, with some caveats. First, high-efficiency furnaces have exhaust fans, which control the flow of the air and gases, allowing them to be more precisely directed. They are smaller and more compact, so air and energy are not wasted. They have smaller flue pipes, which also serve to reduce leakage.

The highest efficiency furnaces also contain a second heat exchanger, which condenses the flue gases even further, thus reducing the amount of energy needed to heat the air. They also have a double-sealed combustion chamber.

Resources

• More about high efficiency gas furnaces