Gas Furnaces

A furnace is an enclosure in which heat is produced by burning fuel. Furnaces are commonly used to warm a building, and in some parts of the world, to smelt metal. In the U.S., natural gas is the most common source of fuel for furnaces. Other sources include coal, oil, wood, and liquefied petroleum gas (lpg). The vast majority of modern homes use gas furnaces for heat.


What Is a Gas Furnace?

A gas furnace is made up of several basic components including a:

  • Cabinet
  • Blower
  • Burner assembly
  • Gas valve
  • Heat exchanger.

Gas furnaces work by burning natural gas or propane in a sealed or open chamber to heat an exchange. A fan or blower distributes the warm air throughout ductwork to heat an interior space. Several accessories may be added to gas furnaces for greater efficiency including air filters or cleaners and a humidifier, to name a few.

Energy Efficient Gas Furnaces

In the late 80s to early 90s, the federal government issued minimum efficiency standards for new gas furnaces. In 1992, the minimum-efficiency standards for new gas furnaces took effect. An Annual Fuel Utilization Efficiency (AFUE) rating is a measure of seasonal performance. New gas furnaces require at least a 78% AFUE, while older furnaces typically have a 55 to 65% AFUE. Power combustion gas furnaces have an 80 to 82% AFUE, and condensing gas furnaces have a 90% AFUE.

According to the American Council for an Energy-Efficient Economy, new gas furnaces are categorized in one of two classes of efficiency:

  • Power combustion - also known as non-condensing models; equipped with larger fans for use with A/C systems; recommended for use in warmer climates
  • Condensing - recommended for use in colder climates.

Gas Furnaces Styles

There are several different gas furnace styles on the market today. Gas furnace styles are based on the design of the duct system and the direction of airflow. The different styles include:

  • Horizontal - used mostly in attics or crawlspaces; the air is distributed sideways
  • Counterflow - air ducts are installed under a slab; the air is distributed from top to bottom
  • Lowboy - blowers are located next to the combustion chamber; the air is distributed upwards
  • Highboy - blower are located at the base of the furnace; the air is distributed upwards.