Gas Furnace Types and AFUE Efficiencies

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    Gas Furnace Types and Efficiency Ratings

    Repairman with Digital Infrared Thermometer Checks Gas Furnace Output Temperature
    BanksPhotos / Getty Images

    The gas furnace in your home is a critically important appliance and one you depend on to work reliably, efficiently and quietly. In this tutorial, we'll explain the various types of gas furnace options you have available, what makes them different, and what you can expect from them in furnace efficiency.

    Types of Gas Furnaces

    There are different types of forced air furnaces because of the different technologies used to meet energy efficiency and user comfort goals. Furnaces fall into two...MORE broad category types:

    • Conventional Furnace
    • Condensing Furnace

    These furnace types operate very differently. Conventional furnaces exhaust combustion gasses fast and hot so that the gasses will exit the chimney flue before cooling and condensing. As a result, the furnace heat exchanger does not collect as much heat from the fuel combustion process as possible.

    With the advent of the condensing furnace, that all changed. Condensing furnaces capture heat even after the combustion exhaust gasses have "cooled" and condensed. They do this by using two heat exchangers, one for primary heat exchange and the other to handle the corrosive condensed exhaust gasses of water and carbon dioxide (which form carbonic acid). The exhaust gasses are depleted of heat until the water condensate drips out of the furnace's heat exchanger and the flue gasses escape from a plastic PVC pipe instead of a chimney.

    Furnace Burner and Blower Operation

    Within the two broad categories—conventional and condensing—furnace types break down even further according to the operation of the burner and blower, using the term "stage." The stage is used to refer to the operation of the furnace's burner and blower, and, indirectly, the level of sophistication of the technology controlling the burner and blower. These stages include:

    1. Single-Stage Furnace
    2. Two-Stage or Dual-Stage Furnace
    3. Modulating Furnace

    A single-stage furnace is the least expensive, and the label means that the burner and blower has one "on" stage. A two-stage or dual-stage furnace has electronic controls that allow the burner flame and burner to be at a high and a low setting, depending on the level of heat required. The modulating furnace has electronic controls for the burner and blower motor that allow very fine adjustments to the burner setting and blower motor speed and modulates them to always keep the temperature of the room very close to the thermostat setting.

    Furnace Efficiency Ratings

    The existing furnace in your home, or maybe one you are considering purchasing, can be organized into these AFUE efficiency categories:

    • Low-Efficiency Furnace: 55% to 72% AFUE (obsolete technology)
    • Low-Efficiency Furnace: 78% AFUE (minimum AFUE allowed for new furnaces)
    • Standard/Mid Efficiency Gas Furnace: 80% (up to 82%) AFUE
    • High-Efficiency Gas Furnace: 90% to 98% AFUE (Energy Star approved)

    The metric used to measure furnace efficiency is called the AFUE rating. AFUE is an acronym for Annual Fuel Utilization Efficiency, and it measures the amount of fuel converted to heat in the space in proportion to the amount of fuel which enters the furnace. The higher the AFUE, the more efficient the furnace.

    Homes today are required to have an AFUE rating of at least 78%, but furnaces of this low AFUE are typically found in manufactured homes. For a furnace to meet the DOE's Energy Star program, it must be a high-efficiency furnace with an AFUE of 90% or higher.

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