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Gas Furnace Types and Efficiencies

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Gas Furnace Types and Efficiency Ratings
Gas Furnace Types and Efficiencies

Trane XT90 High Efficiency Furnace

Trane

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 I’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 broad category types:

  • Conventional Furnace
  • Condensing Furnace
These furnace types operate very differently. Conventional furnaces exhaust combustion gases fast and hot so the gases 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 gases 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 gases of water and carbon dioxide (which form carbonic acid). The exhaust gases are depleted of heat until the water condensate drips out of the furnace's heat exchanger and the flue gases escape from a plastic PVC pipe instead of a chimney.

Furnace Burner and Blower Operation
Within the furnace type categories of conventional and condensing, furnace types break down even further according to the operation of the burner and blower with the term "stage." Stage is used to refer to 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:

  • Single Stage Furnace
  • Two Stage or Dual Stage Furnace
  • Modulating Furnace
A single stage furnace is the least expensive and means the burner and blower has one "on" stage. A two stage or dual stage furnace has electronic controls that allows the burner flame and burner to be on 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 allows 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 efficiency categories (more information found in the following pages):

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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