Conventional vs. Condensing Gas Furnaces
The technologies used in forced air furnaces fall into two broad category types:
Conventional furnacesCondensing furnaces
These furnace types operate very differently. Conventional furnaces represent an older design that exhaust combustion gases up a chimney flue very quickly before they have a chance to cool and before moisture can condense out of them. As a result, the furnace heat exchanger fails to collect the maximum amount of heat from the fuel. 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 gasses 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 mortar or metal chimney.
Furnace Burner and Blower Operation
Within these two broad categories—conventional and condensing—furnace types break down even further according to the operation of the burner and blower. The term “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:
Single-stage furnaces: This is the least expensive, and the label means that the burner and blower has one “on” stage. Two-stage or dual-stage furnaces: This type 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.Modulating furnaces: This type has electronic controls for the burner and blower motor that allow very fine adjustments to the burner setting and blower motor speed, modulating them to keep the temperature of the room very close to the thermostat setting at all times
Furnace Efficiency Ratings
Finally, a furnace can be categorized according to AFUE (Anual Fuel Utilization Efficiency) rankings.
55 to 72 percent AFUE low-efficiency furnaces: Now obsolete for new furnaces, furnaces with this low AFUE rating are still found in many houses. 78 percent AFUE low-efficiency furnace: Until Jan. 1, 2015, this was the minimum AFUE allowed for new furnaces. Standard/mid-efficiency gas furnaces: This group includes furnaces with 80 percent (up to 83 percent) AFUE ratings. High-efficiency gas furnace: These are Energy Star-certified furnaces with 90 to 98 percent AFUE ratings.
The AFUE measurement measures the amount of fuel converted to heat in the space in proportion to the amount of fuel that 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 percent, but furnaces with ratings this low are typically found only in manufactured homes. For a furnace to meet the Department of Energy’s Energy Star program, it must be a high-efficiency furnace with an AFUE of 90 percent or higher.
55 to 72 percent AFUE Furnaces (Low Efficiency)
These forced-air furnaces are now obsolete for new construction but were very common in the 1960s through the 1980s. Their characteristics include:
Non-compliant AFUE (for new furnaces) Non-electronic, standing pilot ignition Single-stage furnace Single-speed blower Natural draft exhaust Heavy cast-iron heat exchanger Sometimes were oil furnaces converted to natural gas
78 percent AFUE Furnaces (Low Efficiency)
In 1987, the National Appliance Energy Conservation Act set the minimum efficiency level for furnaces at 78 percent AFUE, which remained in effect until Jan. 1, 2015. Although an improvement over earlier furnaces, these are still considered low-efficiency furnaces by today’s standards. Their characteristics include:
78 percent AFUE Was typically used in manufactured/mobile homes, space heating applicationsElectronic ignitionSingle-stage furnaceSingle-speed blowerSteel tube heat exchangerThe natural draft that creates a flow of combustion gasses.
80 to 83 percent AFUE (Standard/Mid-Efficiency)
In 2007, the Department of Energy proposed a new minimum standard of 80 percent AFUE, set to go into effect in 2015. However, since furnace manufacturers had already met and exceeded this goal well before 2015, most consumers now install furnaces that exceed this minimum requirement, many opting for high-efficiency Energy Star furnaces. The characteristics of these furnaces:
80 percent is minimum AFUE allowed for new furnaces as of 01/01/2015 Some companies offer models up to 83 percent AFUE Electronic ignition Single- or two-stage design Single- or variable-speed blowers Steel tube heat exchanger Natural draft creates a flow of combustion gases
90 to 98 percent AFUE (High Efficiency, Energy Star Approved)
Introduced in 1992, the Energy Star program was a joint endeavor by the U.S. Environmental Protection Agency and the U.S. Department of Energy. It sought to identify and promote energy-efficient appliances aimed at reducing greenhouse gases. Consumers have enjoyed a variety of tax benefits and rebate opportunities by choosing efficient Energy Star models of furnaces and other appliances. Over the years, furnace manufacturers have met pace with the steadily increasing efficiency requirements for Energy Star qualification. Currently, Energy Star certification for furnaces carries different AFUE ratings for southern climates and northern climates. In 2012, for example, the Southern Energy Star qualification for furnaces was set at 90 percent AFUE, and 95 percent for northern states. In 2018, the DOE proposed 97 percent as the minimum AFUE rating to qualify for Energy Star certification, but that decision is still pending. Characteristics of these high-efficiency furnaces include:
90 percent to 98 percent AFUEMost carry Energy Star ratings (90 percent AFUE for South, 95 percent for North) Currently, 98.7 percent AFUE is the highest rating available, offered by RheemCondensing furnace designElectronic ignitionSingle, two-stage or modulating furnacesVariable-speed blowerSteel tube main heat exchangerSecondary stainless heat exchanger for condensing flue gasesSealed combustion chamber
There are nearly 800 furnaces available that carry the Energy Star approval. Virtually every major furnace manufacturer offers Energy Star models.
