Types of Furnaces: A Comparison

Forced-air furnaces are the most popular heating choice in the United States. Between these, consumers have many choices. Furnaces can either run on natural gas or oil. They come with different blower settings and “stages.” They are rated by AFUEs, the meaning of which most people do not understand.

Learning furnace types and ratings is not difficult, however. In this blog, we break down the differences between models and explain a number of industry-specific terms.

Fuel comparison: oil vs. Natural gas furnaces

The two most common fuels for furnaces are oil and natural gas. Between these, natural gas is more common. Overall, these systems work much the same, except for the fuel they burn. The lifespan for these furnaces also runs about the same number of years.

When comparing prices, natural gas furnaces will cost about 10 to 25 percent more than oil furnaces. However, oil prices vary greatly from year to year. As recently as 2011, it cost up to three times as much to heat a house with oil as with natural gas. Natural gas prices, on the other hand, have regularly declined. Natural gas prices in Indiana, specifically, are among the lowest in the nation, according to the U.S. Energy Information Administration. So overall, natural gas furnaces will cost less than oil furnaces.

Oil furnaces do have some advantages over natural gas. For one, they can be combined with a hot water system to heat water. They are also safer than natural gas. Heating oil produces far less carbon monoxide than does natural gas. Also, if the heating oil leaks, it does not pose an environmental threat. A natural gas leak, on the other hand, is cause for an emergency, as it can ignite and cause a fire.

Types of furnaces: conventional and condensing

Outside fuel, furnaces fall into two categories: conventional and condensing. Conventional furnaces and condensing furnaces work using the same concept: they burn fuel, which produces heat and warms air, which blowers distribute throughout a structure. They differ, however, in what they do with the gasses (the fumes or exhaust) produced from combusting fuel. Conventional furnaces quickly vent the hot gasses out a chimney or double-walled vent pipe before these gasses cool. Because of this, the heat in these gasses escape, and about 20 percent of the energy potential goes to waste.

The best modern furnaces are condensing units. Condensing furnaces, available since 1982, capture the gasses produced when the fuel burns and collect the gasses’ heat as they cool. To do this, they utilize a primary and secondary exchanger. The primary heat exchanger exchanges heat from the majority of the gasses. The second handles corrosive gasses like water and carbon dioxide. After cooling, gasses escape via PVC, and water condenses and drips out of the furnace. Overall, this process makes these furnaces about 10 percent more efficient than conventional furnaces.

Blower and burners

Furnaces can further be broken down in terms of their stages. Stages refer to the ability of a furnace to control its burner and blower, the fan which moves the air.

Gravity furnaces

Gravity furnaces are old, octopus-style furnaces found in basements. These antiquated furnaces work with air’s natural flow, rather than blowers, to circulate air. As the furnace heats air, that warm air rises into upper levels of the house. The air eventually cools, and as it does, it returns through floor vents back to the furnace, where the cycle begins anew.

Manufacturers no longer produce gravity furnaces. Due to their inefficiency, they cost about twice as much to operate as modern forced air furnaces. They also heat buildings unevenly. Warm air from these systems was usually routed up through the middle of the house, which made the center of house warm and edges cold. Also, because the warm air was directed toward the upper stories, lower floors and basements tended to feel colder.

Single-stage furnaces

Single-stage furnaces are typically the least expensive type of modern furnaces. They are most commonly paired with conventional furnaces. In a single-stage furnace, the burner and blower have only one stage, or setting: on and off. Single-stage furnaces average about 80 percent efficiency.

Two-stage furnaces

Modulating furnaces have electronic controls for the burner and blower that allow for fine, 1 percent adjustments to better keep rooms at thermostat settings. Modulating furnaces reach up to 98.5 percent efficiency and are top-of-the-line products.


Furnaces are further broken down by their efficiency. Efficiency for furnaces is measured by AFUE, which stands for Annual Fuel Utilization Efficiency. AFUE is a measurement of the amount of fuel a furnace converts to heat in seasonal situations and takes into account peak and part-load situations. AFUE calculations also consider start ups; cool downs; other operating losses; and electricity used for the blower, inducer fan, and other controls. AFUE measurements do not take into account energy lost during ventilation, which can reach up to 35 percent. AFUE ratings range between 0 and 100.

Low-efficiency furnaces (obsolete)

Before efficiency regulations came into existence in the 1980s, manufacturers produced furnaces with efficiencies as low as 56-70 AFUE. In these old furnaces, the pilot light stayed on continually, as the furnaces had no electronic starts. They were equipped with only single-speed blowers, had natural draft exhausts, and utilized heavy, cast-iron heat exchangers. Low-efficiency furnaces such as these may have been conventional oil furnaces that were converted to natural gas.

Standard/mid-efficiency furnace

Standard or Mid-Efficiency Furnaces have most commonly have efficiencies between 80 and 83 AFUE, although they can have efficiencies as high as 89. 80 AFUE is the minimum efficiency rating allowed by Electric Code of Federal Regulations for new gas furnaces. Weatherized gas furnaces must have an AFUE rating of at least 81.

Mid-efficiency furnaces are the most common furnaces sold to consumers. Unlike low-efficiency models, these furnaces do have electronic ignition (and thus no pilot light). They come either as single or two-stage models and have either single or variable speed blowers. Also unlike low-efficiency models, mid-efficiency furnaces have steel tube heat exchangers and an exhaust fan that controls the flow of combusted air and gasses. They may, however, have a natural draft for gasses.

High-efficiency furnace

High-efficiency furnaces come with a 90-98.5 percent AFUE. In Southern states, these models can receive Energy Star approved. Unlike Mid-range furnaces, high-efficiency models are always condensing units. They as single, two-stage, or modulating models. They may have variable-speed blowers, and they have a sealed combustion tube chamber.

High-efficiency furnaces are the most expensive gas-model furnaces available, but they also save homeowners the most money month-to-month on their heating bills.

If you are considering a new furnace, Patriot Heating and Air Conditioning can help. We offer both conventional and high-efficiency units and can help determine which unit and style best meets your needs. Call or contact us today.

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