Save Money with a New, Energy-efficient Furnace
(Page 2 of 4)
December 2008/January 2009
By Dan Chiras
Conventional natural-draft furnaces are the least efficient of all furnaces. Those manufactured before 1992 have efficiencies below 78 percent. Many are only 55 percent to 65 percent efficient, which means they convert only 55 percent to 65 percent of the fuel they burn into heat. The rest is wasted.
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High-efficiency furnaces produce a lot more heat from the fuel they burn, saving homeowners substantially over the long haul. Most high-efficiency gas furnaces are induced-draft models — so named because they contain an energy-efficient electric fan. It draws air from outside the home into the combustion chamber and propels exhaust gases from the combustion chamber out of the house via the flue pipe. The efficiency of induced-draft furnaces results from the use of more efficient heat exchangers, but also from an electronic ignition, which eliminates the need for a constantly burning pilot light.
The most efficient gas furnaces on the market today are condensing models. These furnaces contain a second heat exchanger, which extracts additional heat from the flue gases, cooling them until the moisture they contain condenses. (Condensation of moisture releases additional heat.)
Both condensing and non-condensing furnaces can be equipped with sealed combustion chambers. This feature prevents dangerous exhaust gases, such as carbon monoxide and nitrogen dioxide, from entering our homes.
When shopping for a new furnace, look for Energy-Star qualified models. Their efficiencies, which are listed as annual fuel utilization efficiency (AFUE), run from approximately 83 percent to 97 percent. As a rule, the induced-draft furnaces have efficiencies in the 80 percent range, and induced-draft condensing furnaces are in the 90 percent range.
Oil Furnaces
Home furnaces in many colder parts of the country burn fuel oil, a petroleum product. It’s injected into the furnace’s combustion chamber through a nozzle that produces tiny droplets that mix with air to promote combustion.
Most Energy Star oil furnaces boast efficiencies in the 83 percent to 86 percent range, with condensing oil furnaces at about 95 percent. All of these are much higher than oil furnaces made before 1992. Older furnaces are typically only 50 percent to 60 percent efficient.
Although condensing models are more efficient, they’re not common. Fuel oil contains many more contaminants (such as sulfur) than natural gas or propane. Condensing out of the combustion gases, these contaminants produce a corrosive liquid that can damage the internal components of an oil furnace. For this reason, contractors don’t often recommend these models.
While sealed combustion chambers are widely used in high-efficiency gas furnaces, few high-efficiency oil furnaces come with this feature. Experience has shown that cold outside air drawn into the combustion chamber of an oil-fired furnace reduces combustion efficiency and may impede start-up.
