Modern condensing furnaces are the most energy efficient heating systems available. An easy way to choose among the most efficient furnaces is to look for Energy Star® label. Energy Star® furnaces must have an AFUE rating of 90 percent or greater, and it will have a sealed combustion chamber.
Furnaces are rated by their Annual Fuel Utilization Efficiency (AFUE), which must be posted on the furnace’s yellow Energy Guide label. If your furnace has an old-fashioned pilot light that you can see constantly burning inside the combustion chamber, or if it is more than 25 years old, it probably has an AFUE rating of 60 to 70 percent. That means that more than one-quarter of the fuel you purchase is wasted, and never gets used to heat your home. You should consider replacing this type of older equipment.
You have two efficiency choices when shopping for a new gas furnace. A mid-efficiency furnace (80% plus) should save you between 15 and 20 percent of your current heating costs. A high-efficiency furnace (90% plus) furnace should save you between 20 and 25 percent.The first step up in modern furnaces is to equipment with an AFUE rating of up to 80 percent or slightly more. This furnace is equipped with electronic ignition and a draft fan, and it draws combustion air from within the home. This type is often called an 80-plus furnace. With the price of natural gas rising, these 80+ furnaces aren’t the best choice.
The best choice is to choose a furnace with an AFUE rating of over 90 percent. Condensing furnaces recover extra heat by extracting water from the combustion gases within a special corrosion-resistant heat exchanger. Like the 80-percent-plus furnaces, they are equipped with electronic ignition and a draft fan. They also draw combustion air from outdoors, providing an extra margin of safety by eliminating the possibility of back-drafting and carbon monoxide poisoning.
Be sure your furnace is sized correctly to your home’s heating load. This sizing should account for any improvements you’ve recently made to the building shell, so your new furnace may require smaller capacity than your old one.
See The Homeowners Handbook to Energy EfficiencyorResidential Energy: Cost Savings and Comfort for Existing Buildings Chapter 6 for more information. For professional information about heating systems and energy efficiency seeSaturn Hydronic Systems Field Guide.