Annual Fuel Utilization Efficiency (AFUE)
The Annual Fuel Utilization Efficiency (AFUE) rating is an indication of how well a furnace converts energy into usable heat. The rating is expressed as a percentage of the annual output of heat to the annual energy input to the furnace.
- Low Efficiency: AFUEs below 71 percent.
- Mid-Efficiency: AFUEs between 71 and 83 percent.
- High Efficiency: AFUEs of 90 percent and above.
Ratings between 84 and 89 percent are not common but keep in mind that acidic condensate, harmful to the furnace, forms at these percentages. Public Law No. 100-12, passed in1987, requires that all gas furnaces manufactured after January 1, 1992, have a minimum AFUE of 78 percent.
Atmospheric Vent Combustion
If a chimney is available, furnaces with this system are the least expensive to install. Atmospheric vent furnaces have AFUEs of 60-65 percent when equipped with standing pilots, and AFUEs of 63-70 percent when equipped with electronic ignition systems.With special vent dampers, atmospheric vent units can achieve AFUEs of 78-80 percent.
Condensing (or Recuperative Units)
These units are super efficient with some designs reaching AFUEs of up to 97 percent. Unlike conventional forced air furnaces, condensing units capture most of the water vapor and heat contained in hot flue gases that would normally escape up the chimney. The escaping gases then pass through a second heat exchanger and condensate is expelled. The heat exchangers are made of corrosion resistant stainless steel, and many have lifetime warranties. Exhaust is cooler than that of conventional furnaces and can be vented with PVC piping.
Downflow or Counterflow Furnace
These units have a blower at the top to draw air into the furnace. Heated air is blown out at the bottom. This type of furnace is used to supply floor duct systems.
An electronic ignition eliminates the need for an energy-wasting standing pilot. Fuel is used only when needed and the pilot is ignited with an electric spark.
The heat exchanger is a metal chamber in the furnace that houses a gas burner. The flame produced by the burner heats the chamber. When the outside of the chamber becomes hot, the air surrounding it is warmed and used to heat the house. Improved designs enhance efficiency and run much more quietly.
Horizontal Flow Furnace
Air travels horizontally from one side of the heater, across the heat exchanger, and hot air is blown out the other side. This type of furnace is installed in areas with limited head room, such as attics or crawl spaces. They can also be installed below floors or suspended below ceilings.
In this system, combustion is produced by a blower. The blower pushes the combustion gases through the vent, and regulates the amount of combustion air. Power combustion furnaces do not require a draft hood. This reduces off-cycle losses and improves efficiency. Many power combustion furnaces operate at 78-80 percent AFUE. When equipped with an additional heat exchanger, they can operate at AFUEs of 90-96 percent.
(These models are listed under condensing furnaces.) Pulse combustion is produced by self-perpetuated “pulses.” This unique system mixes air and fuel in a sealed combustion chamber. A spark ignites the mixture, and the resulting increase in pressure closes the gas/air inlet valve. The combustion products are forced through an exhaust pipe and the pressure in the combustion chamber drops, re-opening the inlet valve. The next combustion cycle is ignited by the heat remaining from the previous cycle. This process repeats itself about 60 times per second. Furnaces with this combustion system have AFUEs from 91-97 percent. Exhaust gases, at 100-200 degrees Fahrenheit, are cool enough to vent through PVC piping.
Sealed combustion systems draw in all the air used for combustion from the outside, and exhaust gases are direct vented to the outside. Since cold outside air is not mixed with the warm indoor air during combustion, efficiency is enhanced. Furnaces with this type of combustion system have an AFUE range of 70-80 percent.
These units have blowers at bottom that draw air into the furnace and blow the air out through the top.. These heaters can be installed in utility rooms, closets, or basements.
The vent damper is a “flapper” device installed in the flue. When the heat demand has been met, the damper closes, trapping residual heat for circulation in the home. When heat is needed, the damper opens before the burners are ignited to allow combustion fumes to escape. The damper remains open only as long as the burners are on. Burners cannot ignite if the damper is closed.
Installing a New Furnace to Reduce Heating Costs
Even if your older furnace is running fine, you can save large amounts of money by replacing it with a new model. You can even save on your air conditioning by choosing the right furnace. Compared with a 17-year-old furnace, a new furnace can save the average person hundreds of dollars per year. Based on the efficiency of your old furnace, a new furnace can cut your utility bills almost in half. By installing a new system along with a digital thermostat, you will notice even room temperatures, saving energy AND money.
You can choose from two basic designs of furnaces: condensing and non-condensing. The condensing models (this refers to the type of heat exchanger used) are the most efficient and the best choice for most homeowners. The efficiencies of condensing models range from about 90% to over 95%. These models are very efficient, and so little heat is lost in the flue gases that a chimney is not needed. The gases are exhausted by a 2-inch-diameter plastic pipe through an outdoor wall. With no need for a new chimney liner, a condensing furnace is often cheaper to install.
Some models also offer sealed combustion for better efficiency. The combustion air is drawn in from outdoors through another plastic pipe instead of being drawn from inside your house. Being sealed, there are fewer indoor drafts, less noise and less chance of hazardous back drafting.
For the ultimate in comfort and efficiency, but at a higher initial cost, is a two-stage heat output furnace with a variable-speed blower. This type of blower is needed if you want the best central air-conditioning. The gas burners operate at low heat levels, allowing the furnace to run with fewer on/off cycles. The blower will run quietly and switch to higher heat in very cold weather. If your budget allows, we recommend installing an air cleaner, providing allergy relief for your family.