Replacement Gas Furnace

A conventional gas furnace can waste 30 cents of every fuel dollar. Older furnaces can waste even more. Replacing an older gas furnace can save you energy dollars while also improving comfort.

Efficiency Ratings

Gas furnace efficiency is expressed as an Annual Fuel Utilization Efficiency or AFUE. The AFUE represents the percentage of the heat available in the fuel that is converted to useable heat for your home. So, for example, a furnace rated at 80% AFUE will deliver 80% of the heat available in the fuel. The remaining 20% is lost as exhaust up the flue or chimney.

New gas furnaces incorporate several features that make them more efficient than older models. These features include:

  • Electronic ignition - new furnaces use either electronic spark ignition or hot-surface igniters to eliminate the wasted gas from a constantly burning pilot light.
  • Induced draft - a small fan is used to draw combustion air through the system, reducing the amount of air needed for safe combustion and improving efficiency.
  • Automatic vent dampers close the flue pipe when the burners are off and reduce the amount of heated inside air drawn up the exhaust flue.

High Efficiency vs. Mid Efficiency

New gas furnaces are typically categorized as either "high-efficiency" or "mid-efficiency". Both are more efficient than older, conventional gas furnaces. High-efficiency furnaces, also called "condensing" furnaces, typically have AFUEs of 90% or higher. Mid-efficiency furnaces have AFUEs between 78% and 83%. Which type is best for your home depends mainly on your climate and how much heating you use annually. While high-efficiency furnaces are more expensive than the standard type, the additional cost can often be offset by the increased energy savings, particularly in colder climates or larger homes where annual heating costs are substantial.

High Efficiency Condensing Furnaces

Condensing furnaces incorporate a second heat exchanger to recapture much of the heat that would ordinarily be lost up the exhaust flue. They extract so much of the heat that the water vapor in the exhaust condenses, releasing additional heat and leaving behind condensed water (5 to 6 gallons per day on average) that is then drained or pumped away.

The exhaust gases from a condensing furnace are too cool to rise upward through a conventional exhaust flue, so they are typically vented horizontally through a wall with plastic piping. Most condensing furnaces draw their combustion air supply from outdoors, through another plastic pipe. A small fan draws the combustion air through the system and also helps propel the exhaust out through the vent pipe. Using outside air for combustion helps ensure safe, efficient operation even in very tight homes with low ventilation rates.

Pulse Combustion Furnaces

Unlike other types of gas furnaces, pulse furnaces burn gas in small explosive pulses, 60 to 70 times per second, much like an automobile engine. Pulse combustion allows the maximum amount of heat to be extracted from the burning fuel. These furnaces can achieve AFUE ratings as high as 97%. Like most condensing furnaces, they draw combustion air from outdoors and their exhaust is vented horizontally through a wall. While pulse furnaces are more expensive than even high-efficiency condensing furnaces, they can be cost-effective in very cold climates or homes with high heating requirements.

Other Features to Look For

  • Multi-stage burners match the furnace's heating output to the heating requirements of the home under the full range of outdoor weather conditions. In milder weather, the burner puts out less heat (and uses less gas), saving energy and enhancing comfort.
  • Multi-speed and variable-speed blowers reduce electric use by matching the blower speed to the heating demands of the home. The blower runs for longer periods of time but at lower speeds, providing a more even and comfortable heat. This also helps minimize wear on the system by reducing on/off cycling.

The EnergyGuide Label

New residential furnaces are required to display an EnergyGuide label, which compares annual fuel costs with those of other similar sized units. (An example of the information included on the EnergyGuide label is shown here).

Related Topics: