During the past few years, manufacturers have used several innovations to boost efficiency. One early advancement was the move from the standing pilot light, which burns gas even when the furnace is dormant, to electronic spark ignition that fires the furnace on demand. Yet another step forward is "hot surface ignition," a method said to be more reliable than the electronic spark. Rather than using a spark plug that can corrode or be fouled by dirt or bugs, hot surface ignition ignites the gas mixture with a coil that glows white hot.

Lennox Industries' Pulse21 furnace uses a technology called "pulse combustion." With this system, a refined spark system ignites an air-and-gas mixture in the combustion chamber 60 to 70 times per second, delivering heat at up to 96.2 percent AFUE (Annual Fuel Utilization Efficiency).

Most furnace models operate at AFUEs of either 78 percent to 80 percent or at 90 percent and higher. The higher-efficiency models are "condensing gas furnaces." They run exhaust gasses through a second heat exchanger to extract and use available heat that's otherwise sent up the flue. These models pull out nearly all of the heat, sending cool exhaust out the flue (or vent, in this case) and leaving behind condensed water in the heat exchanger and vent. This condensate, 5 to 6 gallons per day on average, is drained or pumped away.

A key feature to look for when buying a condensing gas furnace is a long-term warranty on the heat exchanger; the best types are built to resist the corrosive effects of moisture and chemical buildup for the life of the house. One bonus of condensing furnaces is that they may be vented out through a wall with inexpensive PVC pipe, an important feature that saves money and the hassle of routing a flue up through the roof.

The combustion side of furnaces, the mixing of combustion air with fuel, has also enjoyed technological advances. High-efficiency furnaces keep close control over the amount of air mixed with gas. For example, York's Stellar Plus furnace, with an AFUE of 92 percent, uses sealed combustion, bringing all combustion air from outdoors and mixing it with the fuel at a controlled rate to maximize heat from the fuel it consumes.

Gas valves have become more sophisticated, too. Carrier's Weathermaker Infinity has a two-stage gas valve that warms up the furnace quickly and then drops back to a more economical flow.

Many gas-fired high-efficiency furnaces also save on the electricity required to power the blower motor, though this savings is not factored into the AFUE rating. They do this by coupling a sophisticated, programmable thermostat to a variable-speed motor. Unlike a conventional system—where the furnace goes on, blows hot air into the house at full force for a few minutes, and then shuts off—a variable-speed or "variable-capacity" system runs the blower for longer periods at lower speeds. It provides quieter and more even and comfortable heat than a conventional furnace and doesn't consume electricity unnecessarily because it rarely runs at full speed.

For homeowners who want to upgrade rather than replace an existing furnace, Emerson Electric offers an exclusive replacement blower motor called the Mark Z that can be set by the homeowner on a continuous, slower-than-normal speed. It offers three settings: 525 RPM for saving energy, 900 RPM for heating, or 1100 RPM for air conditioning. According to the company, the slower circulation speed reduces energy consumption by 75 percent.

Furnace models with the highest efficiency ratings, those with an AFUE greater than 90 percent, combine several advancements into one package. For example, Carrier's Weathermaker Infinity (AFUE of 93.5 percent) is a condensing gas furnace with a microprocessor control board that automatically adjusts the furnace output and blower speed for optimum efficiency based on information from the thermostat. It comes equipped with General Electric's electronically commutated motors (ECMs). With programmable controls, these cut electrical consumption from a typical 625 watts per hour to 80 watts per hour. This variable-speed system also saves on electric bills when an air conditioner is added to the unit.

Zoned heating is one of the hottest new concepts in efficient energy usage. With a zoned system, you can independently control the air flow sent to various rooms or zones in your home, directing heating or cooling where you want it at various times of the day. To make this possible, a system needs a special, multi-zone programmable thermostat and a few motorized dampers. For best results, the furnace's output should be variable. The motor in the Lennox Pulse21V, for example, can be controlled over an infinite range of speeds, automatically adjusting the amount of heating or cooling delivered throughout the house according to the need.