Inside This Article:
How Furnace Efficiency Is Measured
How Much Does a Furnace Cost?
Energy-Saving Furnace Design
Electronic Pilot & Condensing Gas Furnaces
Zoned Heating Systems
How to Size a Furnace
Furnace Flow Direction
Is it time for you to buy a new high-efficiency furnace? Sky-high energy bills, diminishing resources, and environmental concerns have brought a great deal of attention to the topic of home energy efficiency in recent years–especially when it comes to heating. Homeowners are struggling to spend less, use less, and pollute less without giving up the warmth and comfort they’ve come to cherish. If you intend to stay in your home for a few years, upgrading from an old, inefficient furnace to a new, high-efficiency model may save you money in the long run while improving your comfort.
Over 35 million homes in America are heated by natural gas-fired, forced-air heating systems. Unfortunately, many of these households have been sending 30% or more of their energy dollars up the furnace flue, and, in doing so, each has pumped up to 4 tons of carbon dioxide, the “greenhouse gas,” into the atmosphere every month. Old forced-air furnaces operate at very low efficiencies—some taking advantage of only half the fuel they burn. In an effort to curb this waste and pollution, the U.S. Department of Energy (DOE) instituted standards at the beginning of 1992 that required every new furnace to turn at least 78% of its fuel into heat. Manufacturers responded with models that meet and sometimes far surpass this.
How Furnace Efficiency is Measured
The measurement for efficiency is called an Annual Fuel Utilization Efficiency (AFUE) rating. All furnaces now come posted with this rating, generally in the form of a yellow “Energy Guide” label. AFUE ratings run from the 78% minimum to 98.2%. 
Though most makers list their furnaces as “high-efficiency,” the DOE refers to units with an AFUE higher than 90% as “high-efficiency” and 80% to 83%-AFUE models as “mid-efficiency.” Yellow Energy Guides list estimated annual operating costs for furnaces under specific conditions; they’re meant for comparison only.
How Much Does a Furnace Cost?
How much does it cost to replace an old furnace with a new, high-efficiency model? That depends upon the circumstances. If the new unit can be connected to existing ductwork and exhaust flue, the new unit should run from about $3000 to $4500, installed. The higher the efficiency of the furnace, the higher the price. But keep this in mind: Though high-efficiency models represent about $500 to $1,000 more in material costs than mid-efficiency units, they pay you back with energy savings and qualify for tax credits. The payback period depends on the price of the system, local energy costs, your climate, and the difference in efficiency between the old and new furnace.
HomeTips Pro Tip: When buying a system, you should ask your dealer to help you figure savings and payback. Please note: Natural gas or propane furnaces with an AFUE of at least 95 qualify for a federal tax credit of $150.
Energy-Saving Furnace Design
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. "Sealed combustion," which means bringing all combustion air from outdoors and mixing it with the fuel at a controlled rate, maximizes heat from the fuel.
Gas valves have become more sophisticated, too. A two-stage gas valve 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 and doesn’t consume electricity unnecessarily.
Furnace models with the highest efficiency ratings, those with an AFUE greater than 90 percent, combine several advancements into one package. For example, the Lennox SLP98V, which has up to 98.2% AFUE, automatically adjusts the speed to match the heating requirements, captures maximum heat with a stainless steel secondary heat exchanger, and can interact intelligently with a thermostat for optimal efficiency. The Carrier Infinity® ICS has up to a 95% AFUE; it has 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. Both of these are very high quality products and worthy of consideration.
Electronic Pilot & Condensing Gas Furnaces
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.
The higher- efficiency models are “condensing gas furnaces.” They run exhaust gases through a second heat exchanger to extract and use available heat that’s otherwise exhausted. These models pull out nearly all of the heat, sending cool exhaust out and leaving behind condensed water. This condensate, 5 to 6 gallons per day, 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.
Zoned Heating Systems
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 multizone programmable thermostat and a few motorized dampers. For best results, the furnace’s output should be variable so it can adjust the amount of heating or cooling delivered throughout the house according to the need.
How to Size a Furnace
The size of a furnace is critical to efficiency. A system that’s too large wastes energy warming up and cooling down as it continuously cycles off and on, and a furnace that’s too small will fail to warm the house on really cold days. A properly sized system is designed as closely as possible to the needs of the house; on the coldest days of the year, it should run almost continuously.
The size of a heating system is measured by its “BTU” per hour input or output capacity, depending upon the manufacturer. A BTU (British Thermal Unit) is equal to the amount of heat required to raise 1 pound of water 1 degree F.
Input capacity refers to the amount of gas burned without taking into consideration the percentage of heat lost through the flue. Output (or “bonnet”) capacity refers to the total usable heat generated.
Most manufacturers make several sizes of each model: for example, 40,000-, 50,000-, 60,000-, 80,000-, and 100,000-BTU input capacities. Choosing the right size and type is a job for a qualified central air conditioning contractor or dealer. Proper size should be based on energy-loss calculations figured from your house size, window areas, insulation levels, and related features. You can’t just replace a furnace with a high-efficiency one that’s the same size unless you match output capacities; high-efficiency furnaces often have smaller input capacities than their inefficient predecessors because they generate more usable heat.
Furnace Flow Direction
Furnaces come in “upflow,” “downflow,” and “horizontal” models designed to accommodate basement, attic, or limited-space installation. An upflow furnace is generally placed in the basement; as its name suggest, the heat from it flows upward. A downflow furnace typically goes in the attic. A horizontal model usually goes in an attic with very limited headroom.
