Almost 35 million homes in America are heated by natural gas-fired forced-air heating systems, by far the most popular form of central heating. Most of these households have been sending 30% or more of their energy dollars up the furnace flue and, by doing that, each has pumped up to four tons of carbon dioxide, the "greenhouse gas," into the atmosphere every month. Most conventional forced-air furnaces operate at very low efficiencies -- some taking advantage of only half the fuel they burn.

To make matters even worse, as your furnace sends energy-heated air up the chimney, it must draw new air from somewhere. Somewhere is around your windows, doors, electrical outlets, and any other place where exterior air can squeeze in. If it can't replace the discharged air, fumes will back up into the house, causing a dangerous or fatal situation. New air sucked into the house is cold, and works against your heating efforts. This can be minimized, or entirely avoided with very high-efficiency direct-vent heating systems (more later). If you have a traditionally vented fireplace, you may have noticed how cold the outlying rooms get when it is in use. That's because the fireplace discharge is sucking outside air into those rooms. Your furnace can do the same thing, but it just isn't as noticeable. 

Heating units built during the 1960s or earlier have low Annual Fuel Utilization Efficiency (AFUE). Gas-fired furnaces and boilers can have AFUEs as low as 50 percent; oil-fired units can be around 55 percent. If you think in terms of dollars, I translate this to mean that for every dollar of fuel being burned, only 50 to 55 cents is used for heat. The remaining 45 to 50 cents goes up the chimney or is vented from the house as exhaust. Only in the late 1980s and early 1990s did the minimum AFUE on gas- and oil-fired systems change considerably. Today, minimum AFUEs are 78 for fossil-fueled warm-air furnaces and 80 for fossil-fueled boilers. Efficiency ratings today go as high as 97%.

Do I Really Need to Replace my Furnace?

A definite---maybe.

If your furnace or boiler is older than 20 years, it is probably a good investment to replace it with a high-efficiency model.  Also consider a replacement now if your system is one of the following:

• Old coal burner that was previously switched over to oil or gas

• Old gas furnace without electronic ignition. If it has a pilot light, it was probably installed prior to 1992 and has an efficiency of about 65% or less efficient.

• Old gas furnace without vent dampers or an induced draft fan (which limit the flow of heated air up the chimney when the heating system is off).

If your furnace or boiler is 10–20 years old, and you are experiencing discomfort or high utility bills, hire a highly-qualified home performance or heating contractor who can help you evaluate your existing system. Often it will be more cost-effective to improve house insulation and air-tightness, repair or insulate ductwork, or tune up your system.

 

How Can I Save, If I Decide to Keep my Current Furnace?

You can substantially control you fuel costs by maintaining your system..

• Clean or replace air filters regularly. You will get better quality filtration with a pleated, close-textured filter. But because of their efficiency, they may need to be replace more often. In any case, air filters need to be replace at a minimum of once each month to maintain fuel efficiency. 
   

• Clean Vents. Warm-air supply and return vents should be kept clean and should not be blocked by furniture, carpets, or drapes. Cleaning air ducts will not usually improve efficiency, and in fact, if not done correctly, can cause a lot of particulate matter to enter your home.
   

• Keep baseboards and radiators clean and unblocked by furniture, carpets, or drapes.
   

• Bleed trapped air from hot water radiators, following proper procedures. Call a contractor if not entirely sure of what to do, since water or steam can be under pressure, and cause severe burns.

• Get a tune-up from a heating contractor. Oil-fired systems should be tuned up and cleaned every year, gas-fired systems every two years, and heat pumps every two or three years. Regular tune-ups not only cut heating costs, but they also increase the lifetime of the system, reduce breakdowns and repair costs, and cut the amount of carbon monoxide, smoke, and other pollutants pumped into the atmosphere by fossil-fueled systems.
   

• Seal your ducts. In homes heated with warm-air heating, ducts should be inspected and sealed to ensure adequate airflow and eliminate loss of heated air. It is not uncommon for ducts to leak as much as 15-20%. And, leaky ducts can bring additional dust and humidity into living spaces. Thorough duct sealing can several hundred dollars but can cut heating and cooling costs in many homes by 20%. Some homeowners choose to take on duct sealing as a do-it-yourself project. Start by sealing air leaks using mastic sealant or metal tape and insulating all the ducts that you can access (such as those in attics, crawlspaces, unfinished basements, and garages). Never use duct tape, as it is not long-lasting. Also, make sure that the connections at vents and registers are well-sealed where they meet the floors, walls, and ceiling. These are common locations to find leaks and disconnected ductwork.

• Check for wasted fan energy. If your furnace is improperly sized or if the fan thermostat is improperly set, the fan may operate longer than it needs to. If you're getting a lot of cold air out of the warm-air registers after the furnace turns off, have a service technician check the fan delay setting.