From the 1950s through the early 1980s, most furnaces had AFUE efficiency ratings of about 65 percent. That meant approximately 35 percent of the heat the furnace produced was lost up the fluepipe. Over the last couple of decades the AFUE rating of all furnaces has risen to the point that some boast 97-percent efficiency.
Currently, national standards require that furnaces yield a minimum of 78-percent efficiency. It is possible to purchase one with nearly any efficiency rating between 78 percent and 95 percent, depending on what features are most important to you and how much you can afford.
The furnaces at either end of the spectrum differ markedly in how they operate. It's important to know the difference between how each functions before talking with a contractor about possibly replacing yours.
Comparing Furnace Types
A basic fact about all forced-air furnaces is that they need to mix air with the fuel they burn to combust that fuel properly. How a furnace gets that air is one dividing line between lower- and high-efficiency furnaces. Lower-efficiency furnaces draw combustion air from the room in which they are installed; high-efficiency furnaces draw combustion air directly from outside the house. Why is this important?
Every time the burner on a standard-efficiency furnace starts up, it draws air into the combustion chamber. That air is burned along with the fuel and sent up the fluepipe. The air comes from within the house -- and therefore has been previously heated by the furnace. And as the furnace draws in air to burn, new air has to come from somewhere to replace that which is being burned up and vented outside. Additional air is drawn into the house through cracks and gaps in the exterior walls and ceiling. This creates dry conditions (exterior air is extremely dry in the winter), drafts, and inefficiency, because the incoming cold air has to be heated by the furnace.
By comparison, a super-high-efficiency, or "sealed-combustion," furnace draws its combustion air from outside the house via a PVC plastic pipe. Since the combustion air is coming directly into the firebox from outside, room air is not being burned up and vented out the flue. Because there is no demand for additional air drawn from within the house, there is no continual influx of cold, dry air from outside. The result is much greater efficiency, fewer drafts, not as many problems with dry air, and a warmer, more comfortable house.
It should be said, however, that building codes require an air-intake pipe that provides outdoor air in the vicinity of fuel-burning heating and water-heating equipment. This allows the burners to draw air from outside the house for combustion, but the intake is not as controlled as it is with sealed combustion units. And many older homes with equipment installed before the code requirement lack an air-intake pipe.
Assessing Furnace Speeds
While 95-percent-efficiency furnaces offer homeowners the greatest benefit in terms of energy savings, lower AFUE-rated units have also been upgraded to provide better comfort and efficiency. One feature showing up on many sub-90-percent newer furnaces is two-stage or variable speed burners.
Older furnaces had only one firing capacity; the burner was either on or it was off. On a chilly -- but not cold -- day, that meant the furnace might come on for only a few minutes and then shut off again, having quickly raised the indoor temperature. Running a furnace that way is inefficient. It can be compared to starting a car to drive only a few blocks, then shutting it off only to turn it on again to drive a few more blocks. Cars and furnaces are both more efficient when they can run at their optimum operating temperatures.
Two-stage and variable-speed furnaces use sensors to control the flow rate of the fuel through the burner. On chilly -- but not cold -- days, the furnace runs at the low setting, but for a longer period of time. This allows it to operate at the most efficient temperature and without the many stops and starts that create inefficient burning. On cold days it burns at full capacity to accommodate the more demanding heat load. Most two-stage and variable-speed furnaces run at low settings approximately 90 percent of the time.
In addition to saving money, running the burner and blower longer at low settings distributes air in rooms more evenly and pushes more air through the furnace filter, which results in cleaner air.
In recent years, furnace manufacturers have been making furnaces quieter as well as more efficient. Virtually any newer model will be quieter than one ten years old or older. The interiors of the metal cabinets are lined with sound-absorbing material, and blower-fan blades are engineered and balanced to reduce noise.
Two-stage and variable-speed furnaces are especially quiet. A burner firing at low capacity produces less noise than one firing at full throttle, and the blower-fan speed can be reduced to integrate with the lower heat output. The combination hushes the sound of the air rushing from the heat registers.
Other Furnace Considerations
While furnaces have evolved over the years to offer greater comfort and efficiency, they have also become more complex. Instead of simple controls and moving parts that could be repaired or replaced by virtually anyone -- even in some instances a handy homeowner -- the furnaces on the market today are run by computers. And instead of a simple service call to a local furnace installer if your furnace breaks down, it requires a visit from a highly trained technician.
That simple $20 part replacement is a thing of the past, too. If a computer motherboard needs replacing in a modern furnace, the charge can run into hundreds of dollars.
The good news is that new furnaces are reliable and durable, and most offer generous warranties.
Replacing your boiler or air-conditioning system also can reduce your utility bills. In the next section, we will review tips on replacing your boiler or AC.