Variable Speed Furnace

• The term "variable" applies to the speed of the motor in your furnace. Variable furnace motors utilize intelligent technology to process data from the blowers of the furnace to adjust the motor speed to the appropriate rate to keep your home's temperature consistent. Variable speed furnaces come in two types: variable speed, two stage, two pipe; and variable speed, two stage. The benefit of having a two-pipe variable furnace is that it will re-circulate air from indoors instead of outdoors. A single pipe warms new, outdoor air. Therefore, it has to work harder. Homes that feel drafty often have a single pipe variable speed furnace.
  
  

Variable Motor Speed Efficiency

• A variable speed, two stage, two pipe furnaces have an Annual Fuel Utilization Efficiency rating of at least 90 percent. In comparison, the AFUE for variable speed, two stage motors with one pipe start at 80 percent. The 10 percent difference will show up in your energy use and consequently, your energy bill. So, if you want to reduce the amount of wasted energy, you should look for a furnace with the highest AFUE rating that you can afford.
  
  

Single Stage Furnace Burners

• Once upon a time, single stage furnaces ruled because they were the only choice. But that is no longer the case. Single stage furnaces are inefficient, old-school models. They run, and keep on running (sort of like the Energizer Bunny) until they reach their full British Thermal Unit capacity. For example, once a single stage, 100,000 BTU furnace comes on, it continues to heat air until it has exhausted every one of its 100,000 BTUs. Suppose you had an oven and your only choices switch choices were "on" and "high." That's akin to how a single furnace operates.
  
  

Single Stage Burner Efficiency

• Single stage furnaces are sorely lacking in efficiency. They operate under an "all or nothing" proposition. When the thermostat signals a single stage furnace, and says -- "OK, it's time to turn on the heat," the single stage furnace burners come on at full power, and keeps running at full power. Single stage furnaces lack the technology intelligence and mechanical capabilities to take a break. On the other hand, two-stage furnaces are much more efficient (and intelligent). They have two operation modes: full power and half power. They start at half-power, and check back with the thermostat to see how rooms are heating up. If, and only if, the thermostat says--"not enough," does the two-stage furnace up its game and increase the flame on the burners to full power. A two-stage furnace wins over a single-stage furnace if you want a furnace that's intelligent and efficient.