There are a few different types of furnaces that you have to choose from. These include 80% efficient (80 percenters), 90% and above efficient (90 percenters), naturally vented, power vented (induced), direct vent, multi-speed, variable speed, single stage, dual stage. All of these terms can get confusing. So go together and some are mutually exclusive.

• 80% furnaces are the traditional furnace. They have the metal exhaust pipe that generally goes up through the walls and out through the roof.
  • They can be either naturally vented like the lennox whisper heat models. They don't have an inducer motor. Generally they are quieter but do have drawbacks such as more susceptible to backdrafts like a wood burning fire place. Natural vented furnaces are not very common now a days. Most furnaces today are power vented and have inducer motors for this purpose. Offer many advantages in the venting and size determinations.
  • 80% furnaces today will either have a multi-speed blower or variable speed blower. The blower is fan in the furnace that circulates the air throughout your ducts.
    • Multi-speed simple means the blower speed can be adjusted to best meet the your duct system air flow desired temperature drop. It is set manually by the heating installation tech at the time of installation and can be adjusted by your heating service technician as needed. Two stage furnaces have two blower speeds that the technicians can set. Two stage discussed below.
    • Variable speed means that the furnace can automatically regulate the blower speed to match the heat called for. Can save a lot on your energy bills but the initial cost of the equipment is more than the multispeed.
    • Two-stage - The furnace basically has a high and low setting. When it is not as cold out and less heat is need to warm up your house the furnace can run in low speed saving you money on your utility bills. When more heat is called for the furnace will can either start immediately in high or switch over from low to high depending on how the thermostat is set and how long it is taking to heat up the house. To be considered two stage the gas valve must be 2-stage so it can have a high and low flame setting, the blower must be either a variable or multispeed motor.
• 90% furnaces are the "new great thing" although they have been around quite awhile. When you hear 90% it means that the furnace has an AFUE or efficiency rating of 90% or higher. Basically they have a second heat exchanger to recover more of the heat before exhausting the combustion flue gases out the vent.
  • These furnaces are generally vented with PVC pipe out the side of the house although the PVC can run out the roof as well. These can have a single or two pipe vent. If they have two pipes they are called direct vent because outside air used for combustion is sucked directly into the furnace
  • These furnaces can have single stage or two stage gas valves, multi-speed or variable speed blowers same as the 80% furnaces discussed above.
  • They will always be power vented by an inducer motor because the second heat exchanger makes the exhaust gases too cold to naturally rise up and out the vent like an 80% furnace.