How Two-Stage Furnaces Work

Stage One

• Stage one is when the furnace is exerting its lowest heating capacity. In many cases, this may only be about 50 percent of what it's capable of producing. For example, if the temperature outdoors is 58 degrees Fahrenheit, and you have the thermostat set at 68 degrees, a 100,000 BTU (British Thermal Unit) capacity might start in stage one, using only 50,000 BTUs, and continually blow enough warm air through the duct and ventilation system until the room temperatures reach 68 degrees. During stage one, the motor of the furnace is not running at full blast. Instead, a steady, but gentle airflow can be felt. The system "understands" that it doesn't need to exert all of its power just to achieve a 10-degree increase in the indoor temperature.

Stage Two

• Stage two is when the furnace is giving its all and running at full blast. During stage two, the motors kick into high speed, forcing air through the ductwork and vents faster and with a stronger airflow. For example, if the outdoor air temperature was minus 16 degrees Fahrenheit, and the thermostat was set at 68 degrees, the temperature differential would be 84 degrees. In this case, a 100,000 BTU furnace would start with stage two, ramping up to exert every BTU of heat it is capable of generating.

Intelligent Technology

• Two-stage furnaces utilize what is called "intelligent technology." This term is used to describe how the furnace and the thermostat communicate with each other, as co-workers and components of your home's overall HVAC system. The thermostat actually directs the two-stage furnace and tells it when to use stage one and when to use stage two. On a day when it is brutally cold outside, the thermostat will tell the furnace that it needs to go straight to stage two. As the home starts reaching the desired room temperature, such as 60 degrees when the desired temperature is 68 degrees, the thermostat will signal the furnace to retreat to stage one. The intelligent technology feature of two-stage furnace systems keep indoor temperatures more consistent than the wild on and off fluctuations that you would feel with a single-stage furnace system.

Air Filtration and Recirculation

• You can also choose a two-stage, two-pipe furnace systems to improve air filtration, recirculate air and increase efficiency. A two-pipe system uses a second pipe to recapture cool air inside of the home, heat it and recycle it back into rooms. This makes two-stage, two-pipe systems even more efficient. For example, if the indoor air is 60 degrees Fahrenheit, and the temperature is set to 68 degrees, air from the second pipe only has to be heated 8 degrees. Because the recirculated air passes through the system, twice, it also gets filtered twice -- helping to improve the air quality inside of the home.