Single Stage Furnace

The gas flame is either ON or OFF. The furnace waits until the temperature in the house gets cold to send a powerful flow of warm air to heat up the house. Once the temperature reaches the desired level, the thermostat shuts the system down and restarts the cycle.

Single stage furnaces are the least efficient systems in terms of energy, but they are the lowest cost to purchase.

Two-Stage or Dual-Stage Furnace

This type of furnace keeps the flame “on/high”, “on/low” or “off”. By adding this second flame the system makes a dramatic difference in energy cost. Dual-stage furnaces are quieter and slightly higher in cost.

Modulating or Variable Furnace

This is the most comfortable gas furnace you can buy. A modulating furnace will prevent the temperature fluctuations typical of single- and dual-stage furnaces that leave some rooms with cold spots where the heat doesn’t last between furnace cycles or hot spots where heat builds up. You will also avoid the periods of cold before the furnace kicks back on. A modulating furnace automatically makes slight continual adjustments, offering you consistent, comfortable temperatures throughout your home and operates more quietly and efficiently than other kinds of gas furnaces.

Variable gas furnaces are the most energy efficient furnace with 98% AFUE.

In the illustration below, compare the efficiency levels of a single-stage, dual-stage and modulating furnace when you set your home temperature to 22 degrees Celsius.