A single stage furnace is a type of gas furnace that has only one heating output setting. It operates at full capacity whenever it is turned on, providing the maximum amount of heat output that it is capable of producing. When the thermostat signals the furnace to turn on, the gas valve opens, and the burner ignites, heating the heat exchanger. The blower fan then circulates the heated air throughout the home.

Single-stage furnaces are generally less expensive than two-stage or modulating furnaces and are easier to install and maintain. They are a good option for those who are looking for a simple and cost-effective heating solution. However, they may may not be as energy efficient as other types of furnaces.